Normal Physiology

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YEREVAN STATE MEDICAL UNIVERSITY AFTER M. HERATSI

NORMAL PHYSIOLOGY

HANDOUT FOR FOREIGN STUDENTS

YEREVAN – 2008

YEREVAN STATE MEDICAL UNIVERSITY AFTER M. HERATSI

Ter-Markosyan A.S., Harutunyan K.R., Arakelyan K.P., Avetisyan K.A.

NORMAL PHYSIOLOGY HANDOUT FOR FOREIGN STUDENTS

Editor: professor Khudaverdyan D.N.

YEREVAN Publishing house of the Yerevan State Medical University after M. Heratsi 2008 UDC 612 (07)

Normal Physiology (Handout for Foreign Students) / Ter- Markosyan A.S., Harutunyan K.R., Arakelyan K.P., Avetisyan K.A. -Yerevan, YMSU, 2008 - 330 pp.

Editor: professor Khudaverdyan D.N. Reviewers: Khanbabyan M.V., Professor of the Human and Animals’ Physiology Department of the Yerevan State Pedagogical University after Kh. Abovyan, Doctor of Medical Sciences Hakobyan N.S., Professor of the Human and Animals’ Physiology Department of the Yerevan State University, Doctor of Biological Sciences English language editor: Bisharyan M.N.

In the handout are represented the main parts of physiology, which correspond to the syllabus of the normal physiology course. It will be useful for foreign students of medical and biological high schools.

The handout is adopted by Methodical Comission for Foreign Students of theYerevan State Medical University after M. Heratsi.

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CONTENTS

Introduction 10 Chapter 1. Physiology of Excitable Tissues 16 1.1.General Physiology of Excitable Tissues 16 Bioelectrical phenomena. Historical outline 17 Resting potential 19 Action potential 22 Passive and active transport of ions 25 Changes of excitability during excitation 27 Comparative characteristics of the local (LP) and action (AP) potentials 28 Parameters of excitation 29 The effect of direct current on excitable tissues. The law of stimulation polarity. Physiological electrotone 32 1.2.Physiology of Nerve Fibre 34 Classification of nerve fibres 34 Laws of excitation conduction in the nerve 36 Mechanism of impulse conduction in the nerve fibres 37 1.3.Physiology of Neuromuscular Transmission 39 Mechanism of excitation transmission in neuromuscular synapse 40 The properties of the neuromuscular (chemical) synapse 42 1.4.Physiology of Muscles 43 Skeletal Muscles 44 Types of the muscle contraction 45 Muscle single contraction 45 Tetanus and summation of contractions 46 Ultrastructure of myofibrils 47 Mechanisms of muscle contraction and relaxation 49 Work and force of the muscle 51 Some peculiarities of smooth muscles 52 Chapter 2. Physiology of the Central Nervous System 53 2.1.General Physiology of the Central Nervous System 53 Structural and functional elements of the CNS. Neuron and glia 53

Interneuronal communications 59 Reflector character of the CNS activity 62 Inhibition in the CNS 64 Nervous centre’s properties 69 Principles of coordination in the CNS 73 2.2.Special Physiology of the Central Nervous System 79 Spinal cord 79 Hindbrain 84 Midbrain 87 Cerebellum 89 Diencephalon. Thalamus and hypothalamus 92 Chapter 3. Higher Nervous Activity 97 3.1.Conditional and Unconditional Reflexes 97 Rules for building conditional reflexes 99 Components of unconditional and conditional reflexes 99 Mechanisms of conditional reflex producing 100 3.2.Cortical Inhibition 102 Analysis and synthesis 104 Mutual induction of excitation and inhibition 105 3.3. Types of the Higher Nervous Activity 105 3.4. Sleep 107 Chapter 4. Physiology of the Vegetative Nervous System 115 General characteristics of the vegetative nervous system 116 Comparative analysis of the somatic and the vegetative nervous systems 120 Properties of the vegetative ganglia (synapses) 121 Mechanism of impulse conduction in vegetative synapses, their mediators 123 Vegetative reflexes 127 Chapter 5. Physiology of the Endocrine System 128 5.1. General Characteristics of the Endocrine Glands 129 Structure, properties and action mechanism of hormones 131 5.2. The Hypothalamo-hypophysial System 136 The hypothalamo – extrahypophysial system 139 The hypothalamo- neurohypophysial system 140 The hypothalamo-adenohypophysial system 141 5.3. Special Physiology of the Endocrine Glands 146

Physiology of the thyroid gland 145 Physiology of the parathyroid glands 148 Physiology of the adrenal glands 150 Physiology of the sex glands (gonads) 154 Physiology of the pancreas 159 Chapter 6. Physiology of the Blood System 162 6.1. Internal Medium of the Body 162 Composition and properties of blood 162 Composition and properties of lymph 166 6.2. Blood Formed Elements 168 Erythrocytes 168 Leucocytes 172 Thrombocytes (platelets) 176 6.3. Blood Coagulation 177 Vascular-platelet hemostasis 178 Coagulation hemostasis 179 After-phase of hemostasis 180 Anticoagulation mechanisms 180 Regulation of blood clotting 181 6.4. Blood Groups and Rh-factor 183 Blood groups 183 Rhesus-factor 185 Chapter 7. Physiology of the Cardio-vascular System 187 7.1. Physiology of the Heart 187 The heart conductive system and automatism 188 The phase analysis of the heart cycle 194 Methods of investigation of the heart activity 196 Cardiac muscle physiological peculiarities 201 Regulation of the heart functional activity 204 7.2. Physiology of the Vascular System 217 General principles of the structure and functioning of the vascular system. 217 The main indices of hemodynamics 220 Arterial pulse 226 Regulation of blood circulation 229 Chapter 8. Physiology of the Respiratory System 235 External respiration 235 Lung volumes 241

Lung ventilation 243 Gas exchange in alveoli and tissues 244 Gas transport by the blood 249 Regulation of respiration 252 Respiration under various circumstances 260 Chapter 9. Physiology of the Digestive System 266 Functions of the digestive system and types of digestive processes 266 Digestion in the mouth 268 Digestion in the stomach 273 Digestion in the small intestine 280 Digestion in the large intestine 288 Absorption 291 Motor activity of the gastro-intestinal tract 293 Periodic activity of the digestive organs 298 Chapter 10. Metabolism of Energy and Thermoregulation 301 10.1. Metabolism of Energy 301 10.2. Thermoregulation 307 Chapter 11. Physiology of the Excretory System 311 Morpho-functional characteristics of the kidneys 311 Kidney blood supply 314 Formation of urine 316 Urine excretion and micturition 325 Endocrine function of the kidney 325 Regulation of kidney function 327 Literature 329

Introduction

The term “Physiology” generates from the Greek words – physics (nature) and logy (studying). Physiology studies the main processes of alive organism’s activity, its organs, tissues, cells and their structural elements in conjunction with external environment. So the goal of physiology is to reveal the organs’ functions, intra-organic connections, and interactions between the organism and surrounding medium which is the necessary condition for its existence. Physiology and other disciplines. Physiology is in close connection with other sciences and firstly with such morphological disciplines as anatomy, histology, cytology, since structure and function are interconnected and determine each other. As during the activity of cells the convection of substances and energy proceeds, physiology is based on the rules of physics and chemistry that brings to the development of separate disciplines – biophysics and biochemistry. Physiology is the theoretical base of general pathology and clinical medicine. Without deep comprehension of the heart, lungs, digestive system and other organs’ function it is impossible to reveal the pathology. Achievements of physiology are of great use in medicine, e.g. Pavlov’s works on the physiology of the digestive tract organs serve as a basis for gastroenterology and dietology. In its turn medicine gives a clinical material for physiological study, e.g. different clinical manifestations of endocrine impairments enable the further study of the inner secretion glands’ function.

The connection of physiology and cybernetics is the point of interest and helps to reveal the general principles of function regulation of their interaction using artificial modelling of biological phenomenon. Physiology is an experimental science. The investigator intervenes artificially in organism function. In 1628 English physician William Harvey using the experimental method of investigation, published the data of his observations in the small booklet “Anatomical investigations of the heart and blood motion in animals” and settled the basis for the great science - Physiology. Harvey is considered to be the founder of Physiology. The main methods of physiological investigations. The main methods are the observations and experiment. Experimental methods are divided into two types – acute, that was widely used by Harvey at the beginning of the 17th century; and the chronic one, that was deeply worked out in Pavlov’s investigations. Acute method or vivisection is performed in immobilized animal by means of narcotic preparations or otherwise, during which they perform section and the further investigation of the interesting organ. But this method has its disadvantages. First, it is impossible to carry out the experiment for a long time, as well as to use that animal repeatedly; second, the organ activity is studied beyond its connection with other ones and out of normal physiological conditions. It is the analytical method of investigation. But, in spite of these disadvantages, all the main information about functions of majority of organs has been gained with the help of this method, and up to now it has remained as one of the main investigation methods.

In case of chronic method the animal undergoes different surgical operations in the aseptic and antiseptic conditions. After recovering under the normal physiological conditions the function of the organ in the whole organism is studied using the adequate stimulus. The opportunity of multiple repeating of the experiment on the same animal is an advantage of the chronic method. The chronic method is a synthetic one. Clinical and functional tests, as well as the control of human functions in space also refer to the chronic method. The most implementing methodical ways are the extirpation of an organ, transplantation, denervation (cutting of the nerves) or the pharmaceutical inhibition of the organ activity by different types of solutions, the content of which is chosen by the investigator. Owing to the scientific achievements in the field of physics, the electrophysiological methods, the registration of bioelectrical phenomena in alive cells (ECG, EEG, evoked potentials and others) have been developed. That is a rather subtle type of investigation. Along with them graphical registration of experimental data that have been used since the 19th century (mechanocardiography, pneumography, miography etc.) also remain applicable. The main physiological regularities. From the physiological point of view, the organism is an independent unit of the organic world that is capable to regulate its functions and to react on different changes of external environment. Adaptation of the organism to altering conditions of external environment or to its own requests is carried out by

12 physiological functions and particularly by regulation or self- regulation processes. The self-regulation of the organism function, as well as interconnection between organs and systems take place in two ways, humoral and neuronal. Humoral mechanism is phylogenetically older one and is propagated by chemical substances, hormones, as well as biological active substances and metabolic products. The main property of the chemical stimulus is the absence of the specific address and it influences on all the cells. But in spite of it the sensibility of cells may be diverse, i.e. the pancreatic hormone insulin acts on the diversity of cells, changing (increasing) the permeability of membranes to glucose, especially in fatty tissue and muscles. The hormone of the thyroid glands, thyroxin, evokes the energy changes in the cells of the organism, particularly in the heart and CNS. Neuronal mechanism is the most important way of regulation. The neuronal impulse has an appropriate address, i.e. from the motoneurons of the definite segments of the spinal cord thoracic part the impulse passes along the nerves to the intercostal muscles; from the cervical segments to the diaphragm, etc. Reaction is got faster by the neuronal way, than by the humoral one. The both regulation types are interconnected; the chemical substances influence on the neuronal cells, altering their functional state (CO2 – on the respiratory centre), on the other hand, the function of the respiration regulating nervous centres ensures the certain concentration of O2 and CO2 in blood. The higher the evolution level of animal, the more is the neuronal mechanism part in regulation.

Self-regulation of physiological functions is realized automatically by means of the feedback mechanisms. It means, that regulated organ itself stimulates the regulatory mechanism keeping on its own function of the definite optimal for the organism level. An increased level of glucose in blood causes enhanced production of insulin, that regulates the glucose level in blood. According to Claud Bernard “All vital processes have only purpose to maintain the constancy of the internal medium, that is the necessary element for the beneficial life” (1878). In 1929 by American physiologist Cannon the assumption of the internal medium constancy was developed and introduced in physiology as “homeostasis” notion. Homeostasis is a relative constancy of the internal medium and several physiological functions of organisms (blood circulation, composition of blood, metabolism, thermoregulation, etc.). Maintenance of the blood content constancy, which together with the lymph and the intercellular fluid makes up the internal medium of organism, is of a crucial importance. Among the parameters of the internal medium the more stable are: blood pH – 7.36 – 7.4; the osmotic pressure - 7.6 atm, ionic interrelations (Ca – 9-11 mg/% , Na – 0.8%; P – 3-4 mg%); the inter-products and the final metabolic product concentration (nitrogen – 40 – 60mg%; urea – 30mg%, etc.) the nutritional substances concentration (glucose – 80 – 120mg%, proteins – 7.2%, etc). In such a manner in spite of continual income into organism from outside and produced during metabolism osmotic active products, the osmotic pressure value remains at a definite level (7.6 atm.). It occurs

14 due to regulation mechanisms and is changed only in pathology. The mechanisms providing homeostasis are called homeokinetic ones: e.g. in salt content increase special osmoreceptors send the impulses flow to the supraoptic nucleus of hypothalamus stimulating the antidiuretic hormone’s (ADH) production. Influence of ADH on the kidneys stimulates the

H20 reabsorption to the blood, restoring the Posm. In case of H20 content increase in blood, i.e. Posm. decrease, the impulse flow to the hypothalamus is reduced and ADH is produced in less amount, and consequently the kidneys excrete water from plasma, re-establishing the Posm. value. In impairment of the homeostasis constants the pathological state develops.

CHAPTER 1. PHYSIOLOGY OF EXCITABLE TISSUES

1.1. GENERAL PHYSIOLOGY OF EXCITABLE TISSUES

All cells of alive organism possess irritability, but only definite cells of alive organism possess excitability. Irritability is the property of the tissue to pass from physiologically resting state into the active state under internal or external influences. This process is called irritation and correspondingly the influences are called irritants or stimuli. Classification of stimuli. They are classified by the following criteria: 1) by nature, e.g. physical (mechanical, electrical, light, etc.), chemical (salts, acids, bases, etc.), biological (bacteria, viruses, their toxins, etc.). Of these stimuli the electrical stimulus is the most preferable in physiological experiments, because it is universal for all excitable tissues; it has a short latent period of action, does not evoke postreaction and can be dozed; 2) by strength (threshold, subthreshold, superthreshold). The threshold is the lowest strength of stimulus required to give a response by tissue. The subthreshold stimulus is lower than the threshold one; the superthreshold stimulus is higher than the threshold one. The superthreshold stimulus can be maximal, optimal, pessimal; 3) by location (external and internal), but this subdivision is relative, e.g. CO2 and HCl can be external as well as internal stimuli; 4) by biological significance (adequate and inadequate). The adequate stimulus is accepted by the tissue

16 adapted during the evolution process to definite one (e.g. light is the adequate stimulus for photoreceptors). Inadequate stimulus is not specific for the given receptor, but can influence on it (e.g. the mechanical stimulus brings to the light sensation in photoreceptors). Excitability is the capacity of the tissue to respond to the action of stimuli by definite reactions. Excitation is manifested by a number of common and specific processes. The common processes are the following: generation of local and action potentials, increased utilization of O2 by cells, intensification of energetic and metabolic processes, changes of viscosity and pH in cytoplasm and so on. The specific ones are: specific reactions for the given excitable tissue, and are represented by contraction of the muscle, the secretion process in the glandular tissue, impulse conduction in the nerve fibre. The classical excitable tissues are nervous, muscular and glandular.

Bioelectrical phenomena. Historical outline

The theory of “animal electricity” arose in the middle of the 18th century. There were represented data that some fishes (electrical rays) strung their preys with a strong electrical shock. The founder of “animal electricity” theory was Italian scientist Galvani (1791). The first experiment of Galvani. To study the physiological influence of electrical discharges of lightning on the alive tissue Galvani used a preparation of frog hind legs linked with the spine (Figure 1, A).

A. B. Figure 1. Bioelectrical phenomena. A) Galvani`s first and second experiments; B) Matteucci’s secondary tetanus experiment.

After suspending this preparation on a copper hook from an iron railing of a balcony, he noticed that the muscles of the frog’s legs, swinging under the influence of the wind, were contracting each time they touched the railing. He concluded that this phenomenon was caused by the “animal electricity”, being generated in the spinal cord and transmitted through the chain, formed in a result of the leg being touched the railing. This conclusion was opposed by Volta, who found that the source of electricity were two different metals (copper and iron). The second experiment of Galvani. To deny Volta’s objections Galvani carried out the second experiment using the neuromuscular preparation contented of n. ishiadicus and m. gastrocnemius. During preparing the neuromuscular preparation he refused metal instruments and used the glass ones. When he threw the nerve along the muscle, the muscle

18 contracted. Thus he proved that electricity could arise in the tissue. Galvani’s second experiment is the classical experiment of electrophysiology. The next experiment of this field was Matteucci’s experiment. Matteucci’s secondary tetanus experiment. He used two neuromuscular preparations (Figure 1, B). He contacted the nerve of the second preparation with the muscle of the first one and excited with rhythmical stimuli of the first preparation nerve. In this case the contraction of the first preparation as well as of the second one occurred. For the first time it was shown that excitation (action potential) was capable to be spread through alive tissue.

Resting potential

It is known that in physiological resting conditions the inner side of membrane is charged negatively and the outside one, positively, i.e. the living cell membrane is polarized. This difference of potentials is called resting potential, which is equal to -60 - -90 mV. The resting potential is measured by means of micro- or macro-electrode techniques. The microelectrode is micropipette, filled with 3 M solution of KCl. The microelectrode is input into the cell and another, indifferent electrode is applied to the surface of membrane. Both electrodes are connected with register apparatus, oscillograph or voltmeter (Figure 2). As soon as the microelectrode pierces the cell membrane the register apparatus registers the resting potential. In macroelectrode registration

19 the definite part of membrane is damaged, to which the electrode is applied. The indifferent electrode is applied to the membrane of intact surface. But the potential, being measured by this method usually does not exceed -30 - -50 mV, because the liquid flowing out from the tissue shunts the recording system.

Figure 2. Мeasurement of the resting potential by means of microelectrode technique.

The origin of resting potential. Some scientists, e.g. Osvald, Chagovets tried to explain the origin of resting potential using Arrenius electrolytic dissociation theory. But only in 1902 Bernstein gave the complete ionic theory, which was proved experimentally by Hodgkin, Huxley and Katz in 1952. In compliance with this theory two factors are responsible for the arising of resting potential: membrane selective permeability for K+ and ionic asymmetry between exterior and interior media of the cell. In the cell cytoplasm K+ concentration is 40-50 times higher than that in the extracellular liquid. In contrast to it the extra-cellular Na+ concentration (in 10-12 times) and Cl- concentration (in 30-40

20 times) exceed the intracellular Na+ and Cl- concentration. The permeability of the membrane to these ions is represented by the following relation: K+: Na+: Cl- = 1 : 0,04 : 0,45. If the resting membrane is permeable to potassium ions alone the resting potential would correspond to the equilibrium potential for potassium, as defined by Nernst’s equation:

RT K0 E0 = In = −97.5mV, nF Ki where E0 is the resting potential;

R is the gas constant; T is the absolute temperature; F is Faraday number;

K0 and KI are the cell outer and inner concentrations of potassium, respectively. The experimentally obtained value of the resting potential is lower than that calculated by Nernst’s equation, which is explained by the fact, that the membrane is permeable not only to K+ but to Na+ and Cl- though to a lesser degree. The following model experiment explains the resting potential origin. The vessel is divided by an artificial membrane, which is selectively permeable to K+, but 2- impermeable to anions, e.g. SO4 . Both parts of the vessel are filled with K2SO4 solution. But the concentration of this solution in the right part is higher than that in the left one. Owing to the existence of the concentration gradient potassium ions begin to diffuse from the right part to the left one. The negative anions, to which the membrane is impermeable, are

21 accumulated at the membrane surface in the right part of the vessel. By their negative charge they keep potassium ions electro-statically on the membrane surface in the left part of the vessel, which causes polarization of the membrane, i.e. the resting potential. The direct evidence of the competent of Bernstein’s theory was obtained by Hodgkin and his co-workers on a squid giant axon. The cytoplasm was squeezed out of the axon and the collapsed membrane was filled with an artificial saline solution. In cases, when the concentration of potassium in the solution was close to the cellular concentration, the normal value (-60 - -90mV) of resting potential was established across the membrane. A reduction of potassium concentration in the internal solution led to reduce of the membrane potential. These experiments demonstrate that K+ concentration gradient is the principal factor that determines the value of the resting potential in a cell.

Action potential

A rapid variation of the excitable tissue membrane potential during excitation is called an action potential. The action potential can be also registered by micro- and macroelectrode methods, i.e. by the intra- and extra-cellular leads. For a long time physiologists supposed that the action potential is the disappearance of the resting potential. But detail investigations showed that the amplitude of the action potential exceeds the value of the resting potential by 30-50 mV. During the development of the action potential the resting potential

22 does not simply disappear, as it was previously believed, but the membrane potential is reversed, so the inner surface of membrane becomes positively charged in relation to its outer side. The action potential can arise under the threshold and superthreshold strength action. The main property of the action potential is the capacity to propagate along the excitable tissue without decrement. Phases of action potential. On the curve of the action potential we can differentiate the local potential (Figure 3, phase 1), which is the first phase of excitation. If the local potential reaches to the critical level of depolarization (Ecr.), the action potential arises. Ecr. = E 0 + ∆V,

where E0 is the resting potential; ∆V is the excitation threshold. The first phase of the action potential is the depolarization (ascending part of the curve), during which the membrane potential reduces to 0 (Figure 3, phase 2), and then reappears (Figure 3, phase 3), but by an opposite sign, i.e. reverts, reaching to +30 mV (peak of the curve or spike). This part of action potential curve is called overshoot. After reaching the spike (Figure 3, phase 4) the membrane potential returns to its initial level. On the curve of the action potential it is represented as the repolarization phase (Figure 3, phase 5; descending part of the curve). Sometimes the action potential is accompanied by after- potentials. Two types of after-potentials are distinguished: depolarizing (Figure 3, phase 6) (negative) and hyperpolarizing (Figure 3, phase 7), positive after-potentials, after which the

23 membrane potential finally returns to its initial level (Figure 3, phase 8).

3 5

1 6 8 7

Figure 3. Phases of action potential.

Ion mechanism of the action potential origin. The reason for the action potential appearance is the changes of ion permeability of the membrane (Figure 4). In excitation the membrane permeability to sodium sharply increases, which is considered as the depolarization phase. The inflow of sodium ions is the auto-regenerative process. Na-channels are the potential-dependent ones. Sodium ions inflowing the cell change the membrane potential, and more sodium channels open, i.e. sodium provides self-regulation. Afterwards the inactivation of sodium channels, as well as activation of Na-K–

24 pump and K-channels are observed (spike). The repolarization phase of the action potential is conditioned by K-channel activation, which is reduced during the negative after-potential phase and reactivated during the positive after-potential phase.

Figure 4. The ionic mechanisms of action potential.

Passive and active transport of ions

During the discussion of ionic mechanisms of resting and action potentials’ origin mechanisms it could be noticed that across the membrane the passive and active transport of ions took place: the passive outflow of K+ brings to arising of the

25 resting potential, and the inflow of Na+, to generation of the action potential. The passive flow of ions occurs along the concentration gradient by the potential-activated ionic channels. We distinguish Na-, K- and Ca ion-selective channels. K-channels possess the greatest selectivity. Their diameter is 0.3 nm and they can transfer only K ions, but Na channels transfer Na, as well as K ions, so they have less selectivity, than K ones. All ionic channels consist of the transport system of the protein nature and the so-called gate mechanism. The gates assume two positions: they are either fully closed or fully open. The gates are of two types: activating and inactivating, the function of which is controlled by the membrane potential. When the activating gates are open, the channel is in an active state, in closure of the inactivating gates, the channel is inactivated. Potential-dependent channels are characterized by definite kinetics. Activation kinetics of Na-channels is faster than that of K ones. The activation of Ca- channels as well as their inactivation takes place very slowly. The ionic channels have specific inhibitors, e.g. K-channel inhibitor is amino-pyridine and tetra-ethyl-ammonium, Na- channel inhibitor is tetrodotoxin, Ca-channel inhibitor is verapamil. The ion active transport is realized opposite to ionic gradient with the energy expenditure. This type of ion transport is performed by ion pumps, e.g. Na-K-pump provides the exchange of Na+ and K+ against the concentration gradients of these ions. Na-K-pump is activated at action potential generation, when the intracellular concentration of Na+

26 increases. But it functions also in resting state and keeps the ion asymmetry between the interior and exterior cell medium. Due to this pump the outflow of 3 Na ions and inflow of 2 K ions are observed, so this pump is electro-genic and participates in resting potential arising. This pump is inhibited by ouabain.

Changes of excitability during excitation

The excitability of the tissue is changed during the excitation development (Figure 5). There is a certain relation between the action potential phases and excitability. At the development of a local potential the excitability rises (Figure 5 a), because the membrane potential approaches a critical depolarization level. The depolarization phase of the action potential coincides with the so-called absolute refractory period, when the tissue loses excitability. In this phase the tissue does not respond to any maximal stimulus (Figure 5, b). During the repolarization phase of the action potential the excitability is gradually recovered. This period is known as the relative refractory period (Figure 5, c), during which the tissue can respond to a superthreshold stimulus. The after-depolarization phase is accompanied by the increased excitability (supernormal excitability or exaltation, Figure 5, d), since the membrane potential is closer to the critical level, than in the resting state. In this phase the tissue can respond to the subthreshold stimulus (∆V1<∆V).

-70

Figure 5. Changes of excitability during excitation

In contrast, the after-hyperpolarization phase is accompanied by the decreased excitability (subnormal excitability), because of the deviation of the membrane potential from the critical level (Figure 5, e). In this phase the tissue can respond to the superthreshold stimulus (∆V2 > ∆V).

Comparative characteristics of the local (LP) and action (AP) potentials

1. The AP arises in response to the threshold strength stimulus; the LP appears under the influence of the subthreshold stimulus (stimuli, which account for 50-75% of the threshold value). 2. The AP is propagated in an unlimited distance without decrement; the LP can spread only on 2-3 mm. 3. The AP obeys the “all or none” law, i.e. the same amplitudes of AP to response of threshold and superthreshold

28 stimuli occur; the LP obeys the “strength’s relation” law, i.e. the more is the strength, the more is response. 4. The AP is accompanied with the excitability decrease, the LP, with the excitability increase. 5. The AP can’t be summed; the LP is summed. 6. The AP generation is conditioned by the regenerative process of sodium inflow the cell; at arising of LP the initial increase in the permeability to sodium is not sufficiently high to induce fast regenerative process. 7. The AP evokes specific visual effects: contraction, excretion, impulse conduction, etc.; the LP is invisible.

Parameters of excitation

In order to evoke excitation it is necessary to have an alive tissue and stimulus. But the stimulus might be characterized by the strength, action duration and steepness of the strength’s increase. The main condition of excitation is the stimulation of tissue by the threshold strength stimulus and only after the membrane depolarization till the critical level the action potential can be generated. If the steepness of the strength’s rise reduces, the action potential amplitude decreases, or no action potential appears at all. The reason of this phenomenon is an increase of the depolarization critical level, as a function of the ionic inactivation processes development in tissue. This phenomenon is called an accommodation or adaptation of tissue to stimulus. In 1863 during the mollusk’s smooth muscle excitation Fick noted that besides the strength its action duration is also

29 very important. The threshold strength of any stimulus within certain limits is inversely proportional to its duration. Mathematical dependence between the threshold and the duration of excitation is inferred, which is described by the empirical formula, determined by the properties of the tissue: a i = + b , t where i is the current strength; t is its duration; a and b are constants. This dependence can be expressed by the curve, which has been studied in detail in experiments on various excitable tissues by Hoorweg, Weiss and Lapicque (1892-1909). The curve has a form of an equilateral hyperbola (Figure 6). The values of strength are along the ordinate axis and the values of its duration along the abscissa. The threshold strength of the current expressed in millivolts or milliampers is called rheobase. The minimal time during which the current equal to the rheobase must act to induce an action potential is designated by the term “utilization time”. It implies that further prolongation of the influence of the current has no effect, so it is called “useless time”.

b time a

Figure 6. The curve of Hoorweg, Weiss, Lapicque. 1- rheobase, 2 - double rheobase; a- utilization time; b- chronaxie.

Due to the inverse dependence between the strength value and its duration intensification of the current leads to a shortening of the minimal time of stimulation, but not without limits. As we can see, the stimulation by very short-termed stimuli, the strength-time curve becomes parallel to the axis of ordinates. It means that they produce no excitation, however strong they may be. In practice it is difficult to determine the utilization time. For that reason, Lapicque proposed the term chronaxie, the least time, required for the current equal to double rheobase to evoke excitation. Chronaxie shows the excitation generating speed. There are constitutional and subordinary chronaxies. The constitutional one is the property of the isolated tissue, while the subordinary one is governed by organism's regulating influence, particularly CNS.

The measure of the chronaxie is widely used not only in experimental research, but also in clinical practice and it has a diagnostic significance. Lability. It means functional mobility. It is the velocity of elementary reactions underlying the excitation. It is also the parameter of excitability. The criterion of lability is the maximum number of the action potentials, that can be reproduced by tissue in the unite time correspondingly to the excitation rate. It can be expressed by the formula: 1 L = , R where R is the absolute refractory period (refractoriness). It is an inversely dependence between L and R. The values of the lability of some excitable tissues are:

Lnerve = 500imp/sec, Rnerve= 1/500 = 0.002 sec;

Lmuscle= 200imp/sec, Rmuscle= 1/200 = 0.005sec;

Lsynapse= 100imp/sec, Rsynapse = 1/100 = 0.01sec.

The effect of direct current on excitable tissues. The law of stimulation polarity. Physiological electrotone

An electrical generator with two electrodes, anode and cathode can serve as a source of the direct current. When an excitable tissue is stimulated by the direct current, excitation arises only at the cathode at the moment the circuit is closed; while at the moment of the circuit is open, excitation arises at the anode. These facts are united under the law of polarity that was discovered by Pfluger in 1859. The phenomena developed at the cathode and anode are explained by the passive changes

32 of the membrane. At the moment the circuit is closed at the side where the cathode is applied to the tissue, the negative charge on the outer surface of the membrane appears, i.e. the membrane passive depolarization occurs, which leads to the excitability increase and even action potential generation. In contrast, in this case at the anode the membrane is hyperpolarized and the excitability decreases. At the moment the circuit is open the local electrical current rises between the depolarized and hyperpolarized membrane parts, which causes an increase of excitability (depolarization) at the anode and decrease of excitability (hyperpolarization) at the cathode. The phenomena developed at the cathode and anode is called cathelectrotone and anelectrotone and generally physiological electrotone. There is a neutral point between cathode and anode, where the membrane charge is not changed and is called an isoelectric point. All these phenomena are the result of the passive processes, which is proven by the experiments with using ionic blockers. The latters are not effective in the development of cathelectrotonic and anelectrotonic processes. All these phenomena are observed during the short action of cathode and anode. During the prolonged action of cathode the excitability decreases, although the depolarization of membrane occurs. It is conditioned by the depolarization critical level increase (more than depolarization degree), connected with the sodium inactivation process. This phenomenon is called cathodic depression.

1.2. PHYSIOLOGY OF NERVE FIBRE

Nerve fibre is the axon of the neuron, which diameter makes up 10-20 µm and length above 1 m. The main function of the nerve fibre is the transduction of a nerve impulse. But due to microtubules, neurofilaments transporting fibres, included into cytoplasm, the nerve fibres can perform the transporting function realizing the convey of mediators, enzymes from the soma to the nerve ending, or in the opposite direction, from periphery to the soma.

Classification of nerve fibres

Nerve fibres are divided into two groups: myelinated and unmyelinated ones. The unmyelinated fibre consists of an axis cylinder, which encloses axoplasm with microtubules, neurofilaments, mitochondria and so on. The myelinated nerve fibre has the same structure, but its axis cylinder is covered by the myelin sheath, which is produced by Schwann cells. Schwann cell wraps itself in many times around the cylinder. But the myelin sheath is interrupted at regular intervals, leaving uncovered some parts of the axis cylinder membrane, which are called Ranvier’s nodes. In 1937 Erlanger and Gasser classified the nerve fibres into 3 types by their excitability, impulse conduction velocity, duration of the action potential, diameter: A, B and C. A type in its turn is divided into 4 groups: Aα, Aβ, Aγ and Aδ. All the nerves of A type are myelinated. Aα is the motor nerve fibre, innervating the skeletal muscle (d=12-22µm, velocity of

34 impulse conduction, v= 70-120m/sec). Aβ, Aγ and Aδ are the afferent fibres, which start from the pressure-, pain- and thermo- receptors. Aγ also innervates an intrafusal muscle fibres of muscle spindles. B type fibres are slightly myelinated and serve as preganglionic fibres of the vegetative nervous system. Their conduction velocity is 3-18 m/sec, diameter is 1- 3µm. A distinctive feature of these fibres is the absence of the after-depolarization phase: the repolarization phase of the action potential passes directly to the after-hyperpolarization. C type fibres are unmyelinated fibres (d=1 µm, v=0.5-3 m/sec.). These fibres are the postganglionic fibres of the sympathetic nervous system. They have a long-lasting after-depolarization, accompanied by still more prolonged after hyperpolarization. Action potential of nerve trunk. The above mentioned classification was made in a result of a detailed study of the nerve trunk action potential properties. This action potential depends on the strength of the stimulus applied. A weak stimulus causes a weak response, as the stimulation is augmented, the potential amplitude increases reaching the maximal value. In contrast, the action potential of the isolated nerve fibre belongs to the law “all or none”, i.e. it does not depend on the stimulus strength. These regularities are observed, when recording electrodes are applied to the nerve near stimulating electrodes. The increase of the distance between these electrodes up to 10-15 cm is accompanied by the breaking up of the total action potential into several separate potentials of the nerve fibres, which have different parameters.

This phenomenon is observed, because the velocity of different nerve fibres’ conduction is dissimilar and arrivals of impulse to the recording electrodes via these fibres are not simultaneous. At first the stimulus arrives at the record apparatus via A type fibre. On the nerve trunk action potential curve several spikes differ that are the spikes of A, B, C nerve fibres action potential spikes. So the electric response of the nerve trunk is the algebraic sum of the action potentials of its individual fibres.

Laws of excitation conduction in the nerve

The propagation of impulse along the nerve belongs to some rules or laws. 1. Anatomical and physiological continuity of a nerve. Impulse conduction is feasible in anatomical and physiological intactness of the nerve. That is why cutting, crushing of the nerve, as well as some inhibitors’ influence on the nerve, lead to the complete or partial disturbance of conduction. 2. Two-way conduction. Upon stimulation excitation is propagated along the nerve fibre in both directions. This could be proved by the following experiment. Two recording instruments are applied on the nerve, which register the action potential, arising in excitation of the nerve’s certain part between them. 3. Isolated conduction. Conduction of impulse along the nerve fibres, included into the same nerve trunk, is realized separately. This phenomenon is elucidated by the fact that the resistance of nerve fibre membrane is more than that of the inter-fibre liquid. So the action potential can’t jump from one

36 fibre to another. Due to this phenomenon the impulse acts only on that organ, which is innervated by the given nerve fibre, providing normal functioning of the whole organism. 4. Conduction without decrement. Nerve impulse (action potential) is conveyed along the nerve fibre without decrement (attenuation), because the action potential is generated newly in all points or certain parts of the nerve fibre.

Mechanism of impulse conduction in the nerve fibres

In unmyelinated nerve fibres an excitation is propagated continuously point by point (Figure 7). Excitation of the certain part of the fibre leads to depolarization of the membrane and action potential generation. Local current arises between the excited and unexcited parts, which causes the generation of the action potential in unexcited part of the nerve fibre. In myelinated nerve fibres the mechanism of impulse conduction is the same (by the local currents). But the local current arises between excited and unexcited Ranvier’s nodes and the action potential can be generated only in these nodes, because the sodium channels are located only in these parts of the nerve fibre membrane. The density of Na-channels is very high, 10000 channels per 1 µm2 of the membrane. So the action potential can be conducted by the saltatory manner, by jumping. The reliability factor is very important in this mechanism. A R = AP = 5 − 6, f ∆V

37 where AAP is the action potential amplitude; ∆V is the excitation threshold.

In this condition (when Rf =5-6) the impulse can jump over 1-2 nodes and provide normal conduction even when some anesthetics or Na-channel inhibitors act on these nodes.

D P

P D Figure 7. The mechanism of impulse conduction in myelinated and unmyelinated nerve fibres (“P”- polarization and “D” – depolarization of the membrane).

Myelinated nerve fibre has some advantages in comparison with unmyelinated one: 1) the conduction velocity of myelinated fibre is higher and is proportional to its diameter, whereas, the conduction velocity of unmyelinated fibre is proportional to square root of its diameter; 2) it is more economical in energetic aspect (energy saving), because only in Ranvier’s nodes Na-K pump functions; 3) fatigue of myelinated nerve could be observed rarely, because of the lower energetic expenditure.

1.3. PHYSIOLOGY OF NEUROMUSCULAR TRANSMISSION NEUROMUSCULAR SYNAPSE

The term synapse means connection (junction). This term was proposed by Sherrington. The structural formation ensuring the transmission of impulses from the motor nerve fibre to the muscle is called neuromuscular synapse. It is a classical chemical synapse, because the specific chemical substance, transmitter (mediator) provides the transmission of excitation. In 1936 Dale demonstrated, that this transmitter was acetylcholine. Synapse consists of three main elements: the presynaptic membrane, the postsynaptic membrane and the synaptic cleft (Figure 8).

Figure 8. Structure of neuro-muscular synapse. 1- nerve ending;2-vesicles of acetylcholine; 3-mytochondria; 4-synaptic cleft; 5-postsynaptic membrane; 6-cholinoreceptors; 7-sarcoplasmatic reticulum; 8-myofibrils.

The presynaptic membrane is electro-excitable, where the potential-dependent Ca-channels are located. In contrast, the postsynaptic membrane is chemo-sensitive. There are chemo-excitable

39 channels, which are coupled with the specific choline- receptors. There is also a huge amount of cholinesterase enzyme, breaking down acetylcholine. The presynaptic membrane is the membrane covering the nerve ending, which is represented as a specific neurosecretory apparatus, enclosed the acetylcholine vesicles. In the neurosecretory apparatus the electrosecretory coupling process occurs. The postsynaptic membrane is related to the muscle fibres. The synaptic cleft makes up 20-50 nm and filled with a liquid, which composition is close to the blood plasma.

Mechanism of excitation transmission in neuromuscular synapse

The action potential arriving at the presynaptic ending evokes presynaptic membrane depolarization, in result of which Ca-channels become open and Ca ions enter the nerve ending. These ions are bound with calmodulin and the formed complex activates the Ca-calmodulin dependent proteinkinase, which provides the phosphorylation of vesicle and presynaptic membrane proteins, ensuring the activation of these structures. Due to the interaction of the vesicles and the presynaptic membrane acetylcholine is released into the synaptic cleft and interacts with the choline-receptors. Owing to this interaction the chemo-sensitive channels become open and sodium ions pass into the muscle, evoking the depolarization of the postsynaptic membrane and formation

40 of so-called end-plate potential (EPP) or excitatory postsynaptic potential (EPSP). The properties of EPP are similar to the local response: it depends on the amount of a linked acetylcholine (the more the acetylcholine the more open channels are and the more is the membrane depolarization level); it could be summed; it is not propagated. Afterwards local current arises between the postsynaptic membrane and the neighboring part of electro-sensitive membrane of the skeletal muscle, which leads to depolarization. When the latter reaches the critical level, the action potential arises. Thus the action potential (electrical stimulus) of the motor nerve fibre, which has been transformed into the chemical stimulus in the synapse, is newly retransformed into the action potential of the skeletal muscle. In case of successive nerve impulses it is necessary to remove the preceding portion of acetylcholine before the arrival of each next impulse to produce normal stimulation. This function is accomplished by cholinesterase. Choline formed in the breakdown of acetylcholine is brought back to the nerve ending by a special transport mechanism. This portion of choline is used for re-synthesis of acetylcholine. Miniature potentials. It is known that a weak depolarization of the postsynaptic membrane occurs not only upon stimulation, but also at resting state. Depolarization leads to arising of the so-called miniature potential. Its amplitude (0.5 mV) is 50-80 times smaller, than EPP generated by a single impulse. At rest acetylcholine is released from 1-2

41 vesicles. Each vesicle (quantum) contains 2000 acetylcholine molecules. The miniature potentials usually arise at the frequency of about one per second. Inhibition of the neuro-muscular synapse. The inhibition of the neuromuscular synapse can be realized in the pre- and postsynaptic membranes. The inhibitors of Ca- channels (e.g. verapamil) inhibit release of acetylcholine. On the level of the postsynaptic membrane the inhibition occurs upon the influence of inhibitors of acetylcholine receptors, e.g. curare (poison). The acetylcholine-receptor has a higher affinity to this poison, than to acetylcholine. In case of this interaction synaptic transmission is blocked. Curare-like substances (ditilinium, diplacin) are widely used in clinics, when it is necessary to switch off the respiratory muscles and ensure an artificial respiration. The next group of synaptic inhibitors is the group of substances blocking the cholinesterase. Upon the action of these inhibitors (eserine, proserine, prostigmine) acetylcholine remains bound with the receptors and the new impulses couldn’t pass across the synapse.

The properties of the neuromuscular (chemical) synapse

All the chemical synapses, including the neuromuscular synapse, have the following properties: 1. One-way conduction, which is explained by the presence of a large synaptic cleft and electrically unexcitable postsynaptic membrane.

2. Synaptic delay. It makes up 0.2-0.5 msec, which is connected with the low mobility of chemical processes, occurred in the synapse. 3. Rhythm transformation. It is conditioned by the fact, that the lability of the synapse (100 imp/sec) is less, than that in the motor nerve (500 imp/sec). 4. Synaptic facilitation. Each impulse conduction across the synapse is easier, than the previous one, because each impulse leaves a trace both on the presynaptic- and postsynaptic membrane, exactly on the ion channels. 5. Synapse fatigue. In a frequent and long-term stimulation of the synapse inhibition of conductivity is observed, which is explained by weakening of acetylcholine resources and decrease of acetylcholine receptors’ sensitivity. 6. High sensitivity to temperature, hypoxia and chemical factors.

1.4. PHYSIOLOGY OF MUSCLES

Vertebrates and humans have three types of muscles: striated muscles of the skeleton, cardiac muscle and smooth muscles of the internal organs, vessels and skin. Cardiac muscle has many differences compared to skeletal muscle, although it is also striated. Skeletal muscles Functions and properties of striated muscles. Skeletal muscles constitute an active part of the supporting system and ensure the following functions: 1) posture support;

2) they provide movements of separate parts of the body relative to each other; 3) owing to them the body movement occurs in a space; 4) heat production. They produce 60% of the organism’s total heat. Skeletal muscles possess three very important properties: excitability, conductivity and contractility. In comparison with the motor nerve fibre the skeletal muscle fibre excitability is less, therefore although the critical level of depolarisation in the muscle and nerve fibres is nearly equal (-50mV), the RP in muscle fibre is more negative (-90 mV), than that in nerve fibre (-70 mV). So the excitation threshold (40 mV) in muscle fibres is higher than in nerve ones (20 mV). This comparative electronegativity of the RP in muscle fibres is explained by higher permeability of their membranes to Cl-. The AP amplitude in the muscle fibres is 120-130 mV, and its duration, 2-3 msec. Excitation spreading velocity along the muscle fibres makes up 3-5 m/sec. The skeletal muscle obeys the law “strengths’ relation”. It is conditioned by the fact, that the muscle consists of enormous separate muscular fibres of different excitability and as the stimulus strength grows up more amounts of myofibrils involve into the contraction process. In result the muscle response gradually increases. If the stimulus strength achieves such a value, when all the fibres are excited, the maximal contractile response occurs and the further strength increase

44 does not matter. Unlike the whole muscle, the isolated muscle fibre obeys the “all or none” law.

Types of the muscle contraction

There are two types: 1. Isotonic, in which myofibrils are shortened, but the tension is not changed. It occurs when only one end of the muscle is fixed. 2. Isometric, in which the length of myofibrils remains constant, but their tension increases. It occurs when both ends of the muscle are fixed. Altogether natural contractions in the organism are never purely isotonic or purely isometric, since the muscle is shorted under the load, but simultaneously change its tension. Exceptionally the isometric is the contraction of the heart ventricles. Mixed type of contraction is called auxotonic, that can be concentric, when the muscle length is reduced, while its tension is increased; and excentric, that is characterized by increase of length and tension.

Muscle single contraction

Stimulation of the muscle or the motor nerve fibre by a single stimulus evokes the single muscle contraction. But the muscle reacts to the individual stimulus not immediately. There is a latent period of 0.01 sec that is followed by the mechanical response of the muscle that can be registered. The latent period corresponds to the term of the AP inducing. In this period the muscle is in refractory period, during which the processes of

45 preparing contraction, excitation conduction and physicochemical alterations of the muscle occur. Beginning with the middle of the contraction (shortening) period (it lasts 0.04 sec) up to the final of the muscle relaxation (that lasts 0.05 sec) the muscle is in the phase of exaltation (increased excitability). Totally the single contraction duration is 0.1 sec.

Tetanus and summation of contractions

In natural conditions in the organism a skeletal muscle usually receives number of impulses from the nervous system. Stimulation of the muscle by rhythmic stimuli leads to a strong and long-term contraction, known as tetanic contraction or tetanus. Amplitude of the tetanic contraction exceeds the maximal value of a single contraction in several times. Tetanus can be of 2 types: complete (smooth) and incomplete (toothed). Tetanic contractions of the muscle are the result of the summation of individual contractions that can occur in two possible ways. If the second stimulus is applied before the first contraction has reached its peak, the second contraction will fully merge with the first, forming a single summation peak (so-called complete summation, smooth tetanus). If the second stimulus is applied when the muscle has already begun to relax after the first contraction, the peak of the second contraction will be separated from that of the first (incomplete summation, toothed tetanus). At complete summation the interval between the first and second stimuli must be less than 0.05 sec, while for the second type (incomplete summation), it must be longer than 0.05sec. To produce tetanus artificially the muscle is

46 subjected to a great number of stimuli following one another at the frequency required for summation. With a relatively low frequency incomplete tetanus (toothed tetanus, Figure 9,a) is obtained, and with a higher frequency, complete tetanus (smooth tetanus, Figure 9,b). Generally tetanus can be observed if the interval between stimuli is longer than the refractory period and shorter than the whole duration of the contraction response, so that the second stimulus will act on the muscle before it has relaxed after the first stimulation. After termination of the tetanic stimulation muscle fibres do not relax completely and their initial length recovers but some time later. This phenomenon is called post-tetanic or residual contraction.

Figure 9. Toothed (a) and smooth (b) tetanus.

Ultrastructure of myofibrils

The muscle fibre is a multinuclear formation that has special contractile organelles, myofibrils. The myofibril consists of 2500 protofibrils that are polymerized molecules of actin and myosin proteins. Actin is a thin filament (d=5nm) and consists of two twisted strands. Tropomyosin is located between strands. Tropomyosin is bound with troponin, which has high affinity to Ca2+. Myosin is a large, complex molecule

47 consisting of the tail, neck and head regions. Its diameter is 10 nm. Each head possesses ATP-ase activity. The structural and functional unit of the muscle is sarcomere, limited by Z- membranes (Figure 10). Actin is supported on the Z-membrane and its ends enter spaces between myosin thick filaments.

A Band

Figure 10. Ultrastructure of myofibrils.

The area of the myofibril containing only actin filaments and Z-membrane is called I-band (isotropic). This band is light upon the light microscope. The region of both thin and thick filaments possesses polarized light double refraction. It is dark and is called A-band (anisotropic). In the middle of A-band there is a light line H, where are only myosin filaments. So under the light microscope in the skeletal muscle regular altering dark and light striations are apparent, which give striated appearance to the skeletal muscle.

Conductive system of the muscle. Myofibril membrane at Z-membrane level is invaginated, forming so-called T- tubules (T-system) (Figure 11). On the both sides of the T tubule there are cisterns of the sarcoplasmic reticulum. T- tubule with two adjacent cisterns composes triad. Ca2+ concentration in the cisterns makes up 10-4M.

Mechanisms of muscle contraction and relaxation

The initiator of the muscle contraction is an AP. It spreads along the membrane and into the depth of T-tubule. Ca2+-channels of the cisterns are open and Ca2+ ions pass to the myoplasm (Figure 11). Ca2+ concentration in the myoplasm becomes 10-6M (at rest 10-8 M) and Ca2+ ions interact with troponin. Troponin alters its configuration promoting the tropomyosin displacement into the depth of the space between the actin strands, which enables interaction of actin and myosin. The heads of myosin are attached to the actin, forming cross-bridges, which due to the neck hinge special mechanism provide the displacement of actin towards the middle of the sarcomere for the distance equal to 1 per cent of its length. Then the heads step aside from the actin and are reattached to the next region of the actin, providing its further displacement. The head detachment from the actin occurs in the presence of ATP and ATP-ase of myosin’s head. This mechanism of contraction is called the “mechanism of filament sliding”. In contraction the I-band and the H-line are reduced, while the A- band remains constant (Figure 10).

Figure 11. Conductive system of muscle and mechanisms of muscle contraction. After contraction of the muscle its relaxation comes. This process is opposite to contraction. Owing to the Ca2+-ATP-ase Ca2+ ions return into the cisterns against their concentration gradient in result of which Ca2+ is detached from troponin and tropomyosin is lifted up on the strand surface between the actin filaments. The actin and myosin interaction desists. As it is seen energy is required both for contraction and relaxation of the muscle. The third way of ATP consumption in the muscle is for the Na-K-pump work provision.

Work and force of the muscle

In contraction the muscle, overcoming the load, fulfils work equal to: A=F× h, where F is the strength; h is the muscle shortening degree. The strength of contraction depends on the peculiarities of the given muscle, particularly on its cross section. There are maximal and absolute muscle strengths. A measure of maximal strength is the value of maximal load that could be lifted up by the muscle, or the value of maximal strength that could be developed by the muscle during isometric contraction. Muscle absolute strength is the maximal strength, which acts on the unit area of its physiological section. The physiological section is the section that is perpendicular to all myofibrils (Figure 12).

Figure 12. Cross sections of various muscles.

Work fulfilled by the muscle is governed by the regularity named as a “rule of average loads”. Pursuant to this rule the maximal work is carried out in average loads.

Some peculiarities of smooth muscles

In comparison with the skeletal muscles in the smooth muscles troponin – tropomyosin complex is absent. During the muscle contraction the actin and myosin interaction is provided by phosphorylation of the myosin head through the calcium - calmoduline complex. Relaxation of muscle is realized due to dephosphorylation of the myosin heads. Herein, the Ca-ATP- ase is weakly exhibited and the Ca2+ back into the cisterns thereby Na-Ca exchange occurs. The energy expenditure here is about 300 times less than that in the skeletal muscles.

CHAPTER 2. PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM

2.1. GENERAL PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM

It is known that the main condition for existence and for providing integrity of the organism and environment is the organism’s ability to be adapted to the external medium conditions. The important role in this providing belongs to the neuronal system. In general, the main functions of the central nervous system (CNS) are: the regulation and coordination of all the organs’ and systems’ activity, the organism’s effective adaptation to the continuously altering external medium conditions and, as a result, the living organisms’ aimed behaviour formation. Strictness of these regulatory influences is due to the existence of a two-way circular connection between the nerve centres and the peripheral organs. The neuronal system owing to its neuronal principle of the structure and the reflector mechanism of activity differs from other regulatory systems by very high promptness of reactions and preciseness of signals that provides a great effectiveness of the organism’s reactions.

Structural and functional elements of the CNS. Neuron and glia

The base of the present conception of the neuronal system structure and function is the neuronal theory of Ramon

53 y Cajal and Charles Sherrington. This theory considers the brain as a functional unity of neurons that are separate structures and only contact to each other. So, the morpho- functional unit of this system is the neuron. The main function of neurons is perception of the afferent signals, their elaboration, and transmission to other neuronal cells and to the executive organs. Structure of neuron. Each neuron consists of a body (soma) and special processes: one long, the axon, and numerous short ramifying ones, the dendrites (Figure 13). The soma is covered with the protoplasmatic membrane and contains all necessary cellular organelles (nucleus, ribosomes, Golgi apparatus, mitochondria, etc.). The soma performs synthetic function (e.g. synthesis of neuromediators). The function of the axon is to convey impulses from the soma to other cells or to other peripheral organs. The axon is an alone long process that does not give collaterals at the beginning and ramifies only at the contact site with the innervating cell or organ. The axon has the following compartments: 1. Axon hillock (the site of its origin from the soma) and the initial part of the axon, which over the 50-150 µm has no myelin sheath. The initial part with the axon hillock is called the initial segment. The specific feature of this segment is its high excitability and an ability to generate the action potential. 2. Axon’s cylinder, which performs the conductive and transport (e.g. synthesized in the soma neuromediator is conveyed by the axon) functions.

Figure 13. Structure of neuron.

3. Presynaptic terminal, in which the mediator vesicles are located. 4. Dendrites are numerous ramifying processes, which function is the reception of impulses arriving from other neurons and their conduction to the soma. Dendrites with a large amount of contacting endings and a big perception surface fulfil the leading role in the neuron’s perception of information. Classification of neurons and their electrophysiological properties. Neurons are classified by their

55 different properties. All neurons differ by their shape, structure, size and function. By shape they can be round, oval, multangular, pyramidal, etc. By size they vary from 5 µm to 150 µm. By amount of processes they can be unipolar, bipolar, pseudo-unipolar and multipolar. By their significance (physiological role) they are subdivided into: 1) afferent, or sensory, or receptory neurons; 2) intercalated, or contact, or interneurons, 3) efferent or motor neurons. The afferent or sensory neurons in vertebrates are represented in the spinal ganglia as pseudounipolar round cells. The afferent neurons ensure the signal perception and conduction from periphery to the centres, and are named as centripetal ones. The afferent neurons are divided into primary-afferent and secondary-afferent ones. The primary- afferent neuron precepts the excitation by its receptors, in which due to the changes of their ion permeability and membrane depolarization the receptory potential (RP) arises. The latter provides the generation of spreading action potential (AP). The secondary-afferent neuron obtains the excitation from the special sensitive neuron, in which RP arises. This potential promotes the mediator release, providing the generation of so-called generatory potential (GP) in the secondary-afferent neuron ending. This potential getting a definite critical value generates the spreading AP. RP and GP on their physiological significance are similar to the local potentials and give the similar effects of summation, relief, they can’t be spread, etc.

The effector or motor neurons give rise to an axon that goes out of the CNS and finishes in the effector structures (for example muscles, glands). These neurons have a lot of ramifying dendrites. Impulses arising in the effector neuron spread mainly centrifugally and provide the realization of different final reactions. Among these motoneurons of the spinal cord are studied more detailed. They are characterized by efferent discharges of low frequency. This ability has an important physiological significance. It is enough for the allied peripheral effect (e.g. the muscle group general contraction), maximally economical and excludes the opportunity to block the peripheral effect due to the pessimum phenomenon (e.g. pessimum in this rate does not arise). This low intensity arises because of the prolonged hyperpolarization (positive after potential) of the soma followed by each AP generation. Its duration for the spinal motoneurons is about 100-150 msec. The intercalated neurons are characterized by a huge number (90% of all neurons) and small sizes. Moreover, their processes do not leave the CNS. They are located between the afferent and the efferent neurons. Interneurons can be divided into the excitatory and inhibitory by evoked effects on other neurons. The critical level of depolarization a little prevails its resting potential and the threshold is low. The AP duration is rather short (0.5 msec), the hyperpolarization is not significant and is of a short duration. All this determines the characteristic specificity of interneurons, the ability to generate discharges of high frequency. The characteristic feature of the interneurons is their ability to rhythmical and spontaneous activity.

Systemic interaction of all the mentioned neuronal populations provides the regulation, coordination and realization of motor acts in a living organism. AP of neuron. At first the AP arises in the axon hillock. It is connected with the fact that the excitation threshold is different in different parts of the neuron: it is the lowest in the axon hillock region (due to a big amount of Na+ channels); it is the highest in the soma and dendrites. In direct excitation of the neuron its AP has two peaks: 1) the peak of the initial segment (IS) and 2) the peak of the soma and dendrites. In the condition of antidromic excitation the AP has three peaks: 1) the peak of the myelin; 2) the peak of the IS and 3) the peak of the soma and dendrites. Glia. The main types of glia are: olygodendroglia, astroglia and microglia. The glia is another compartment of the CNS and consists of the glial cells, which like neurons have a soma and many processes, but they have other specialization. They make a network, matrix, in hinges of which the neuronal cells are located. They make generally the main mass of the brain. On the one side the processes of the glial tissue attach to the neurons and on the other side to the vessels. So, it is supposed, that the glial cells have not only the support function, but also provide the transport of nutrients to the neuronal cell, as well as remove the waste products from it (metabolic function). Glial cells have also other functions: the buffer (due to the high permeability to K+ ions they regulate K+ concentration in intercellular liquid), the isolating (Schwann

58 cells compose the myelin sheath), the defensive (microglia performs the phagocytosis).

Interneuronal communications

The process of transduction of the neuronal impulse from one neuron to another or organ proceeds by means of the asserting contact (synapse) between them. All synapses are classified by the mechanism of impulse transmission (electrical, chemical and mixed), structure (axosomatic, axodendritic, dendrodendritic, axoaxonic, etc.) and functional specialization (excitatory, inhibitory). The electrical synapses in general are represented in invertebrates and partly in vertebrates, but the chemical synapses compose the main part of synapses in the CNS of higher animals and humans. In electrical type of synapses the synaptic cleft is practically absent (it makes up 2-4 nm), inside which the protein bridges are situated. The impulse transduction, realizing along these bridges, in general resembles the conduction mechanism in nerve fibre. This type of structure determines also the peculiarities of their functioning: 1) the synaptic delay is practically absent; 2) the two-way conduction; 3) the generator for current arising in the postsynaptic membrane is located in the presynaptic membrane, from which it spreads on the postsynaptic membrane electrotonically (passively). In this connection these electrical synapses are called electrotonical ones.

There are also mixed synaptic contacts in the CNS that combine both chemical and electrical synaptic elements. Chemical transmission of excitation. This mechanism of transmission in the chemical synapses of CNS is similar to that of the neuromuscular synapse. The neuromediator’s extrusion is carried out by the special electrosecretory mechanism in compulsory participation of Ca2+. The electrosecretory mechanism includes: a) the presynaptic ending depolarization by AP arriving; b) activation and opening of the potential-dependent calcium channels; c) Ca2+ ions entrance into the ending; d) interaction of Ca2+ ions with neuromediator vesicles and their activation; e) interaction of the vesicles with membrane, their rupture and the mediator pouring out into the synaptic cleft. The mediator interacting with the postsynaptic membrane receptors causes postsynaptic membrane depolarization. The investigations of English physiologist Katz have shown that this depolarization (excitatory postsynaptic potential - EPSP) is connected with Na+ and K+ ion permeability strict increase (more for Na+). The EPSP is a basis for the AP generation and neuronal impulse spreading. If the mediator evokes the hyperpolarization of the postsynaptic membrane, conditioned by K+ or Cl- permeability increase, the inhibitory postsynaptic potential (IPSP) arises. The main functional properties of the chemical synapse, by which it is distinguished from the electrical synapse, are: 1) relatively long-lasting synaptic delay (0.2-0.5 msec); 2) one- way conduction; 3) development of active postsynaptic

60 potential on the postsynaptic membrane (i.e. the neuronal impulse generator arises on the postsynaptic membrane); 4) postsynaptic potentials in the chemical synapses can be both excitatory and inhibitory, while in the electrical synapses – only excitatory; 5) rhythm transformation; 6) synaptic fatigue; 7) high sensitivity to temperature changes. Chemical mediators (transmitters). In the neuronal system many chemical substances have a mediator role. The basic criteria of the mediator function of substances are: a) their synthesis and accumulation in the presynaptic terminals (vesicles); b)capacity for being released under the influence of a nerve impulse; c) the similar molecular and ionic mechanisms of action on the postsynaptic membrane of a substance which is released by a nerve impulse and that of artificially applied to the postsynaptic membrane; d)existence of the special mechanism of their removal from the postsynaptic membrane and synaptic cleft. The same mediator can react with various receptors. Different cholinoreceptors being combined with acetylcholine give rise to different effects (excitatory or inhibitory). There are different receptors to catecholamines (α− excitatory and β − inhibitory). The ability of the same mediator to evoke various changes of permeability in various postsynaptic membranes is a reason for EPSP and IPSP arising in neurons. Simultaneously the same neuron can serve as a site of attachment for both inhibitory and excitatory mediators and so generates the IPSP or EPSP. But glycine and γ- amino-butyric

– acid (GABA) are classical inhibitory mediators that evoke only IPSP. Principle of Dale. In all endings of the same neuron the same mediator is secreted. Therefore the cholinergic, serotonergic, dopaminergic, adrenergic and GABA-ergic neurons are denominated.

Reflector character of the CNS activity

A reflex is a regular reaction of the organism to a change in its external or internal environment effected through the CNS. Classification of reflexes. Reflexes can be classified according to a number of attributes. According to biological importance (nutritional, defence, sexual, orientation), to the location of the reflexogenic receptors (exteroceptive, interoceptive, proprioceptive), to the parts of the CNS, involved in their performance (spinal, bulbar, mesencephalic, cortical, etc.), to the character of response, or organs involved (motor, secretor, vasomotor), etc. Reflex arc. The reflex arc (Figure 14) involves receptors (1), afferent nerve fibre (2), nervous centre (3), efferent nerve fibre (4), effector organ (5). The simplest reflex arc can be represented schematically as formed by two neurons only, an afferent and an efferent, with a synapse between them. Such a reflex is called bineuronal or monosynaptic reflex. Monosynaptic reflexes occur very rarely.

Figure 14. Spinal reflex arc.

They may be exemplified by myotatic reflexes (knee jerk, Achilles-tendon reflex, etc). The reflex arcs of most reflexes include not only two neurons (afferent and efferent) but also one or more interneurons. These reflexes are known as multineuronal or polysynaptic. Excitation is conducted considerably more slowly in nerve centres (because of the presence of synapses and the synaptic delay) compared with the nerve fibres. The reflex time or latent period of reflex, i.e. the period between stimulation and response, consists of: excitation of receptors (A), conduction of excitation by the afferent nerve (B), transfer of excitation to the effector neuron

(C), conduction of excitation by the efferent nerve (D), conduction of excitation from the nerve to the effector organ, and its latent period (E). Thus, the reflex time R is the sum of the time of all these processes: R=A+B+C+D+E. Period C is called true or reflex central time. In man the quickest reflexes are the tendon reflexes (monosynaptic reflexes), e.g. the knee reflex takes only 20 -23 msec, the central time of this reflex is 3 msec. But the winking reflex takes rather longer, 50-200 msec, its central time is 36-186 msec (polysynaptic reflex).

Inhibition in the CNS

The phenomenon of the central inhibition was discovered by Sechenov in 1863. It was shown, that incision of the frog brain at the thalamus level and NaCl crystals application on that site lead to spinal reflex time significant prolongation or reflex entire eradication. It was determined by Turck’s method (measuring of the reflex time for withdrawing the hind legs from a solution of sulfuric acid by frog) (Figure 15). He made the conclusion about existence of specific neuronal centres in the thalamic region producing an inhibitory influence on the spinal centres. Afterwards new facts of inhibition in the CNS were discovered. Goltz showed that the reflex of the frog leg withdrawal in response to immersion in an acid solution could be inhibited by simultaneously strong mechanical stimulation of the other leg by squeezing with pincers even after removal of thalamus.

Figure 15. Sechenov`s experiment (central inhibition).

Facts of the central inhibition were out of doubt and got a common acceptance, but concerning to the mechanism of its derivation there were contradictory views. Some researchers (Sechenov) supposed that there were special structures in the central nervous system performing the function of inhibition. Others (Goltz, Sherrington) opposed this maintaining that inhibition resulted from a conflict between several excitations. Only owing to the electrophysiological research methods rapid development of the cellular mechanisms underlying the inhibition process became clear. Inhibition is an independent nervous process, which is caused by excitation and manifested by the suppression of another excitation. In distinct from the excitation process, which is manifested in a view of the local

65 potentials and spreading action potentials, the inhibition process is manifested only in a local form that never spreads. There are two principally different ways of neuronal activity inhibition: 1. The inhibition is a result of special inhibitory synapses activation (primary inhibition). 2. The inhibition arisen in the neuron is secondary as a sequence of its excitation. This type of inhibition appears in a high rate (pessimal rate) stimulation of the cell or it may be connected with the hyperpolarization processes followed by excitation. Primary inhibition. By means of the microelectrode examination, it has been established, that the primary inhibition of neuron activity could be a result of the postsynaptic membrane’s property changing, as well as a result of a special mechanism blocking excitatory synapses at the level of the presynaptic structures. We differentiate two types of inhibition - the postsynaptic and the presynaptic. Postsynaptic inhibition. It is most common in the CNS. It has been established that there are so-called inhibitory neurons in the CNS, e.g. Renshaw’s, Wilson’s cells (spinal cord) and Purkinje’s cells (cerebellum). The axons of the inhibitory neurons secrete inhibitory transmitters, glycine or GABA. These mediators cause postsynaptic membrane hyperpolarization and IPSP arising (direct inhibition). The ionic mechanism of the above-mentioned inhibitory action on the postsynaptic membrane consists in the following: the inhibitory mediator brings about the postsynaptic membrane

66 permeability increase for K+ (outflow) or Cl- (inflow) ions that finally lead to hyperpolarization. Another type of the postsynaptic inhibition is the recurrent inhibition (Figure 16) that can be studied on α- motoneurons. α-motoneuron being excited sends stimuli not only to periphery (muscle), but also by collateral branch to Renshaw’s cell, which forms the inhibitory synapse on the body of motoneuron. Renshaw’s cell secretes the inhibitory mediator (GABA) that acting on the motoneuron through the synaptic contact, inhibits it. This phenomenon is of importance, since it prevents the motoneuron from strong excitation. The particular case of this phenomenon is the inhibition of inhibition. In this case Renshaw’s cell contacts with another inhibitory neuron – Wilson’s cell, which in its turn contacts with α-motoneuron. Wilson’s cell has a background (spontaneous) activity, by means of which it keeps the motoneuron in an inhibited state. Renshaw’s cell being excited (by motoneuron collateral) causes the inhibition of Wilson’s cell and α-motoneuron will escape from the inhibited state (inhibition of inhibition or facilitation). Presynaptic inhibition. As its name implies, this type of inhibition is localized in the presynaptic elements and realized in the axoaxonic synapses (Figure 17).

Figure 16. The recurrent Figure 17. The presynaptic postsynaptic inhibition. inhibition.

The mediator secreted in these synapses evokes the axon membrane depolarization because of Cl- - pump activation. The formed state results in a partial or complete block of conduction of impulses (decrease of the spreading AP amplitude) to the nerve endings. On the other hand the inhibitory mediator can disturb the Ca2+ ions entry into the presynaptic membrane, which can inhibit the secretion of excitatory mediator.

Secondary inhibition.

Pessimal inhibition. The activity of a nerve cell can be inhibited without participation of special inhibitory structures. In that case, inhibition develops in the excitatory synapses as a result of a strong and prolonged depolarization of the post synaptic membrane under the influence of nerve impulses arriving too frequently. So a state similar to Werigo’s cathodal depression in the cell develops.

Inhibition following the strong excitation. A discrete type of inhibition is the one developing in a nerve cell after termination of excitation and which appears when excitation is followed by a strong after-hyperpolarization of the cell membrane. The excitatory postsynaptic potential arising under these conditions proves to be insufficient to depolarize the membrane, so that spreading of excitation does not occur.

Nervous centres’ properties

The nervous centre is integrity of neurons, which implies a definite reflex or regulates a particular function. It is located in a certain part of the CNS. The localization of nervous centres is determined through experiments with stimulation, or with limited destruction, extirpation, or section of different parts of the brain or spinal cord. Since in the nervous centre neurons are interconnected to each other by means of synapses, the nervous centres first are to have all that properties intrinsic to the synapses particularly. They are the following: 1. One-way conduction of stimuli. It can be illustrated by the following experiment. At stimulation of the dorsal roots of the spinal cord (the afferent nerve fibres) the action potentials will be registered in the ventral roots (the efferent nerve fibres). On the other hand, if the ventral roots are stimulated in the dorsal roots no action potential will be registered. 2. Central delay. Excitation is conducted considerably more slowly in the neuronal centres than in nerve fibres, which accounts for the relatively long duration of reflex time, i.e. the period between stimulation and response. Slow transmission of

69 excitation is consequent to the peculiarities of the impulse conduction across the synapses. The more synapses are involved in the reflex response; the more is the central delay. 3. Fatigue of the nervous centres. In prolonged and more frequent stimulation of the nervous centre, considerably weakened responses or gradually ceased responses will be registered on the efferent way of the reflex. The reason is that the intensive stimuli cause the mediator resources full consumption, as well as decrease of the postsynaptic receptors’ sensitivity. The definite time period is required for the mediator resynthesis and the energetic resources recovering. 4. Transformation of rhythm excitation. In response to an isolated stimulus applied to an afferent nerve the centre sends along efferent nerve fibres a series of impulses following each other in a definite rhythm. Figurally said, they respond to “an isolated rifle shot with a burst of machine–gunfire”. In certain cases it is due to the EPSP long duration, which triggers the second, third, etc. action potentials after the termination of the first. 5. Summation of excitation. Its essence is that a combination of two or more stimulations of peripheral receptors of afferent nerves arises a reflex, whereas each taken separately is not sufficient to elicit the response. Two types of summation are determined: 1) spatial (or simultaneous); 2) temporal (or consecutive). The spatial summation of impulses occurs, when two or more weak (subthreshold) stimuli are acting simultaneously on

70 different receptors belonging to the same receptive field. Either stimulus acting alone will not induce the given reflex, but in combination they will. This phenomenon is explained by summation of EPSPs evoked by subthreshold stimuli. Consecutive summation is the interaction of impulses coming to the neuronal centre one after another at short intervals along the same afferent nerve fibres. It can be induced by applying a series of rhythmic stimuli to an afferent nerve or to the receptive field of a reflex. If each of the stimuli is strong enough to elicit a reflex, their rhythmic application will increase it. If the strength of the stimulus is so calculated to be not enough to cause a reflex when acting alone, the reflex can be elicited by a succession of them. The difference between these two types of summation is that with simultaneous excitation of several neighbouring synapses the EPSPs are summed up spatially, whereas with consecutive stimulation they are summed up in time. 6. Occlusion. It is like to be contradictory to the summation phenomenon represented above. It consists in simultaneous stimulation of two afferent nerve fibres (each of which gives a strong response), producing an effect which magnitude is less than the arythmetical sum of those taken separately. Reason for this phenomenon is the summation bringing about the process of cathodal depression. 7. Posttetanic potentiation. It is a phenomenon of response increase of production in case of preliminarily given tetanic impulses. For example, we give stimuli in time interval of 2-3 sec. and take responses accordingly to this rhythm. The

71 following intensifying of stimuli brings about intensified responses. It can be continued up to 200-600 impulses per second. But in case of immediate returning to the initial rate of stimuli, the intensified responses will be kept. The reason is that the presynaptic ending acquires a property to cumulate much more amount of the mobilized mediator. 8. After-action. Essence of this is the following. Reflex acts do not end simultaneously with the cessation of the stimuli causing them, but after a certain, sometimes comparatively long interval. This phenomenon can be explained by the circulation of nerve impulses through the closed neuron chains of the reflex centre. With links of that kind between neurons excitation of one is conveyed to another (or others) and returns again to the first cell, through the collateral’s of their axons, and so on. Owing to existence of such circular connections the excitation can circulate in a nerve centre for some time until the onset of fatigue in one of the synapses or the neuronal activity arrest by the arrival of an inhibitory impulse. 9. Tone of neuronal centres. Electrophysiological researches have shown that the nervous centres are constantly in state of limited excitation, in tone. It is sustained by the afferent impulses continuously conveyed from peripheral receptors to the nervous centre, as well as by various humoral stimulants (hormones, CO2, etc). The role of afferent impulses in maintaining the tone of neuronal centres is demonstrated by Bronjest’s experiment. Section of the sensory roots of the spinal cord innervating the hind leg of a frog causes a decline of the muscular tone, almost identical with that seen with

72 damage of the motor nerve. In result the leg with decreased tone becomes longer than the other one. 10. High sensitivity of the neuronal centres to oxygen supply and to certain poisons. The brain cells are marked by intensive consumption of oxygen (twice more than the skeletal muscles). Because they consume large amount of oxygen, neurons are highly sensitive to its deficit (hypoxia), so a decrease in oxygen supply to the CNS leads to functional disturbances in the nervous centres. That is why complete or partial cessation of the cerebral circulation (e.g. in thrombosis) entails severe impairments in the neuronal system and the death of nerve elements. Besides, nervous centres possess a selective sensitivity to certain poisons, which are known as nerve poisons. For example, strychnine serves as an inhibitor of the central inhibitory synapses, thereby causes a sharp increase in the excitability of the CNS, particularly of the spinal cord; apomorphine stimulates excessively the vomiting centre; lobeline acts on the respiratory centre and abruptly stimulates it, etc.

Principles of coordination in the CNS

Every reflex is a reaction of the entire central nervous system and is realized by the definite nervous centre. It depends on the CNS condition at the given moment and on the whole aggregation of intercentral relationships and interactions, which in turn encounter through the neuronal and synaptic chains. The same reflex could have a number of

73 components – motor, secretor, vascular and so on; and all these centres act in a coordination way. The main principles of coordination are: 1. Convergence. Impulses reaching the central nervous system along various afferent fibres may converge upon the same inter- and effector neurons. In the spinal and medullar centres convergence is comparatively limited; only stimuli coming from the same reflexogenic field can converge. In contrast, in the higher parts of the CNS (e.g. in subcortical nuclei and the cerebral cortex) there is a convergence of impulses issuing from different receptive zones, for instance, acoustic, optic, the smell and skin receptors. 2. Divergence. It is an opposite phenomenon to convergence. The afferent nerve entering into the nervous centre can form synaptic contacts with several neurons. 3. Irradiation. Impulses arriving at the CNS can induce excitation not only in the neurons of a given reflex centre but also in those of other centres. It occurs mostly in case of strong and prolonged stimulation. To illustrate this phenomenon let us consider the results of the following experiment. A weak stimulus applied to the pads of the animal’s hind leg causes flexion of that leg only, at the talocrural joint. Intensified stimulation causes flexion at the knee joint, in addition, and still stronger stimulation at the hip joint. Further increase in stimulus strength entails in addition to the above mentioned flexion, the extension of the hind leg on the opposite side. Irradiation could be endless and strong if the inhibitory synapses don’t exist. Irradiation is obstructed by numerous

74 inhibitory neurons in various reflex centres. Importance of inhibition is clearly illustrated by injecting 0,1% solution of strychnine. Strychnine blocks the inhibitory synapses and evokes hyperexcitation. Even insignificant touch or another stimulus can induce the most intensive general excitation of the CNS accompanied by convulsions of all the skeletal muscles. 4. Reciprocal innervations. Flexion reflex is accompanied by inhibition of extension reflex, simultaneously the contraction of the extensor muscle and the relaxation of the flexor muscle of the opposite side are observed. This phenomenon is explained as stimulation of the flexion centre causing inhibition of the extensor centre. Afferent nerve exciting the flexor motor centre simultaneously sends the impulses by collateral branch to Renshaw’s cell. The latter being excited brings about the extensor α-motoneuron inhibition. The flexor centre could evoke an excitatory influence on the extensor centre of the opposite side (opposite leg), as well as an inhibitory influence on the flexor centre. 5. Principal of the dominant. Activity of the nervous system is characterized in the natural conditions of the organism by the existence of dominant foci of excitation, which change the action of all other nerve centres and subordinate them to themselves. If during the act of defecation a strong pain stimulus is applied, the flexion reflex of the leg that is normally evoked by this stimulation will not occur. Instead, the defecation reflex will be quicken and intensified. Dominant centres fulfil the most important for that moment reflex.

Dominant foci are characterized by the following basic peculiarities: a) increased excitability; b) stability of the excitation; c) capacity to summate excitation; d) inertia, capacity to remain excited for a period after the stimulus has ended. 6. The common final pathway. The same reflex may be caused by a great number of different stimuli acting on various receptors. For example, the scratching reflex may be caused by stimulation of the skin receptors, of the flexor muscle proprioceptors, of the extensor muscle proprioceptors on the contrary side, or even by acoustic or visual action, if they have been previously combined with the scratching reflex (conditional reflex). So, in this experiment stimuli from different reflector arcs get the same motoneuron (final common pathway). Reflexes which arcs have a final common pathway may be divided into allied and antagonistic. Reflexes coming to compete for the final common path are antagonistic ones. Example for antagonistic reflex is the following: if during the scratching reflex a strong pain stimulus is applied, the scratching reflex will cease, since the final common path for these two reflexes is the same. Those reflexes coming into collaboration with each other and so intensify the final common path are synergistic (allied) ones, e.g. the reflex of saliva excretion is accompanied by excitation both of the taste and the tactile receptors. If these two types’ receptors are excited at the same time, the main reflex (saliva excretion) will be more intensified.

7. The excitation replacement by inhibition, and vice versa (induction). Inhibition is always followed by excitation and this is called consecutive positive induction. Similarly, excitation is followed by inhibition (that will be negative consecutive induction). It is shown, that if the animal’s skin is excited by a weak stimulus, a weak scratching reflex will be produced, which in turn will be inhibited if at this moment the strong electrical stimulus is applied at that site of the skin. But, if we remove that strong stimulus, the scratching reflex will be expressed more intensively. Pavlov studied induction interrelations during the functioning of the conditional reflex, and named these phenomena as the cerebral positive and the negative induction. 8. The”rebound’’ phenomenon. It consists in the following: one reflex could be replaced by another reflex of the contrary meaning. Termination of stimulation causing a strong flexion reflex is followed by a sharp extension of the flexed leg. The reason is that, the extensor centre being in reciprocal relations with the flexor centre becomes excited now. Actually, the flexor centre excitation brings to the extensor centre inhibition, and vice versa. All this is connected with realization of the successive rhythmic reflex. 9. The principle of ,,feedback’’. Not only basic stimuli, but also the so-called secondary afferent stimuli come into the CNS. Any motor act induced by an afferent stimulus is accompanied with the stimulation of the receptors of the muscles, tendons, etc., i.e. of the proprioceptors, from which nerve impulses are conducted to the CNS. When a movement

77 is being performed by a human being under the guidance of vision, or hearing, the proprioceptor impulses are joined by visual or acoustic signal. The significance of secondary afferent impulses for reflex realization and of its coordination is very great. It is successfully shown both by experiments on animals and by clinical observations of patients who have lost one or other of the senses. Patients with impaired proprioceptive sensibility no longer have smooth and accurate movements (their movements become abrupt, uncoordinated). Section of afferent nerve fibres, deafferentiation, results in abnormal reflex performing. In this case intensified irradiation of the nerve impulse will be also observed. For example, the leg deprived of sensibility performs rhythmical movements coinciding with the respiratory rhythm. This type of connection (secondary afferentation) persists in the whole organism between all the organs and systems. Owing to this ,,feedback’’ between the nerve centres and the effector organs the intensity of the excitation of neurons in the centre and the sequence of excitation of its demands are strictly coordinated. This ,,feedback’’ could be positive in case of strengthening of the basic reflex; and negative in case of weakening of the basic reflex. Very often they turn over together, e.g. secondary afferent impulses during the muscular contraction on the one hand excite one centre of motoneurons and on the other hand, inhibit another centre of motoneurons.

2.2. SPECIAL PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM

Spinal cord

The spinal cord is phylogenetically the oldest part of the CNS. It is logged in the vertebral canal. The typical feature of the spinal cord is its segmental structure, which reflects the segmental structure of the vertebrate body. Each spinal segment gives rise to two pairs of the ventral and dorsal roots. The dorsal roots form afferent inputs of the spinal cord. They are formed by central processes of the fibres of first afferent neurons which bodies are located on the periphery in the spinal ganglia. The ventral roots form efferent outputs of the spinal cord. They contain axons of the α- and γ-motoneurons. This distribution of afferent and efferent roots is known as Bell- Magendie’s law. All the neuronal elements of the spinal cord are classified into four principal groups: efferent neurons, interneurons, ascending tract neurons, and intraspinal fibres of sensory afferent neurons. The spinal cord performs the following functions: conducting, reflectory and elementary analysis. The neuronal organization of the spinal cord. Motor neurons are located in the anterior horns. The α-motoneurons innervate skeletal muscle fibres (so-called extrafusal fibres) and ensure muscle contraction. The γ - motoneurons innervate muscle intrafusal fibres, contraction of which causes the excitation of muscle stretch receptors. Combined activity of these two types of neurons provides the motor coordination. In

79 decreased tone of the skeletal muscle the γ- motoneurons cause contraction of intrafusal fibres, ensuring the excitation of stretch receptors, which are located in the nucleus of the fuse. The impulses, coming from these receptors to the spinal cord, activate the α−motoneurons. The latters provide the contraction of muscles (extrafusal fibres), causing the increase of muscle tone. The preganglionic neurons of the vegetative nervous system make up a special group of efferent neurons of the spinal cord. They are located in the lateral horns of the gray matter. Axons of these neurons pass to the ganglion cells of the sympathetic chain, where they are interrupted forming synapses with the second neurons. The latters innervate the internal organs. The interneurons of the spinal cord form rather a heterogenous group of nerve cells, which processes lie within the spinal cord. The interneurons can be excitatory, as well as inhibitory. The Renshaw cells which are excited by the muscle receptor afferent fibres belong to the group of inhibitory interneurons. They participate in reciprocal inhibition of the motor neurons of antagonistic muscles. They take place in postsynaptic (direct and indirect) and presynaptic inhibition. The conducting tracts of spinal cord. As is noted above, the spinal cord contains neurons, which give rise to long ascending pathways to various brain structures. The main ascending pathways of the spinal cord are the fasciculus of Goll and the fasciculus of Burdach. They are included in the composition of the posterior funiculus of the white matter and

80 end in the medulla oblongata. These pathways are responsible for the perception of skin mechanical stimuli. The other ascending pathways begin from the neurons of the grey matter of the spinal cord. Since these neurons are supplied by the synaptic inputs from the first afferent neurons, it was accepted to designate them as the secondary afferent neurons. The main bulk of secondary afferent fibres pass as part of the lateral funiculus of the white matter. Here the spinothalamic tract is located. The axons of the spinothalamic neurons are decussated and through the medulla oblongata and midbrain reach uninterruptedly the thalamic nuclei to form synapses with the thalamic neurons. This tract transmits the impulses from skin receptors. The lateral funiculi contain fibres of the spinocerebellar, posterior and anterior tracts, which transmit impulses from the skin and muscle receptors to the cortex. The pathway of pain sensation is located in the anterior funiculi. Тhe posterior, lateral and anterior funiculi contain propriospinal tracts which provide the functional integration and reflex activity of the spinal cord centres. The spinal cord also receives many descending pathways, which are formed by the axons of nerve cells located in the cerebral cortex, mid-brain and medulla oblongata. The main among them are reticulospinal and vestibulospinal tracts (phylogenetically older). The reticulospinal tract is formed by axons of the brainstem reticular formation neurons, which terminate on the neurons of the grey matter, including motoneurons. The vestibulospinal tract is composed by the

81 axons of the neurons of the lateral vestibular nucleus, Deiters nucleus. These two tracts do not decussate. The evolutionary younger rubrospinal tract begins from the red nucleus of the mid-brain. After decussating, the rubrospinal tract is included in the composition of the lateral white funiculi. The neurons of corticospinal or pyramidal tract (evolutionary the youngest) lie in the motor cortex. The fibres of this tract decussate and as a part of the dorsolateral funiculi pass above the rubrospinal tract. Pyramidal axons make direct contacts with motor neurons. Reflex activity of the spinal cord. The centres of number of reflexes are located in the spinal cord. The tendon and stretch reflexes are the simplest spinal reflexes. The afferent pathway of these reflexes begins from stretch receptors forming synapses directly with the motoneurons. Thus, the arc of these reflexes is monosynaptic and the reflex time is very short. The estimation of reflex time has an important diagnostic significance for nervous diseases. Among these reflexes the patellar, the Achilles reflex, the knee reflex are the most important. The reflexes of the body position have a more complex character. They are polysynaptic reflexes. For that reason the tonic neck postural reflex has been especially distinguished. The proprioceptors of muscles of the neck and fascia covering the spinal cervical segment are the receptive field of the tonic reflexes. The reflex reaction involves the muscles of the trunk and limbs. Tonic neck reflexes arise when the head is turned or

82 bent, which causes stretching of the neck muscles and excitation of the receptive fields. The spinal cord also plays a major role in the reflex regulation of the internal organs and contains of the centres of numerous visceral reflexes. The preganglionic neurons of the vegetative nervous system take part in these reflexes. The axons of these neurons leave the spinal cord through the anterior roots and are interrupted in ganglia. The ganglion neurons, in turn, send axons to the visceral organs (intestine, urinary bladder, vessels, heart, etc.). The centres of defecation and diuresis are also located in the spinal cord. Descending control of the spinal cord activity. The activity of the spinal cord is subjected to the strict control of the overlying parts of the CNS. The impulses arriving at the spinal cord via the descending tracts can exert direct action on the spinal motor centres. That is why in partial, and moreover, in total section of the spinal cord, the activity of the spinal centres caudally to the site of trans-section is sharply disrupted (spinal shock). A unilateral affection of the spinal cord due to trauma or various diseases of the CNS causes the development of a symptom complex which is known as the Brown- Sequard’s syndrome. It is characterized by the disturbed muscle tone and sensitivity, paralysis and vasomotor disorders on the site of affection. Further the reflex activity is gradually restored, and the higher the organization of the nervous system the slower this restoration process is. First, the motor reflexes are restored. Reflexes connected with the activity of internal organs are restored later. Sometimes spinal reflexes are

83 expressed more strongly, than in the norm (hypereflexia), which demonstrates that overlying structures have also an inhibition effect on the spinal cord activity.

Hindbrain

The medulla oblongata and the pons varolii compose the hindbrain. This part performs reflectory and conducting functions. The nuclei of the fifth-twelfth pairs of cranial nerves are located in the hindbrain. The neuronal organization of hindbrain. Similar to the spinal cord, it has efferent neurons, interneurons, neurons of ascending and descending tracts, primary sensory fibres and fibres of conducting tracts passing through the hindbrain in the ascending (rostral) and descending (caudal) directions. Nuclei of the cranial nerves receive afferent impulses from the periphery and send efferent impulses to the muscles, organs thus resembling the spinal neuron centres. The eleventh pair (n.accessorius) and twelfth pair (n. hypoglossus) nerves are purely motor nerves. The tenth pair (n. vagus) and ninth pair (n. glossopharyngeus) nerves are mixed. The eighth pair of nerves is sensory (n. vestibulocochlearis). It consists of two branches, the vestibular and cochlear nerves. The vestibular nerve is formed by the afferent fibres running from the receptors of the semicircular canals. Some of these fibres pass to the cerebellum. The vestibular neurons give rise to the vestibulocerebellar and vestibulospinal tracts. The reticular formation is situated in the medial part of the medulla oblongata. Cells of the reticular formation give rise to the

84 ascending and descending tracts, which form numerous collaterals whose endings make synaptic contacts with various nuclei of the CNS. The pons varolii, which is a continuation of the medulla oblongata, connects the medulla with the midbrain. It consists of nuclei of the seventh pair (n. facialis), sixth pair (n.abducens) and fifth pair (n. trigeminus) nerves. The facial and trigeminal nerves are mixed. The abducent nerve is motor. The medial nuclei of the reticular formation of the pons give rise to the ascending fibres to the midbrain and diencephalons. Reflex activity of hindbrain. The hindbrain is responsible for numerous functions, such as respiration, heart activity, digestion, vasomotor activity, etc., which are vitally important for the body. The hindbrain realizes static reflexes, which are subdivided into the postural and righting reflexes and statokinetic ones. The postural reflexes ensure changes in the muscle tone, when the body position in space is changed. The righting reflexes govern the redistribution of the muscle tone, owning to which the natural posture can be restored in case it has been changed. The vestibular afferent fibres and neurons of the lateral vestibular nucleus, whose axons pass to the spinal cord as a component of the vestibulospinal tract, take part in the performance of these reflexes. The excitation of vestibular receptors causes postural reflexes, i. e. the activation of extensor muscles and inhibition of flexor muscles. The righting reflexes are responsible for the normal position of the head.

The statokinetic reflexes are the most complex. They provide the maintenance of posture and orientation in space when the speed of movements is changed. For example, during the sudden halt of bus or car, the muscles contract to overcome the force acting on a human body. The statokinetic reflexes involve almost all body musculature. They are pronounced in the muscles of the eye. Movement of eye muscles mediates the normal visual orientation during acceleration or slowing down of movement. Along with motor reflexes, activation of the vestibular apparatus causes excitation of the autonomic centres, including the nucleus of the n. vagus. The formation of vestibule- autonomic reflexes cause changes in respiration, heart rate, gastro-intestinal activity, etc. In turn, many motor reflexes are associated with the food intake, chewing and swallowing. Damage to other parts of the CNS may have no symptoms due to great compensatory capacities of the brain, while a minor trauma to hindbrain immediately has a grave or even fatal consequence. Descending and ascending influences. The medulla oblongata and the pons send fibres to the spinal cord, which cause more diffuse non-specific influence on the spinal cord motor centres. Electrical stimulation of the medulla oblongata medial reticular formation causes inhibition of all the spinal motor reflexes owning to the development of postsynaptic inhibition with the help of inhibitory interneurons. It has been concluded that this zone functions as a non-specific inhibitory centre.

The hindbrain reticular formation can have a positive effect on the cerebral cortex, which is associated with involvement of the reticular formation not only of the hindbrain but of the midbrain and the diencephalon. This complex of the reticular nuclei and pathways makes up a functionally united system.

Midbrain

The midbrain or mesencephalon consists of two main parts: the dorsal part known as the tegmentum of the midbrain and the ventral part, the cerebral peduncles. The midbrain contains also the substantia nigra, the quadrigeminal bodies, the red nucleus, the nuclei of the cranial nerves, and the reticular formation. Various ascending pathways pass through the midbrain to the thalamus and cerebellum; the descending pathways from the cerebellum hemispheres, corpus striatum and hypothalamus run to the midbrain neurons and to the nuclei of the medulla oblongata and the spinal cord. Neuronal organization of midbrain. The nuclei of the trochlear (forth pair, n. trochlearis) and oculomotor (third pair, n. oculomotorius) are located in the midbrain. Neurons, which receive signals via the auditory tracts (the primary auditory centre), are situated in the inferior colliculus. The superior colliculus contains cells, which are relay stations for the impulses arriving along the optic tracts (the primary optic centre). The red nucleus contains cells of different sizes. The thickest and having higher conduction velocity axons of the

87 rubrospinal tract arise from the large neurons, which receive signals from the motor area of the cerebral cortex, the nucleus interpositus of the cerebellum and from the nerve cells of the subtantia nigra. The red nucleus gives rise to cells, which axons innervate the spinal centres controlling the upper and lower limb musculature. The substantia nigra is the integrity of nerve cells containing pigment melanin, which imparts the typical dark color to this nucleus. These neurons are dopaminergic. They are lodged in the zona compacta. The other part of the substantia nigra is zona reticulata, which consists of target cells for the projections of the basal ganglia. The latters in turn form synaptic inhibitory contacts with the nuclei of the thalamus, pons, and superior colliculus. Functions of midbrain nuclei. The arcs of orientation, visual and auditory reflexes are closed in the midbrain (inferior and superior colliculi). The nuclei of the quadrigeminal bodies participate in the performance of the guarding reflex. The substantia nigra participates in the complex coordination of movements. The dopaminergic neurons of the substantia nigra send axons to the nuclei of the corpus striatum, where the dopamine controls the complicated motor acts. Damage to the substantia nigra leads to degeneration of dopaminergic fibres causing disorders of movements of fingers and development of muscular rigidity and tremor (Parkinson’s disease). Trans-section of the brainstem below the red nucleus in animals causes significant changes in the distribution of muscle tone of the body. The tone of the extensor muscles is sharply

88 increased. The animal’s limbs are strongly extended, the head is tilted back, and the tail is raised. This condition is known as decerebrative rigidity. This phenomenon is explained by interruption of signal transmission to the spinal cord via the corticospinal and rubrospinal tract activating the motor neurons of the flexor muscles and inhibition of the extensor muscles. As a result, the activity of the vestibulospinal system, which increases the tone of the motor neurons of the extensor muscles, becomes dominating.

Cerebellum

The cerebellum is located posteriorly to the cerebral hemispheres, above the medulla oblongata and the pons. The cerebellar hemispheres consist of anterior and posterior lobs, which are covered by cortex. The cerebellum is connected with other parts of CNS by three pairs of peduncles. Neuronal organization of cerebellum. The cerebellar cortex consists of three layers. The superficial or molecular layer has ramifications of the flask-like or Purkinje cells, which are inhibitory neurons. It has been established that one Purkinje cell forms nearly 200000 synapses due to a huge amount of dendrites. The molecular layer has also parallel fibres, which are the axons of intercalary neurons. The lower part of the molecular layer contains basket cell bodies, the axons of which make synapses with Purkinje cell bodies. The molecular layer also contains a certain number of stellate cells. The second layer is ganglionic layer, which is represented by Purkinje cell bodies. The third layer is granular

89 layer, which contains intercalary neuron bodies. Axons of these cells ascend to the molecular layer where they divide in a T– pattern. The Golgi cells, which axons project to the molecular layer, are also located in the granular layer. The cerebellar cortex receives only two types of afferent fibres: climbing and mossy. They supply the cerebellum with all sensory influences. The climbing fibres, which are axons of the neurons lying in the inferior olives, make synapses with the dendrites of the Purkinje cells. Each Purkinje cell is reached by only one climbing fibres. The characteristic feature of these fibres is the formation of plural synapses with intercalary neurons, which axons reach to the molecular layer form the system of parallel fibres. The latters are in the synaptic contacts with the Purkinje cell dendrites. These synapses are excitatory. Mossy fibres make synapses with the basket cells. The synapses between the basket cell axons and Purkinje cell bodies are inhibitory. They provide effective inhibition of excitatory influences exerted on the Purkinje cells through the axodendritic synapses formed by climbed fibres and intercalary neuron bodies. The mossy fibres make synapses also with the Golgi cells and stellate cells, which like the basket cells are inhibitory neurons. Two types of afferent fibres (climbing and mossy) enter the cerebellar cortex, but only one type of efferent fibres, i.e. axons of the Purkinje cells, leave it. The cerebellum receives information from various sensory systems. Afferent signals reach the cerebellum from

90 the spinal cord, vestibular receptors, inferior olive and reticular formation of the hind-brain. The spinocerebellar tract supplies the cerebellum with information about the muscular apparatus, skin and deeper lying tissues. Neurons of the nucleus interpositus (the globosus and emboliform nuclei) send fibres to the cells of the red nucleus. Synapses formed by these fibres on the rubrospinal neurons are excitatory. Thus, responses arising in spinal motor neurons on stimulation of the nucleus interpositus resemble those arising on stimulation of the red nucleus. Neurons of other nuclei of the cerebellum establish excitatory synapses on the reticulospinal neurons of the medulla oblongata and the pons. Thus, the whole information supplied to the cerebellum is transmitted to the Purkinje cells, which, in turn exert inhibitory influences on the cerebellar nuclei, as well as on the neurons of the lateral vestibular (Deiters) nucleus. Therefore, Purkinje cells through cerebellar nuclei inhibit the activity of the reticulospinal and rubrospinal neurons. Thus, the cerebellum can effectively control most signals transmitted to the spinal cord via the main descending tracts. Functions of cerebellum. Clinical manifestations attendant to disturbances of the cerebellum as well as the effects caused by its stimulation or extirpation testify to the important role played by the cerebellum in static and statokinetic reflexes and other processes involved in the control of motor activity. Their main manifestations are impaired equilibration and muscle tone, tremor, ataxia, asynergy and astasia. Muscular atonia is an inability to maintain posture.

The tremor is characterized by small amplitudes of oscillations, which occur synchronously in various body segments. Ataxia is described by less precise of movement range, speed and direction. Motor reactions lose their smoothness and steadiness. The goal-directed movement (e.g. an attempt to take an object) becomes rough and jerky. In asynergy, the interaction between motor centres of various muscles is deranged. Adiadochocinesia is a variant of asynergy and manifested in the derangement of correct of antagonistic movements, e.g. finger flexion and extension. In astasia movements are swinging and jerky. Total excision of the cerebellum or its anterior lobe in animals increases the tone of extensors; on stimulation of the anterior lobe, the tone decreases. The cerebellum performs a significant role in the regulation of the vegetative functions due to its numerous synapses with the brainstem reticular formation.

Diencephalon. Thalamus and hypothalamus

The diencephalon consists of thalamus and hypothalamus. Neuronal organization of thalamus. The number of thalamus nuclei is forty. They are classified topographically into the following main groups: anterior, intralaminar, medial and posterior. According to their function, non-specific and specific nuclei are distinguished. Axons of the specific nuclei make contacts only with the cells of specific cortical areas. Neurons of the non-specific nuclei first transmit signals into

92 the subcortical structures from where impulses pass to different cortical areas. The non-specific nuclei are a continuation of the reticular formation of the midbrain and by nature they resemble the functions of reticular formation. Fibres of various ascending tracts end on the neurons of the specific nuclei. Axons of these neurons make direct monosynaptic contacts with the neurons of the sensory and association cortex. Cells of the nuclei of the lateral group of the thalamus receive impulses from receptors of the skin, motor apparatus, and from the cerebellothalamic tract. The other group of the specific nuclei is a component of the posterior group and forms the medial and lateral geniculate bodies. The neurons of the lateral geniculate body receive impulses from the primary visual centre of the anterior quadrigeminal bodies. The medial geniculate body neurons receive signals from the auditory nuclei of posterior quadrigeminal bodies. Neurons of the specific nuclei send axons, which are almost devoid of collaterals to the cortex. In contrast, axons of the non-specific nuclei form numerous collaterals. At the same time, fibres passing from the cortex to the neurons of the specific nuclei have exact topography of their endings unlike the widely ramified system of fibres ending diffusely in the non-specific nuclei. Functions of thalamus. All sensory signals, except those arising in the olfactory tract, reach the cerebral cortex only via the thalamocortical projections. The thalamus may be regarded as a kind of a gateway through which the main information about the external and internal environment and status of the

93 body passes to the cortex. Thalamic neurons are relay stations for the afferent signals on the way to the cerebral cortex. In turn, the thalamus receives inhibitory signals from the cortex. The ascending activating influences from the brainstem reticular formation enter the cerebral cortex through the non- specific thalamic nuclei. The system of non-specific thalamic nuclei is involved in the control of rhythmic activity of the cerebral cortex and performs as the intrathalamic integrative system. The cortex reaction is marked by prolonged latent period and intensifies on repeated stimulation. This reaction differs from specific responses of the cerebral cortex by generalized pattern, when extensive cortical areas become activated. Damage of non-specific thalamic nuclei causes disorders of consciousness and emotions. Pain signals cause strong activation of the thalamic nuclei. Latent period of thalamic responses is prolonged and variable. The thalamus is the higher centre of pain sensitivity. Certain thalamic nuclei (dorsal group) exert regulatory influences on the subcortical structures. Thus, the thalamus can play a significant role of a suprasegmentary centre of reflex activity. The talamus is responsible for the locomotion and complex motor reflex control (swallowing, chewing, sucking, etc.). Neuronal organization of hypothalamus. The hypothalamic nuclei are the higher subcortical centres of the vegetative nervous system and governing all vitally important body functions. The preoptic, anterior, medial lateral and posterior areas are distinguished. The preoptic area comprises

94 the medial and lateral preoptic nuclei. The anterior part includes the supraoptic, suprachiasmatic and paraventricular nuclei. The lateral area consists of lateral groups of nuclei. The posterior hypothalamus has the posterior hypothalamic and a large group of mamillary nuclei. The hypothalamus is characterized by extensive and highly complicated afferent and efferent connections. Afferent signals are supplied from the cerebral cortex, thalamic structures and basal ganglia. Main efferent pathways pass to the midbrain and thalamic and subthalamic areas. There is a direct connection between anterior hypothalamus and posterior pituitary, which provides the transport of synthesized hormones (oxytocin and ADH). Functions of hypothalamus. The lateral and dorsal groups of hypothalamic nuclei increase the tone of the sympathetic nervous system. Stimulation of the medial nuclei decreases the sympathetic tone. Experimental evidence suggests the existence of sleep and wakefulness centres in the hypothalamus. The hypothalamus plays a significant role in the thermoregulation. Stimulation of the posterior part causes hyperthermia due to increased heat production (intensification of metabolism). It is the centre of chemical thermoregulation. The anterior part of the hypothalamus is responsible for the physical thermoregulation. The medial area is considered as the centre of satiety and the lateral one, hunger. The activity of these areas is stimulated or inhibited by changes in the chemical composition of the

95 supplied blood. The sensation of thirst is conditioned by activity of the area, located dorsolaterally of the supraoptic nucleus. Destruction of this area causes absence of thirst. It has been established that the emotional and pleasure centres are located in the hypothalamus. The posterior hypothalamus, which is connected with the anterior pituitary, regulates the secretion of adenohypophyseal hormones by means of liberins and statins.

CHAPTER 3. HIGHER NERVOUS ACTIVITY

The conception of higher nervous activity was first suggested by Pavlov distinct from the lowest neuronal activity and is represented by the brain hemisphere cortex activity, as well as the closest subcortex, which together provide normal complex interrelations of the whole organism and the internal and external media.

3.1.CONDITIONAL AND UNCONDITIONAL REFLEXES

The whole integrity of reflector reactions is the convention to divide into the conditional and unconditional reflexes. Unconditional reflexes are congenital and strictly connected with the structure and are functioning from the birth moment, afterwards depending on the given structure maturation and have an adaptation significance to the environment constancy. Conditional reflexes are being obtained during ontogenesis and are connected with acting stimulus. They are individual and have adaptation significance to the altering environment. Differences: I. Unconditional reflexes are congenital; conditional ones are obtained. 2. Unconditional reflexes become stable during phylogenesis; conditional ones arise during ontogenesis.

3. Unconditional ones are connected with structure; conditional ones are connected with acting stimulus. 4. Unconditional reflexes are typical; conditional ones are individual. 5. Unconditional reflexes are constant; conditional ones are temporary. 6. Unconditional stimulus is adequate; conditional stimulus is not adequate. 7. Unconditional reflexes have an adaptation significance to the constant environment; conditional ones – to the environment changes. Classification of reflexes. According to their biological significance the whole totality of unconditional reflexes are divided into: feeding, defence, sexual, orientation reaction (,,what is it?’’ reflex) and parental reflexes. Another interesting classification was suggested by Slonin: 1) reflexes for the internal environment constancy sustain (feeding, maintaining homeostasis); 2) reflexes for the external environment altering (defence, situational); 3) reflexes sustaining the type (sexual and defence); Conditional reflexes are divided into natural (that are worked out on the natural stimulus action, smell and other natural features), artificial (light, bell ringing, geometrical figures, i.e. features not intrinsic to the acting stimulus).

Rules for building conditional reflexes

1. Conditional reflex can be produced in time when the beginning of the indifferent (conditional) signal precedes the beginning of unconditional stimulation. 2. Conditional and unconditional signals have to cohere in time. 3. Stimulus, which will become conditional must not produce a significant unconditional reaction, i.e. physical strength of conditional one must not exceed the strength of unconditional stimulation. 4. The cortex must be active. 5. The cortex must be free from other types of activity. 6. Motivation. 7. Attribute factors. 8. The animal has to be healthy.

Components of unconditional and conditional reflexes

Each reflector reaction is accompanied by vegetative and motor components. For example, unconditional defence reflex on the pain stimulation is manifested by defence motor reaction, changes in breathing, cardiac activity, arterial pressure, blood content, etc. The conditional reflex produced on the unconditional base also serves as a multi component reaction. Herein they differentiate the main specific and secondary non-specific components. In the defence reaction the main component is the movement component, the vegetative changes are secondary. In feeding the main component will be

99 secretion and motor activity of the digestive tract; whereas all the rest components will be secondary ones and they have just a serving role, forming optimal conditions for the main component realization.

Mechanisms of conditional reflex producing

According to Hasratyan, any unconditional reflex is multistage and each unconditional reflex has a cortical representation. With the cortex removal the unconditional reflex does not disappear, but changes in its character take place. In case of acting of two signals having similar strength and producing feeding and defence reflexes, then altering them timely we can work out doubled conditional reflex: upon the feeding reflex not only saliva excretion, but also paw’s pulling off take place. So, between two unconditional reflexes a connection arises. But decortications interrupt this connection. In the conditional reaction forming, e.g. on light signal, first the orientation reaction arises, and on reply to alimentary signal the feeding reflex produces. Multiple combinations in time of these signals create a temporary functional connection between the visual and the feeding centres. Closing of this connection takes place by the dominant principle. Stronger feeding centre (there is a feeding motivation) being excited more impulses attract from the visual centre, and the summation expense becomes more intensive. Now just in case of conditional signal acting (e.g. light) impulses through the existing pathway will arrive from the visual centre to the feeding centre, exciting the latter.

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Ecckles considered that repeated passing of impulses through the neuron system enhances effectiveness of synapses and intensifies the synaptic transmission (synaptic relief). Pavlov previously suggested that the conditional reflector connection was realized transcortically: cortex-cortex. But Hasratyan and other researchers showed, that the vertical dissection of the grey matter does not affect on the conditional reflex producing. Hasratyan proposed the vertical closing of connection: cortex-subcortex-cortex. The afferent impulses arrive by specific and non-specific pathways to the sensory zone of the cortex, and after being worked off there they return to the reticular formation by the efferent ways, and then to the cortical centre of the unconditional reflex. Afterwards this version was ascertained electro-physiologically. An issue of long-term keeping of the worked out conditional reflexes even nowadays remains not solved. The short-term memory is considered to be connected with the neurons functional characteristics augmentation, as well as with the neuronal traps. As for the long-term memory the version of morphological changes in the neurons is suggested: the development of the presynaptic terminals, increasing in number of synapses, changes in the nucleic acids, particularly in RNA contents. But the update knowledge level does not allow submitting a certain hypothesis regarding the memory problem.

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3.2. CORTICAL INHIBITION

No conditional reflex is possible to work out just by stimulation. Herein inhibition is necessary and very important. The orientation is the reaction previously arisen by the light stimulation then inhibited and lets the feeding reflex be produced. Without inhibition the existing excitation will irradiate over the whole cortex. Inhibition is characterized by ceasing or weakening of that or other reflector activity. There are two types of cortical inhibition: conditional and unconditional. Unconditional inhibition. Unconditional inhibition arises since the first display: the feeding reflex becomes inhibited and the orientation reaction arises. This inhibition is called external, because it is formed under the influence of external, non-significant for the given reflector activity stimuli from the other neuronal centres, out of the given reflector arc. In multiple repetitions the orientation reaction disappears and the signal’s inhibitory influence weakens. Such stimuli were called by Pavlov “extinguishing inhibitors’’. External inhibition is explained by the negative spatial induction. Another type of unconditional inhibition is the transmarginal inhibition. The excessive increase in strength or in action duration of conditional stimulus gives an opposite result, leading to weakening and inhibition of the conditional reflex. It has a protective significance and protects neuronal cells from the exhausting influence of strong and prolonged stimuli.

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Both of these types of inhibition are connected with the neuronal system’s natural congenital properties. Conditional inhibition. Conditional inhibition is based on the conditional reflex and appears in those neuronal structures, which participate in the given conditional reflex realization, i.e. within the given reflector arch. So, it is called an internal inhibition. Conditional inhibition arises during ontogenesis and needs processing. Types of internal inhibition. There are 4 types of internal inhibition, depending on conditions expediting the conditional reflex to be not sustained: extinguishing inhibition, differential inhibition, conditional inhibition and delayed inhibition. 1. Extinguishing inhibition. A conditional signal is accompanied and reinforced by an unconditional stimulus, but if a conditional reflex is used alone and is not reinforced by an unconditional stimulus, the stable conditional reflex, previously established, gradually weakens after several applications and finally extinguishes. 2. Differential inhibition. If a conditional reflex is developed to the note ,,re’’, similar sounds (do, re, mi) will also be capable of eliciting a conditional positive reaction. But in case of reinforcing only the ,,re’’ note, generalization of the conditional reflex declines and differentiation of stimuli takes place. The closer parameters of stimuli, the more difficult to develop a conditional reflex.

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3. Conditioned inhibitor. It develops in case, when the stimulus A (light) is constantly sustained, but A+B (light + sound) - not. Previously both A and (A+B) combination evoke a conditional reflex. Further, the (A+B) combination loses its positive significance. ,,B’’ serves as a conditional inhibitor. It contributes additional information to the conditional stimulation and obtains an inhibitory significance. 4. Delayed conditional reflex. If the unconditional stimulus (food) is constantly delayed for a short span after the beginning of the conditional stimulus (1-5 sec), the secretion begins just after the conditional stimulus action onset. If the reinforcement lags behind for 2-3 min, the secretion is delayed by 1-3min. The conditional stimulus first has really an inhibitory significance, but afterwards- a positive one. Deeply delayed stimulus loses its positive significance. The role of internal inhibition is: a) to divide all conditional signals into 2 categories: the positive, which evokes conditional reflex reactions and the negative, causing inhibition; b) to have an adaptation importance making the organism’s reactions more economic and purposeful; c) to bring to inhibition and excitation processes underlying the analysis and synthesis of the cortex activity.

Analysis and synthesis

Analysis is an investigation of specific discrete sides of a whole process. In physiological meaning it is a reaction of the organism to discrete components of a complex stimulus and

104 consists in discrimination between different signals and divisions of (complex phenomenon) its components. Analysis is achieved through the internal inhibition. Synthesis is a common reaction of the organism on several stimuli, and is expressed in the association, generalization and unification of excitations. Synthesis is manifested by the formation of a temporary connection on which every conditional reflex is built.

Mutual induction of excitation and inhibition

Excitation and inhibition may intensify each other, which is called induction. First this phenomenon was discovered by Vvedensky in the spinal cord. Then it was studied by Sherrington, and finally, Pavlov, who found out induction in the cortex. After the concentration of excitation or inhibition, they induct contrary phenomenon on the periphery. Pavlov differed two phases of induction: 1) positive (inhibition intensifies excitation); 2) negative (excitation intensifies inhibition).

3.3. TYPES OF THE HIGHER NERVOUS ACTIVITY

Integrity of the main individual properties of the neuronal system is called the type of the higher nervous activity (HNA). In Pavlov's laboratory it was revealed, that being in the same conditions not in all animals it was possible to develop conditional reflexes, which depended on individual peculiarities of the HNA. As a criterion of the HNA assessment

105 the force mobility and balance of the excitation and inhibition processes were elected. According to these criteria Pavlov elected 4 types of HNA: I type is strong-balanced-moveable (sanguine); II type is strong- balanced-inert (phlegmatic); III type is strong-unbalanced (choleric); IV type is weak (melancholic). Pavlov’s this classification coincides with the temperaments submitted by Hippocrates: sanguine, phlegmatic, choleric, melancholic. There are many other intermediate types of HNA, making variants of these main ones. Type of HNA is built up based on congenital and acquired characteristics during all life. The integrity of the congenital characteristics is named as genotype; of the acquired ones as a result of life experience – as phenotype. In man the significance of genotype and phenotype in forming HNA are approximately equal, since man is a social being. Different conditions during life may change not only behaviour, but all the psychological habits. In any type of HNA it is possible to work out socially useful features of character. Thus, even a weak type being in favourable conditions can attain much and become a more useful member of a society, than a representative of a strong type deprived of constant purpose (e.g. outstanding composer Chaykovsky was melancholic). In Pavlov’s laboratory puppies from the same family were taken and placed into two different conditions – with a poor care and with a good one. They respectively were grown

106 up of a weak and a strong types of HNA. Excessive guardianship and limitation of independence lead to weak, non- initiative people’s appearance.

3.4. SLEEP

Sleep is a period of relative rest for the body and mind, which arises in definite spans of time, and is accompanied by decrease of the work ability level of discrete organs and functions. Sleep is an indispensable requirement for the organism, especially higher animals and man. This requirement is different in different periods of life. Grown up man spends third of his life in a state of periodically recurring sleep. But for every man duration of sleep is individual and depends on habits and temperament. Napoleon and Peter I slept only 5 hours a day, Edison – 2 hours, Kant –7 hours. Sleep has a crucial role in normal physiological activity. Without meal man can not live more than 1 month, but without sleep - more than 1 week. Sleep is a primary state of the organism and wakefulness is the following state. Wakefulness is developed in the evolution process, and the higher organization of matter the completer process of wakefulness. As an evidence of it is the fact of development of wakefulness in child during his growth. Types of sleep. There are several types of sleep: 1) periodic diurnal sleep; 2) periodic seasonal sleep (winter and summer hibernation in animals); 3) narcotic sleep induced by different chemical or physical agents;

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4) hypnotic sleep; 5) pathological sleep, connected with the brain blood supply disorders and affection of its several parts. The first two types are the variants of physiological sleep, and the remaining three occur due to special non- physiological effects on the organism. The objective indices of sleep. 1) loss of active connections between the organism and environment; 2) change in muscular tone; 3) change in vegetative functions depending on the sleep phases (heart activity, arterial pressure, redistribution of blood, metabolism, the body temperature, etc.). Electric activity of the cerebral cortex. Record of electrical potentials from the cortex is called electroencephalography (EEG), which was submitted by Berger. There are 4 main types of oscillations: Frequency Amplitude β 13 Hz 20-25 mV vivacity (eyes are open, man is thinking) α 13-8 Hz 50 mV rest (man is awaken with close eyes) θ 8-4 Hz 100-150 mV sleep δ 4-0.5 Hz 50–300 mV deep sleep

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In synchronic excitation the waves’ amplitude is high; in vivacity desynchronization of waves (activation of cortex) is observed and the β rhythm of vivacity arises. Phases of sleep. According to the contemporary classification sleep has got two phases: the slow and the fast ones. The slow phase of sleep or the orthodoxal sleep lasts 1- 1.5 hours. It is characterized by θ and δ rhythms on EEG, as well as by pattern of synchronous work of neurons. But EEG of different cerebral structures brings about a conclusion, that sleep is not a passive process, but rather active. In experiments on cats it was revealed that even in wakefulness one part of neurons is in active state, and another part, in passive (is inhibited). Some neurons in sleep even intensify their spontaneous activity, but work in another rhythm synchrony. The fast phase of sleep or paradoxal sleep was established in 1952 by the post-graduate of Chicago University, Yujin Azerinsky. It is characterized in EEG by desynchronization and by arising of β−rhythm. A rapid motion of eyeballs occurs. It is called rapid eye movement (REM- phase). After 1.5 hour sleep (slow phase) the REM-phase comes, which lasts 6-10 min and alters by the slow phase again. In such a manner it occurs 4-6 times over the night. Herein, close to waking up this phase is prolonged up to 0.5 hour. In REM-phase man is in deep sleep and to wake up him is more difficult, than in the phase of slow sleep. This period is called fast because in EEG we have a rapid rhythm. That’s why it is called also paradoxal for that although it is deep sleep, the

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β−rhythm of wakefulness occurs. Thus sleep during all night is summed up by the slow and fast cycles of sleep. In man fast sleep makes up 25% of the whole sleep. It is interesting, that in various animals the duration of the REM-phase is different. In rabbits – 3%, in cats – 30%, Cat is a cousin of beast prey, but the latter has not any rights to sleep much, otherwise it will be eaten. The vegetative and the motor components of sleep. Abrupt decrease of the muscular tone (neck, face) during the REM-phase, the blood supply of the brain increases, muscular small contractions, oscillations of the heart activity rhythm, breathing, the metabolism increase, etc. appear. Most interesting is the fact, that fast sleep is connected with dreams accompanied by emotional experience. A waken up person in the fast phase of sleep can retell the dream, but in the slow phase of sleep he can’t, because the slow phase sweeps off the dreams. The vegetative changes in the fast phase are accompanied by the dreams’ vision. If one continuously wakes up an experimental animal in REM- phase, the different disorders will arise: excitation, lack of appetite, fear, hallucinations up to the conscience impediments. If in this case it’s allowed to have a sleep, all disturbances will disappear. Wakening up in the slow phase of sleep does not influence on overall condition. In the slow phase of sleep the motor activity decreases, but in the fast phase it increases. People can speak, cry, etc. during the sleep. Some of them with an increased excitation can go for a walk; they are called somnambulists or

110 sleepwalkers. This arises in the fast phase of sleep. In medicine a whole family of somnambulists is described (6 persons). During nights they gathered together in the dining room to have tea and went separately, but in the mornings they couldn’t remember anything. The somnambulist’s movements are distinguished by easiness. It is considered to be a compound of normal and pathology and refers to sleep anomalies. Physiological mechanisms of sleep. At the beginning of this century a number of theories explaining the sleep nature were proposed. They are the follows: circular, anemic, and hyperthermic. All these vascular theories articulate sleep with the brain blood flow changes. French scientists Legendre and Pueron conducted an interesting experiment trying to explain sleep as a result of special chemical substances’ production (“the poisons”) that by humoral way evoke sleep. The following experiment was performed on a dog. During some days the dog was not allowed to sleep; and on the 10-th day it was killed, after which its brain was studied. It was ascertained that something horrible had happened to the frontal lobe of the brain: the neurons’ shape was altered and their membranes were eaten by leukocytes. Introduction of the spinal liquid of a dog not permitted to sleep for a long time to the awaken dog, evoked the same changes in the brain of the latter. Ten-year investigations of these French scientists, as well as their humoral theory of sleep without any doubt were accepted, although hypnotoxin was not managed to get. Russian scientist Anokhin observing the twins Sasha and Pasha revealed that

111 twins slept at different times in spite of their common blood supply. It was a quite serious fact against the humoral theory. The next stage in the sleep mechanisms’ studying was the theory of subcortical nuclei. During the World War I encephalitis epidemic burst off, the characteristic feature of which was sleepiness. Austrian neuropathologist Economo observing the patients and performing sections and histological studies of the dead bodies’ brains proved and testified that the reason for this pathology was a virus having affinity to the hind parts of the hypothalamus and the upper parts of the brain. This type of encephalitis was named as Economo’s encephalitis. Afterwards it was established, that affection of the mentioned part of the hypothalamus leads to the sleepy condition development. Brazil singer Maria-Luisa-Santes after a car accident was in sleep condition during seven years due to the affection of the upper parts of the brainstem. Further investigations of the Swiss physiologist Hess on the cat’s hypothalamus, as well as Russian neuropathologist Grastchenkov on the patient’s hypothalamus asserted Economo’s conclusion consisting of presence of the sleep and wakefulness centres in the hypothalamus. Pavlov appreciated of the data obtained by Hess, but referred to them with criticism. According to Pavlov, excitation of these subcortical structures brings about sharp reduction of the inflow of afferent signals to the cortex, supporting the latter in a tone (wakefulness). Thus, a cortical theory of sleep was proposed by Pavlov. In his laboratory it was revealed, that a dog fell asleep during the conditional reflex or its inhibition development.

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Pavlov suggested the inhibition and sleep were the same. Weak monotone excitation brought to sleep, and the inhibition irradiated over the whole cortex and even the subcortical structures. But there were some situations, when the inhibition did not capture all the cortex and some “guard centres” remained, by means of which contact with environment took place. Pavlov differentiated two types of sleep: active or internal inhibition; and passive, when the cortex tone decreases after removing afferent impulses. The law of strength relations changes upon the parabiosis type. Botkin was observing lass, who was deprived of vision, hearing and touch feeling and always slept. German neuropathologist Shtrompel described a boy, who was blind in one eye and deaf in one ear and did not feel any pain, when pricking the skin. When they covered his capable eye and capable ear he immediately fell asleep. All these facts prove the functional significance of afferent impulses in the cortex tone maintenance. But, Pavlov’s theory is not universal and can’t explain all types of sleep. A modern theory of sleep, cortico- subcortical theory, was proposed by Anokhin. Independent of the sleep type’s reasons it is arranged on a unique scheme. Discovery of the reticular formation proved Anokhin’s hypothesis. According to Anokhin 3 main structures take part in the sleep formation: the reticular formation (RF), the limbico-hypothalamic centres of sleep and the cerebral cortex. Between the reticular formation and the hypothalamus reciprocal interrelations exist. It is established, that in presence of ascending influence from the RF the cortex inhibits the

113 hypothalamus activity. In the passive sleep fewer impulses come from the RF and the cortex activity weakens. Inhibition of the cortex removes the hypothalamus inhibition (activation of hypothalamus). In this case the RF also becomes inhibited, which causes strong inhibition of the cortex. Another trunk structures (hypnotic zones) take part in the sleep development, too, but the main role here belongs to the thalamus-cortical system. Other zones evoke the regulatory effect on the latter depending on humoral factors and on physiological systems’ activity. All these zones correspond to the slow sleep, but the fast sleep is connected with the reticular nuclei of the pons. In their destruction the fast sleep disappears and the slow one remains. Ideal sleep is accessed when the RF all ascending influences become as less as it is possible. In the RF partial blockage non-deep sleep occurs. Dreams are considered to be the cortex activity without the RF ascending influences; they take place at the limbic system’s and hormones’ expense.

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CHAPTER 4. PHYSIOLOGY OF THE VEGETATIVE NERVOUS SYSTEM

Vegetative functions are the functions performed by the internal organs, blood and lymphatic vessels, blood formed elements and are directed to provide metabolism, growth, development, as well as reproduction of the organism. This term (vegetative) was proposed by French physiologist Bichat in 1800. All functions of the organism have been divided into animal, or somatic, and vegetative. In accordance with this distribution of functions somatic and vegetative regulating nervous systems are distinguished. The somatic nervous system is responsible for the sensory and motor functions of the organism; the vegetative system provides the efferent innervations of all the viscera. Vegetative functions are characteristic both to plants and animals, i.e. they comprise nourishment, reproduction, respiration, etc. Animal functions are intrinsic to animals only. They mostly comprise the moving functions. In 1883 Gaskell proposed another term, the visceral nervous system that was renamed later as the autonomic nervous system by Langley. The meaning of this term was in the independence of this system from the central nervous system. But the concept of “autonomy” is quite conventional. Later it has been shown that this system innervates skeletal muscles providing their trophy. Besides, the centres of vegetative system are located in the CNS and numerous somatic reflexes have vegetative components.

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General characteristics of the vegetative nervous system

In the structural and functional point of view the vegetative nervous system consists of two parts: sympathetic and parasympathetic (Figure 18). Both of them have central and peripheral parts. The central part is located in the CNS. The peripheral part consists of ganglia and nerve fibres. The sympathetic centres are situated in the thoracolumbar part of the spinal cord, in T1-L5 segments. Their neurons’ bodies are located in the lateral horns, but the axons extend from the spinal cord through the anterior horns, forming the white rami communicantes. They are interrupted in the sympathetic ganglia, where the second neurons are located. The axons of postganglionic neurons extend from the ganglia to the peripheral organs either via individual pathways or as part of somatic nerves’ trunk. They extend from the ganglia as delicate grey rami communicantes (their colour is due to lack of myelin sheaths). Summarizing in contrast to somatic nervous system the vegetative system’s efferent pathway consists of two successive neurons, i.e. the preganglionic fibres terminate in the vegetative ganglion and form the synaptic contact with the main effector neurons. The axons of these neurons are called postganglionic fibres. So, the vegetative ganglia serve as migrated neurons from the nervous centres, which were turning out from the CNS during evolution. The true effector neurons are located in the ganglia, but the interneurons are still in the CNS.

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Figure 18. The structural characteristics of the vegetative nervous system.

Depending on localization all the sympathetic ganglia are divided into the paravertebral and prevertebral. The paravertebral ganglia are arranged on both sides of the spinal cord, forming two sympathetic trunks. The prevertebral ganglia are situated further away from the spinal cord, than the ganglia of sympathetic trunk. The majority of sympathetic preganglionic fibres are interrupted in the ganglia of the sympathetic trunk, but some of

117 them pass through the trunk without interruption and are broken in prevertebral ganglia. The sympathetic nerves supply actually all organs and tissue. In contrast, the parasympathetic nerves do not innervate the skeletal muscle, the CNS, most of the blood vessels, and the uterus. The upper centres of the sympathetic system realize the innervations of the organs of the head (via superior cervical sympathetic ganglion), several thoracic segments send the fibres through ganglion stellatum to the chest organs (heart, bronchi), through the solar plexus and superior mesenteric ganglion to the abdominal organs; from the lumbar segments fibres pass through the inferior mesenteric ganglion to the organs of the pelvis. The parasympathetic centres have craniosacral localization. The cranial part is represented by the midbrain, pons and the medulla oblongata. Its neurons respectively are situated in the III pair of craniocerebral nerve nucleus; in the VII, IX, and X pair of nerve nuclei. The parasympathetic fibres extended from these centres form the part of n. oculomotorius (innervation of the pupil), n. facialis (innervation of the sublingual and submaxillary glands) n. glossopharyngeus (innervation of the parotid gland) and n. vagus (innervation of all the thoracic and abdominal organs). The sacral centres’ fibres form the part of the pelvic nerve (innervation of large intestine, urinary bladder, genitals). Parasympathetic ganglia are located within the organs (intramural ganglia) or are close to them (ciliary ganglion, otic ganglion, etc.). The parasympathetic efferent pathway has also bineuronal structure, i.e. the first neuron axon is interrupted in

118 the parasympathetic ganglion, forming synaptic contact with the second neuron. Moreover, the parasympathetic preganglionic nerve fibres are longer than the postganglionic ones. In contrast, the sympathetic preganglionic nerve fibres are shorter than the postganglionic ones. The vegetative innervation of the above mentioned organs is bineuronal. But there are some exceptions: 1. The medulla of the adrenal glands. The sympathetic efferent pathway of this one has one-neuronal structure. The role of the postganglionic fibres is performed by the chromaffine cells of the adrenal medulla, which secrete catecholamines into blood. So, the chromaffine cells produce adrenaline and like the sympathetic postganglionic nerve ending they are able to produce noradrenaline. 2. The metasympathetic nervous system is characteristic for some internal organs (intestine, ureter) that possess automatism. These organs due to own reflex arc (reflexogenic field, the afferent way, the centre (ganglion) and deriving from it the efferent pathway) have in fact three-neuronal innervation, since the outer vegetative part provides the two- neuronal innervation. Afferent pathway of vegetative reflex. The afferent pathways begin from the reflexogenic fields and appropriate receptors. The latters are localized in the internal and external organs. The afferent nerve fibres may pass with somatic nerves or independently. The impulses coming by those pathways reach not only the vegetative centres, but also the

119 hypothalamus and the cortex. All levels of the autonomic nervous system are subordinated to the higher autonomic centres located in the hypothalamus and corpus striatum, which coordinate the functions of many organs and systems of the body and, in turn are subordinated to the cerebral cortex. So, the cerebral cortex is responsible for an integrated body reaction by combining its somatic and vegetative functions into single behavioural acts.

Comparative analysis of the somatic and the vegetative nervous systems

1. The vegetative nervous centres are located in foci, but the somatic centres are situated in all spinal segments. 2. In the vegetative system the effector neuron is located out of the CNS and only the interneuron is in the CNS. In the somatic nerve centres both the effector and the interneuron are in the CNS (Figure 19). 3. The vegetative neurons are situated in the lateral horns of the spinal cord, while the somatic neurons are in the anterior horns (Figure 19). 4. The vegetative innervation is not of metameric pattern in distinct from the somatic one. 5. The conductivity of impulses in the vegetative nerve system is low, because the preganglionic nerve fibres are represented by the B-type very weak myelinated fibres, and the conductivity speed of impulses is 3-18m/sec. The postganglionic nerve fibres belong to the C-type: they are unmyelinated nerve fibres, and the conductivity speed of

120 impulses is 1-3 m/sec. In contrast, all the somatic nerve fibres belong to A-type and their conductivity speed of impulses makes up 70-120 m/sec. 6. The action potential (AP) of the vegetative nervous system is long-term. The AP of postganglionic nerve fibres has negative after-potential that turns into positive after-potential of 300 msec duration. 7. The vegetative nerves possess low lability and excitability, compared with the somatic fibres.

Figure 19. Comparative analysis of the somatic and the vegetative reflector arcs.

Properties of the vegetative ganglia (synapses)

The vegetative ganglia play a significant role in the distribution and propagation of the impulses passing through them. The vegetative ganglia could be represented as neuronal

121 centres and perform peripheral reflexes, e.g. in the cardiac activity. The number of nerve cells in ganglia is much larger (2-30 times) than the number of preganglionic fibres entering them. Each fibre forms synapses with many ganglionic cells (divergence of impulses). It is rather important, because the zone of influence of preganglionic fibre is extended. The same neuron could get impulses from different nerve fibres, so there is also convergence phenomenon. The latter provides reliability of the impulse transmitting. Since ganglia are neuronal unites, some properties of nervous centres have to be intrinsic to them. For example, one-way conduction of impulses, fatigue, spatial and temporal summation, occlusion and so on. Along with this, the vegetative ganglia have also a number of specific features: 1) long synaptic delay (1.5-30 msec), while the synaptic delay in the CNS is only 0.5 msec; 2) long duration of excitatory postsynaptic potential; 3) long-term after-hyperpolarization that results in inhibition after the excitation; 4) rhythm transformation. The rhythm of excitation of preganglionic fibres exceeding the natural frequency of impulses is partially blocked in the synapses, and the postganglionic neuron is excited at a slower rhythm; 5) low lability of the ganglionic synapses. The maximal rhythm of impulse conduction along the postganglionic nerve fibres is 15 imp /sec. In contrast to this, in the motor nerve fibres it makes 500 imp/sec.

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Mechanism of impulse conduction in vegetative synapses, their mediators

It has been revealed that in the preganglionic nerve endings (both for the sympathetic and the parasympathetic) the same mediator, acetylcholine, is produced, and the impulse transmission is realized by means of acetylcholine. This phenomenon was discovered by Kibyakov in 1933 during the experiment with perfusion of the superior sympathetic ganglion. He determined acetylcholine in the perfusion liquid. Being produced acetylcholine bounds to the cholinoreceptors, which could be blocked by curare-like substances (for example ditillinum, etc.), and could be activated by nicotine. So, this type of receptors is called N-type of cholinoreceptors. The mediator of the postganglionic parasympathetic nerve fibre is also acetylcholine. In this case it interacts with another type of receptors, so called M-type cholinoreceptors (the muscarinic ones). They can be blocked by atropine. M- cholinoreceptors have their subdivision too. It has been revealed, that acetylcholine can activate Na+ - chemo-excitatory channels resulting in the excitatory postsynaptic potential formation. These synapses are present in the smooth muscles: intestine, urinary bladder, bronchi. Another type of the M- cholinoreceptors results in activation of K+-channels and hyperpolarization of the postsynaptic membrane, so the inhibitory postsynaptic potential is formed. This type of M- cholinoreceptors is present in the heart, excitation of which

123 causes inhibition of the heart activity (negative chronotropic, negative dromotropic, negative inotropic effects, etc). The mediator of the sympathetic postganglionic fibre is noradrenaline. But there are some exceptions: the cholinergic sympathetic nerve fibres, innervating the sweat glands and the skeletal muscle vessels. However, the majority of the sympathetic nerve fibres are adrenergic ones. There are two types of adrenoreceptors: α and β, which in their turn are divided into α1-and α2-, β1- and β2 -groups. Depending on the type of adrenoreceptor the mediator is combined with, different effects could be observed. In case of binding with α1- adrenoreceptors, which are abundant in the arterioles of the internal organs, the vasoconstrictor (excitatory) effect occurs.

The α2-adrenoreceptors are localized in the presynaptic membrane. In case of noradrenaline binding to these receptors its further production is inhibited. If noradrenaline interacts with β1-adrenoreceptors, which are available preferably in the cardiac muscle, the excitatory effect (positive chronotropic, positive dromotropic, positive inotropic, etc.) is observed. β2- adrenoreceptors are available in the coronary vessels of the heart. In case of binding of adrenaline to these receptors, the inhibitory (vasodilative) effect takes place (it prevents heart attack during the sympathetic over-stimulation). Comparative characteristics of sympathetic and parasympathetic influences. The sympathetic nervous system has a crucial importance in all extreme situations and provides ergotropic regulation (mobilization of all the organ-systems).

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The parasympathetic nervous system performs the regulation of current activities of organs and systems (trophotropic regulation). Stimulation of the sympathetic nerves of a fatigued skeletal muscle restores its working capacity (experiment of Orbeli and Ginetsinsky), so the sympathetic system also possesses a trophotropic function. We can assert that the sympathetic and parasympathetic nerve systems are in close interconnection. But simultaneously they have opposite effects, which are expressed in the regulation of different organs’ and tissues’ activity (Figure 20). 1. The smooth muscles of internal organs. The sympathetic nervous system has an inhibitory influence on the gastro- intestinal tract motor and secretory activity, urinary bladder, bronchi (relaxation because of the stimulation of the β2- adrenoreceptors). Conversely, the parasympathetic nervous system has an excitatory influence on the above-mentioned organs (the effect of M-cholinoreceptors, combined with Na- channels). 2. Sphincters of digestive organs and urinary bladder. The sympathetic nervous system causes contraction of the sphyncters; the parasympathetic one evokes their relaxation. 3. The vascular smooth muscles of the internal organs. The sympathetic nervous system stimulates contraction of the smooth muscle layer in the vessels of the skin and internal organs (the effect of α1-adrenoreceptors). The parasympathetic nervous system causes dilation of salivary gland and tongue vessels (the effect of M-cholinoreceptors combined with K- channels).

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4. The heart activity. The sympathetic nervous system stimulates the heart function (the effect of β1-adrenoreceptors). The parasympathetic nervous system inhibits the cardiac activity (the effect of M-cholinoreceptors combined with K- channels).

Figure 20. The sympathetic and parasympathetic influences on the organs.

The antagonistic effects of sympathetic and parasympathetic systems are also expressed in their action on the pupil. The excitation of sympathetic nervous system leads to dilation of pupil, parasympathetic – to narrowing. Nevertheless in some conditions the sympathetic and the parasympathetic nervous systems are synergists (in stress

126 reaction development, in blood clotting process, etc.). Generally, the sympathetic nervous system inhibits the peristaltic movements of the intestine, but in a very strong stimulation (e.g. during stress reaction) it acts like the parasympathetic system (so-called nervous diarrhea). Both the sympathetic and parasympathetic nervous systems lead to the activation of blood clotting.

Vegetative reflexes

In connection with the given reflex reflexogenic field and effector organ localization all the vegetative reflexes are divided into: 1) viscero-visceral reflexes (the regulation of the heart activity is performed by excitation of the receptors in the aorta, a. carotid, vena cava, etc.); 2) somato-visceral reflexes (decrease of the heart rate when pressure is applied to the eyeballs (Danini-Aschner’s reflex or oculocardiac reflex); 3) viscero-somatic reflexes (the activation of respiratory muscles (breathing) at blood composition changes and chemoreceptors’ excitation); 4) viscero-sensory reflexes (hypoxia in cardiac muscle causes a pain in the subscapular or pectoral region); 5) senso-visceral reflexes (the stimulation of the skin definite areas leads to appearance of vascular reactions in visceral organs. Certain therapeutic methods (acupuncture and physiotherapeutic procedures) are based on this phenomenon.

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CHAPTER 5. PHYSIOLOGY OF THE ENDOCRINE SYSTEM

Alive organism is an integral system that is ensured by the coordinated activity of various regulatory systems. Evolution of organisms occurs in close interconnection with evolution of the regulatory systems. For example, in the simplest organisms (viruses, unicellular organisms) regulation was carried out by diverse chemical products, inter- metabolites. Later, when the multi-cellular organisms appeared, the neuronal system arose. With appearing of vertebrates some part of the neuronal cells obtain the capacity to secrete biologically active substances, i.e. neurosecretory cells appear. Finally, at the recent stages of the development of vertebrates, the endocrine glands appeared which along with the neuronal system fulfilled the regulation of the organism that leads to the organism integrity. Thus, evolution of the regulatory systems goes on in the following direction: intralcellular mechanisms→neuronal cells→neurosecretory cells→endocrine glands. The endocrine glands, being submitted to the neuronal influences, themselves influence actively on the neuronal system and are a compound part of the nervous-endocrine regulation general system.

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5.1. GENERAL CHARACTERISTICS OF THE ENDOCRINE GLANDS

The properties, functions and the biological importance of the endocrine glands are an issue of endocrinology. The term endocrine derives from Greek words: endo – inside and crineum secrete. So endocrine glands, or the glands of inner secretion, are specialized organs or a group of cells having a main function to secrete specific biologically active substances into the internal medium of the organism. Properties of the endocrine glands: 1. They are of very small size (from some mg to 25-35 g). 2. They have an abundant blood supply. 3. They don’t have ducts. The endocrine glands excrete their secret immediately into the blood (Figure 21).

Figure 21. An exocrine gland (left) and an endocrine gland compared by their respective paths of secretion.

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The endocrine glands of humans and higher animals are follows: pituitary (hypophysis), pineal gland (epiphysis), thyroid gland, parathyroid glands, thymus, adrenal glands, pancreas (islets of Langerhans), sex glands, the temporal gland - placenta.

Figure 22. Endocrine glands of humans.

Some organs (kidney, heart and digestive tract) consist of endocrine cells, which are included into the APUD (amine

130 precursor uptake and decarboxylation) system and produce some hormones. The hypothalamus, “endocrine brain” simultaneously is a neuronal and endocrine structure.

Structure, properties and action mechanism of hormones

The specific product of the endocrine gland secretion is called hormone (from the Greek word horma – excitation). By their biochemical structure all hormones are classified into three principle groups: 1) protein–peptide compounds (pituitary hormones, insulin, thyrocalcitonin, parathyroid hormone, glucagon and the hypothalamic releasing factors); 2) steroids (adrenal cortical hormones, sex hormones); 3) amino-acid derivatives (thyroxin, thriiodothyronine, renin, epinephrine and norepinephrine (adrenaline and noradrenaline)). This classification is very important, since the hormone action mechanism depends on their chemical structures. Hormone properties. The most important properties of hormones are: 1. High specificity. It is a unique property, which is conditioned by the hormone chemical structure, function and site of production. Every hormone evokes a definite and specific biological action. Another hormone cannot replace the deficiency of a certain hormone. Every hormone influences on the corresponding organ, tissue or cell, which are named as targets. Along with this it is known, that several biologically active substances – histamine, serotonin, bradykinin,

131 prostaglandins, etc also have a specific action. But these are not called hormones due to their production in different organs and different tissues. Catecholamines (epinephrine, norepinephrine and dopamine) together with the main site of their production have also other sites (peripheral synapses of the neuronal system, some structures of peripheral organs) and cannot be considered as true hormones, so they are usually called hormonoids. 2. High biological activity. Their quite insignificant amounts can sharply change functions of the organs. 3. Distant action. The hormones can act on the organs and tissues being located at a distance from the gland, by which they are secreted. 4. Secretion (property to be secreted). Ahead of being released hormone has to be synthesized. The peptide hormone synthesis proceeds in ribosomes, after which hormone passes and is accumulated in Golgi’s complex. The steroid hormones’ and catecholamines’ synthesis takes place in the cytoplasm and mitochondria. All synthesized hormones are accumulated in the vesicles, which move to the cytoplasmic membrane with the help of microtubules and extrude their composition into the blood. The secreted hormone passes into the inner medium of the organism (blood and lymph). One part of it circulates in the active form (free state), and another, in a combined state with the proteins specific for the given hormone (transcortin for the hormones of adrenal cortex), or with nonspecific plasma proteins (albumins, globulins), or with formed elements (erythrocytes). The complex part with proteins and the part in

132 free state are in dynamic equilibrium and easily convert into each other, if necessary. The combined state of the hormone has a significant physiological benefit: 1) prevents the organism from the abundant accumulation of free state hormones in the blood and protects tissues from their action; 2) serves as a reserve and, if needed, can be decomposed and converted into a free state; 3) protects the hormones’ from the destructive action of enzymes; 4) prevents the low molecular hormones’ filtration through the kidneys. Two factors are effective to increase or decrease the concentration of a hormone in blood. One of these is a rate of hormone secretion into blood; the second one is a rate of removal of the hormone from the blood, which is called the metabolic clearance rate. This is usually expressed in terms of millilitres of plasma cleared out of the hormone per minute.

The biological half-life of hormone (T1/2) is the time, during which the blood concentration of hormone is reduced by half. That could be used to estimate the intensity of hormone consumption. Mechanism of hormone action. The mechanism of hormone action on the target-cell is conditioned by the chemical origin of the hormone. 1. The membrane type of hormone action (Figure 23). It is suitable for hormones with a high molecular mass (protein- peptide hormones) and catecholamines. The high molecular mass of the hormone hurdles its passage into the cell. In this case the hormone action takes place through the hormone- receptor conjunction. The hormone is being combined with a

133 receptor on the outer membrane, forming so-called hormone- receptor complex, which can act on certain intracellular structures only with the help of the intracellular mediators – messengers (cAMP, cGMP, Ca2+ ions, products of the membrane phospholipid decomposition (diacylglycerol, inositoltriphosphate)). The sequence of events in proteinic hormone action is essentially the following: 1. The hormone binds with the membrane specific receptor. 2. The combined receptor-hormone complex activates synthesis of the secondary messenger. 3. The messenger activates the appropriative proteinkynase with the phosphorylation and activation of intracellular proteins.

Figure 23. Membrane and cytoplasmatic mechanisms of hormones’ action.

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2. The cytoplasmic type of hormone action (Figure 23). It begins with the hormone penetration through the membrane, conjunction with the cytoplasmic protein-receptor and the signal transmitting from the originated complex to the nuclear structures that are responsible for the protein synthesis. That is the way of steroid and thyroid hormones’ action. The sequence of events in steroid hormone action is essentially the following: 1. The hormone enters the cytoplasm of the cell, where it binds with a specific receptor. 2. The combined receptor-hormone complex is transported into the nucleus. 3. The receptor-hormone complex acts on the transcription process of the RNA messenger. 4. The RNA messenger diffuses into the cytoplasm, where it promotes the translation process in the ribosomes. Summarizing the issue of hormone action mechanisms it is necessary to note, that the hormone action can be of two ways: functional and morphogenetic. 1. The functional action can be: a) metabolic, changes of metabolic processes in the target-organ; b) kinetic, some functions of organs or processes are expressed only by action of the given hormone; c) corrective, changes of function intensity in organs or tissues;

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d) reactogenic or permissive, changes of the tissue susceptibility to the given hormone action in the presence of another hormone. 2. Morphogenetic action. The morphogenetic action is the stimulation of differentiation, growth and metamorphosis processes in the organism. Methods of study of the endocrine glands’ activity are: 1. Clinical examination of the patients with endocrine hypo- or hyper-functions. 2. Gland removal. 3. Gland transplantation. 4. Determination of hormone in the organism liquid medium. Estimation of the blood or urine concentration of a specific hormone using biological or physiological techniques. 5. Introduction of glandular extracts or pure hormones. 6. Selective damage of gland. 7. Parabiotic method. 8. Comparative assessment of the biological activity of the glandular inflowing and out- flowing blood content. 9. Method of indicated radioactive isotopes. 10. Method of fluorescent antibodies.

5.2. THE HYPOTHALAMO-HYPOPHYSIAL SYSTEM

A close anatomical and functional connection between the hypothalamus and hypophysis is a base for their uniting in the entire hypothalamo–hypophysial system. For comprehension of the physiological mechanisms of functioning

136 of this system it is necessary to imagine its anatomic – histological characteristics. The hypothalamus, containing 32 pairs of nuclei conditionally is divided into 3 parts: anterior, medial, posterior; and thanks to its special neuro-secretory cells it is conjoined with the pituitary functionally. There are two main conjunctions between the hypothalamus and hypophysis (Figure 24).

Figure 24. Hypothalamo-hypophysial system.

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I. The hypothalamo-neurohypophysial system. The anterior hypothalamus, where two types of neurosecretory cells (nuclei) are, the supraoptic nucleus (SON) and paraventricular nucleus (PVN), is connected with the posterior pituitary (neurohypophysis) through the nerve cells’ axons. Posterior pituitary consists of pituicytes that resemble the glial cells and are named as Herring’s bodies. They serve as a reservoir and only two hypothalamic hormones accumulate: anti-diuretic hormone (ADH) and oxytocin. So neurohypophysis is connected with the hypothalamus by nerve cells’ axons. II. The hypothalamo-adenohypophysial system. Hypothalamic posterior and medial parts’ conjunction with the anterior pituitary (which is known as adenohypohysis) and is realized through the abundant vascular (arterial) ramifications that compose so called marvelous network (primary capillary plexus). In the hypothalamic region the nervous network, consisting of the nerve cells’ axons, forms peculiar neuron- capillary synapses on the capillary plexus. Through these structures, the products of neurosecretion of the hypothalamic cells enter the blood and with its flow are conveyed to the anterior pituitary cells to change their function. The blood out- flowing from the capillaries of the hypothalamic region enters the so-called portal vessels of the pituitary. These vessels compose the pituitary secondary capillary plexus, from which the blood outflows into the vein. Pituitary consists of uncolored principal or chromophobe cells (60%) and the colored chromophil cells (40%). In turn the chromophil cells are

138 divided into the acidophil (30-35%) and the basophile cells (5- 10%). It is considered that the main producers of hormones are the chromophil cells, and the principal cells are only their precursors. III. Recent data evident about the presence of the third hormonal system, the hypothalamo-extrahypophysial system of neuroregulatory peptides (encephalin, endorphins, vasoactive intestinal peptide (VIP), substance P, etc.). These neuropeptides partially are produced in the digestive system and other tissues.

The hypothalamo – extrahypophysial system

It includes neurosecretory cells’ axons, that continue into the medulla oblongata, thalamus, hypophysis, limbic system and by secreting neuropeptides (endorphin, encephalin, dinorphin, substance P, vasoactive intestinal peptide (VIP), somatostatin) realize their activity. A more characteristic action of endorphin, encephalin, dinorphin is a morphine-like action. It is considered they are bound with the same receptors of the neuron membrane that provides the morphine (opium) action. That’s why they are named “endogen opiates”. Correspondingly, the morphine antagonists, particularly naloxone, inhibit the endorphin action. The endorphins and the P- substance have a direct relation to pain. They take part in the realization of pain reflex: P-substance is a mediator of pain and promotes the pain perception, whereas endorphins and encephalin mitigate the pain sensitivity. The stronger is the pain, the more are endorphins. Secretion of endorphins and

139 encephalin increases during stress, labor, surgical operations, etc.

The hypothalamo-neurohypophysial system

There are two peptide hormones in this system: ADH and oxytocin, which are secreted in the SON and PVN, but ADH is secreted mainly by SON, and oxytocin - secreted mainly by PVN. Normal, physiological doses of ADH exert antidiuretic action and regulate water reabsorption. The mechanism of the antidiuretic action consists in intensifying water facultative reabsorption by the walls of the renal distal and collecting tubules. The initiator of the ADH secretion is the excitation of the osmoreceptors, which are represented by vacuolated cells. ADH transfers to the neurohypophisis by axons and thereafter is released into blood. ADH circulating in blood influences on its target (renal distal and collecting tubules) and activates the enzymes that destroy cell membrane with creating pores through which water reabsorption is facilitated. In this way the water balance of the organism is regulated. Thus, impairment of the neurohypophisis function brings to diabetes insipidus development. ADH is also called vasopressin, because it causes contraction of the smooth muscles of the vessels (especially of the arterioles) and raises arterial pressure. But this pressing effect is observed only in artificial administration of the hormone large dozes or its hypersecretion. Oxytocin stimulates contraction of the uterus smooth muscles, especially in near-term pregnancy. The presence of this hormone is indispensable for the normal course of labour.

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It also influences on the extruding of already produced milk. There are some data that oxytocin has a stimulating influence on the smooth muscles of vessels and sex ducts and evokes their contraction. Pursuant to recent data in male oxytocin in parallel with ADH participates in the water–salt balance regulation. Natural stimulus for the oxytocin secretion is the irritation of receptors of the nipple and uterus. The way of the reflex is: mechano-sensory afferent impulses from the nipple and uterus → SON and PVN→ neurosecretion of oxytocin → milk secretion or uterus contraction. So the hypothalamo- neurohypophysial system function is submitted to the reflector regulation.

The hypothalamo-adenohypophysial system

In the mediobasal shallow cell nuclei and the posterior hypothalamus regions produce substances or factors, which stimulate or inhibit the release of the hypophysial tropic hormones. Correspondingly these parts are named hypophysotropic zones, and the hormones released there - releasing factors. Herein the hormone production releasing factors of adenohypophysis are called liberins, and the factors, inhibiting the production - statins. Liberins are the followings: 1.Thyroliberin (TL), stimulates the thyrotropic (or thyrotropin) hormone (TTH) production (according to some data the prolactin production too) by adenohypophysis.

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2.Corticoliberin (CL), stimulates the adrenocortico- tropic hormone (ACTH) secretion. 3.Gonadoliberin (GL), stimulates the luteinizing (LH) and the follicle stimulating (FSH) hormone secretion. 4.Somatoliberin (STL), stimulates the growth (somatotropic) hormone (STH) secretion. 5.Prolactoliberin (PLL), stimulates the prolactin (PL) secretion.. 6.Melanoliberin (ML), stimulates the melanocyte- stimulating hormone (MCSH) secretion. Statins are the followings: 1. Somatostatin (SS), inhibits the STH secretion. 2. Melanostatin (MS), inhibits the MCSH secretion. 3. Prolactostatin (PS), inhibits the PL secretion. All hormones of adenohypophysis are divided into glandotropic hormones, which directly act on the peripheral gland activity, and effector ones, which alter some organs’ functions. The glandotropic hormones are: ACTH, TTH, LH, FSH. The effector hormones are: STH, PL, MCSH. Effector hormones. STH is produced by the acidophil cells of adenohypopyisis. It promotes protein synthesis in organs and tissues, especially in tubular bones, and so causes growth in children and younger. STH activates RNA synthesis, which is an essential link in the protein synthesis and intensifies the amino-acid transportation from the blood to the cells. It is of importance to note that STH performs its protein synthesis stimulating action only in the presence of

142 somatomedins, synthesized in the liver, and insulin, as well as carbohydrates. This action is inhibited after removal of the pancreas in animals or exclusion of carbohydrates from food. Large doses of STH activate insulin secretion in young animals, but in adults the pancreatic islets are degenerated and diabetes mellitus develops. Owing to the intensified protein synthesis the blood content of amino-acids is reduced. Retention of nitrogen in the body takes place, as well as that of phosphorus, calcium and sodium. STH mobilizes fats from depots and activates their utilization by tissues. Secretion of STH continues throughout the organism’s life. It is stimulated by the STH – releasing factor and is inhibited by somatostatin. In early childhood hypoproduction of STH brings to sharp retardation of growth, what is called hypophysial dwarfism (nanism). Overproduction of STH in childhood leads to gigantism. Hyperproduction of STH in adult life, when the growth has already been completed altogether, causes enlargement of the body parts, which are still capable to grow. This state is called acromegalia. It is also characterized by insufficiency of the pancreatic insular apparatus, leading to diabetes mellitus. Hypoproduction of STH in adults leads to panhypopituitarism. Panhypopituitarism is expressed by suppression of thyroid and adrenal glands’ function, which leads to dysfunction of sex glands. MCSH or intermedin immediately acts on the skin and retina pigment metabolism. It activates the melanin pigment synthesis in the melanophores. Besides, it promotes the

143 melanophores’ enlargement and the pigment synthesis and distribution throughout the cytoplasm. Melanin pigment in retina or skin has a protective function against direct sun light. Reduced production of the hormone leads to the skin depigmentation. Hyperproduction of it, observed in pregnancy, brings to hyperpigmentation in separate parts of the skin. PL or luteotropic hormone (product of the acidophil cells) has a property to strengthen milk production by the mammalian glands, as well as to stimulate the corpus luteum of pregnancy development. Parenteral administration of this hormone induces intensification of the milk extrusion not only in feeding females, but also in non feeding individuals. PL decreases the utilization of carbohydrates by tissues, but facilitates the glucose transport to the mammary glands. This hormone regulates the water-salt balance. It also is responsible for the mother’s instinct development. In males prolactin regulates activity of spermatozoa, formation of LH-sensitive receptors in sex glands and secretion of androgens. Glandotropic hormones. ACTH is produced by the basophile cells of the anterior hypophysis. It has a direct relation to the adrenal glands’ incretory function regulation. Mostly it refers to the fascicular zone of the adrenal cortex, but ACTH has insignificant action on the glomerular and reticular zones. Secretion of ACTH by the pituitary is augmented in stress. Hypersecretion of ACTH leads to Cushing’s syndrome (moon face, buffalo torso and hirsutism) development, but hyposecretion of it, to Addison’s disease (bronze disease, which is accompanied by metabolic disorders).

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TTH is produced by the basophile cells. It regulates the iodine-containing hormones’ synthesis and secretion by thyroid gland in the following ways: 1) it activates the iodine pump and accumulation of iodine by thyroid tissue; 2) it increases iodination of tyrosine; 3) it intensifies thyroglobulin breakdown in the thyroid gland, which leads to enhanced secretion of thyroxin and triiodothyronine into the blood. Hypersecretion of TTH evokes Grave’s syndrome. GTHs are produced by the basophile cells. They regulate the sex glands’ activity. The FSH stimulates the follicle maturation and estrogens’ secretion in female ovaries and spermatogenesis in male testes Sertoli’s cells. The LH stimulates the ovulation process, corpus luteum formation and progesterone secretion in the ovaries, as well as androgens’ secretion by the interstitial Leydig’s cells in the testes. The regulation of functional activity of the endocrine glands is realized by so-called positive and negative feedback mechanisms mainly at the level of hypothalamus (liberins and statins), and also of hypophysis. Endocrine glands that are not under the regulating influence of the hypothalamo-hypophysial system are parathyroid glands, C-cells of the thyroid gland, pancreas, and the medullar part of the adrenal glands.

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5.3. SPECIAL PHYSIOLOGY OF ENDOCRINE GLANDS Physiology of the thyroid gland

Iodine-containing hormones. The thyroid gland (Figure 25) consists of follicular and parafollicular tissues. The follicular tissue contains of glandular cells, which are characterized by the capacity to absorb iodine from blood by the iodine pump. Their intercellular concentration of iodine exceeds 300 times than that in the blood plasma. Owing to the accumulation of iodine the iodinated monoiodotyrosine (MIT), diiodotyrosine (DIT) and active hormones triiodotyronine (T3) and tetraiodotyronine (thyroxin, T4) are synthesized in the gland. They form a complex with the protein thyroglobulin. T3 and T4 are released into the blood, when thyroglobulin is hydrolyzed by protease produced by the gland cells. In the blood these hormones bind to the specific proteins, carriers (thyroxin-binding globulin, thyroxin-binding prealbumin). But only the free form of hormone is capable to produce physiological action.

Figure 25. Structure of thyroid gland.

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T4-protein complex serves as a reserve from which new active portions of hormone are liberated. T3 is physiologically more active than T4. But T3 effect is observed only during 6-12 hours, while the duration of T4 effect is 12 days. The active agent of T3 and T4 is the triiodothyroacetic acid which is formed in the tissues from T3 more quickly than from T4. Physiological effects of iodine-containing hormones. The characteristic action of these hormones are: 1) metabolism intensification (facilitation of carbohydrates’, fats’ and proteins’ breakdown with increase of oxygen consumption); 2) calorigenic action (decoupling of oxidative phosphorylation, diminish of ATP synthesis, release of the energy in a form of heat and increase of the body temperature); 3) stimulation of the growth and development processes; 4) activation of the CNS function, particularly of the cerebral cortex (mediated by stimulation of the reticular formation); 5) increase of the tissues’ excitability and receptors’ sensitivity to the catecholamines. The regulation of thyroid gland function (production of iodine-containing hormones) is realized by the hypothalamo- adenohypophysial system (thyroliberin, thyrotrophic hormone) by feedback mechanism. Disorders of thyroid function. Malfunction of the thyroid gland (hypothyroidism), developed in childhood leads to cretinism, which is characterized by retarded growth, disproportion of the body, delay of sexual and mental development. This disorder in adulthood causes myxedema,

147 which is accompanied by diminished metabolism, slow thinking and speech, apathy, sexual dysfunction and the body temperature drop. Due to derangement of protein metabolism, the oncotic pressure increases in the tissue, causing retention of water and the body weight increase. When there is a deficiency of iodine required for the thyroid hormones’ synthesis the gland tissue proliferates and goiter develops (endemic goiter). In hyperfunction of thyroid gland Basedow’s or Graves’s disease (thyrotoxicosis) develops which is characterized by the goiter, exophthalmoses, acceleration of the heart rate, high basal metabolic rate and temperature, enhanced consumption of oxygen and food, hyperexcitability of the nervous system and tremor. Thyrocalcitonin (TCT). TCT hormone is a calcium- regulating hormone. It is produced by C cells of the parafollicular tissue. It decreases the concentration of Ca2+ in the blood, because it activates the function of osteoblasts, which promote formation of the bone tissue. TCT also reduces the reabsorption of Ca2+ in kidney and its absorption in small intestine. So, the targets of the TCT action are the kidneys, bones and intestine. It also reduces the reabsorption of phosphate in the kidneys. The regulation of the C-cells’ function is realized by the feedback mechanism regarding the blood Ca2+ concentration.

Physiology of the parathyroid glands

Parathyroid glands (two pairs in man, Figure 26) produce parathyroid hormone (PTH), which is the main

148 calcium-phosphate-regulating hormone. Its targets are the kidney, bone and intestine (the same as that for TCT). But in contrast to TCT, PTH activates the osteoclasts (causes destruction of bone), stimulates the reabsorption of Ca2+ in the kidney and its absorption in intestine. All these processes lead to increase of Ca2+ level in the blood. PTH reduces the reabsorption of phosphate in kidney. So in this function PTH and TCT are synergists. The regulation of the parathyroid glands’ function is realized by the negative feedback mechanism, which is determined by the blood Ca2+ concentration.

Figure 26. Thyroid and parathyroid glands.

Parathyroid glands are vital glands and their hypofunction is accompanied by so-called hypoparathyroidism. The latter is characterized by convulsions of the skeletal muscles (parathyroprival tetany). Parathyroprival tetany develops because of a low calcium level in the blood.

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Parathyroidectomy leads to death, the reason for which is the spasm of the respiratory muscles. Parathyroid hyperfunction (hyperparathyroidism) is accompanied by muscular weakness and destruction of bones (osteoporosis) and teeth.

Physiology of the adrenal glands

The adrenal glands consist of two parts: the medulla and the cortex (Figure 27). The cortex forms three zones: an external glomerular zone (zona glomerulosa), a middle fascicular zone (zona fasciculata) and an internal reticular zone (zona reticularis). The cortical part of this gland produces more than fifty corticosteroids, but only eight of them are physiologically active.

Figure 27. Adrenal glands.

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The glomerular zone secretes mineralocorticoids: aldosterone, corticosterone and desoxycorticosterone. Hormones of the fascicular zone are the glucocorticoids: cortisone, hydrocortisone and corticosterone. The reticular zone produces sex hormones: androgens, estrogens and progesterone. Mineralocorticoids regulate the mineral metabolism. The most active of the mineralocorticoids is aldosterone, which promotes sodium reabsorption in the renal tubules and increases sodium content in the blood. Simultaneously aldosterone inhibits potassium reabsorption in the renal tubules, which leads to potassium decrease in the blood. Secretion of aldosterone is directly dependent upon the level of sodium and potassium in the blood and is regulated by the feedback mechanism (by concentration of sodium and potassium ions). The control of aldosterone release is also realized by the renin-angiotensin system. In decreased blood supply and pressure in the kidney, it produces renin, which activates plasma angiotensinogen, transforming it into angiotensin I. The latter is converted into angiotensin II, which possesses vasoconstrictive effect. Constriction of the vessels leads to the normalization of blood supply and pressure. On the other hand the renin-angiotensin system stimulates secretion of aldosterone with further reabsorption of sodium and water that causes the blood volume and pressure increase. In this point of view renin, angiotensin and aldosterone are included into one integral system: renin-angiotensin-aldosterone system.

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Overproduction of aldosterone causes hypernatriemia and hypokalemia, hypertension, increase of water content in the organism, promotes edema and inflammatory process development in tissues, etc. Hyposecretion of aldosterone evokes hypotension, disorders of mineral metabolism (hyponatriemia and hyperkalemia) and further death of the organism. Glucocorticoids regulate carbohydrate, protein and fat metabolism. Glucocorticoids owe their name to their ability to raise the blood sugar level, which is conditioned by acceleration of the deamination of aminoacids and conversion of their nitrogen-free residues to carbohydrates (gluconeogenesis). They also decrease the glucose utilization in tissues and receptors’ sensitivity to insulin, thereby the glucose level in blood increases. But in the liver they stimulate glycogen synthesis. Glucocorticoids intensify the mobilization of fat from its depots and its utilization in the energy metabolism. They weaken the inflammatory and allergic reactions by inhibiting lymphocytes and eosinophils production and reducing capillaries’ wall permeability. At the same time they inhibit phagocytosis although the amount of neutrophils in this case increases. Glucocorticois are used in clinical practice for treating some chronic diseases due to their anti- inflammatory action. Concentration of glucocorticoids increases in stress reactions. The function of fascicular zone is regulated by the hypothalamo-hypophysial system, particularly by ACTH of adenohypophysis (negative feedback mechanism). Hypersecretion of glucocorticoids leads to hyperglycemia and

152 inhibition of the organism’s immune reaction. The same symptoms are observed in Cushing’s syndrome. Hyposecretion of hormones evokes Addison’s disease, hypersensitivity to stress factors. The sex hormones of adrenal reticular zone perform a very important role in formation of the secondary sex characters in teenage period of ontogenesis, when the function of sex glands is still insufficient. In old age when the function of sex glands has ceased the reticular zone of adrenal cortex again becomes the source of androgens and estrogens. Hypersecretion of sex hormones in girls causes virilism, but in boys, pre-term sexual maturation. The adrenal medulla is composed of the chromaffin cells embryogenically related to the cells of the sympathetic nervous system. The adrenal medulla produces adrenaline (epinephrine) and dimethylated adrenaline – noradrenaline (norepinephrine). Adrenaline and noradrenaline are referred as cathecolamines, because they are derivatives of cathecol. They are also called sympathomimetics, because their effect is similar to that of the sympathetic nerves. Adrenaline realizes several effects: 1. It activates the glycogenolysis in the liver and muscles, increasing the blood glucose level. Glucose is the main source for the energetic metabolism of the muscles and other tissues. 2. Adrenaline stimulates cardiac muscle contraction, has constrictive effect on the arterioles, but causes dilation of the heart coronary vessels. 3. It reduces the functional activity of the digestive system (secretor and motor) and the kidneys. 4. Adrenaline causes contraction of some

153 muscles (radial muscle with dilation of pupil and skin smooth muscle with increase of pilomotor activity) and relaxation of the bronchial muscles. 5. Adrenaline increases the excitability of receptors, which promotes perception of external stimuli by the organism. It also activates CNS. 6. Adrenaline provides trophic function of the skeletal muscle. The effect of noradrenaline is similar to that of adrenaline, but there are some exceptions, e.g. adrenaline relaxes the uterus in pregnancy, while noradrenaline stimulates the uterus contraction, noradrenaline increases the blood systolic and diastolic pressure, but adrenaline increases only the systolic pressure. Thus cathecolamines can bring about an urgent reorganization of functions in order to mobilise the working capacity of the organism in extraordinary circumstances – stress. The function of medullar part of adrenals is regulated by a nervous way, particularly by the sympathetic nervous system. That is why in extraordinary circumstances, when the sympathetic nervous system is excited, the concentration of adrenaline and noradrenaline increases in the blood, which leads to the functional reorganization for a long time.

Physiology of the sex glands (gonads)

The sex glands (Figure 22) are mixed glands, because they perform excretory (formation of spermatozoa and ova) and incretory (synthesis of sex hormones) functions. Sex hormones are divided into two groups: male sex hormones,

154 androgens and female sex hormones, estrogens. Androgens (testosterone, androsterone and dehydro-3-epiandrosterone) are formed in the interstitial tissue of Leidig’s cells. This tissue in the testes is known as the puberty gland. Estrogens (estrone, estriol and estradiol) are formed in the ovaries in the granular layer of the follicles and graafian vesicles and their internal sheath. Progesterone, the hormone responsible for the normal course of pregnancy is produced in the ovarian corpus luteum, which develops in place of the ruptured graafian vesicle (after it has ruptured and discharged its ova). It is also produced by placenta. Sex hormones are responsible for the sexual activity of the organism. They are necessary for sexual maturation and reproduction function, a normal course of pregnancy. They provide development of the secondary sexual characters, specific features distinguishing the male and female organisms. Besides, they have a significant role in the regulation of metabolism of proteins and fats. They stimulate the ossification of the cartilaginous zones in the long bones. Male and female sex hormones have a different action on hemopoiesis. The androgens stimulate the RBC formation, while estrogens inhibit this process. The function of the sex glands is regulated by the nervous and humoral ways. Stress and strong emotions may impair the female sex cycle (emotional amenorrhea). But the most important regulation is realized by the hypothalamo- hypophysial system via feedback mechanism. The adenohypophysial hormones, follicle-stimulating and

155 luteinizing hormones take participation in this regulation. In females the follicle-stimulating hormone promotes the development and maturation of graafian vesicles and secretion of estrogens, in males it provides the development of seminiferous tubules and accelerates spermatogenesis in Sertoli’s cells. The luteinizing hormone stimulates the formation of sex hormones. In females it determines the ovulation and formation of the corpus luteum, which produces progesterone. In males it stimulates Leidig’s cells activity and hormone production. The pineal gland also has a certain effect on the sex maturation. It prevents the organism from the pre-term sex maturation. Menstrual cycle. Puberty describes the series of events in which a child matures into a young adult. These changes include the development of secondary sex characteristics, the growth spurt (peak height velocity) and achievement of fertility. With the onset of puberty in women, menstrual cycle occurs periodically, which is the result of maturation of the hypothalamic-pituitary-ovarian axis and is the cyclic pattern of activity of the hypothalamus, pituitary, ovary and uterus. The produced hormones include gonadotropin-releasing hormone (GRH) or GL from the hypothalamus, which stimulates FSH and LH from the anterior pituitary. The latter stimulates the estrogen and progesterone production from the ovarian follicle. The menstrual cycle lasts for 27-28 days and may be divided

156 into several phases (Table 1, Figure 28) and duration of each phase varies from woman to woman and cycle to cycle.

Table 1. Phases of menstrual cycle. Name of phase Days Menstruation 1-4 Follicular phase (also known as proliferative or pre- 5-13 ovulatory phase) Ovulation (not a phase, but an event dividing 14 phases) Luteal phase (also known as secretory or post- ovulatory phase) 15-28

During the follicular phase release of FSH from the pituitary results in development of a primary ovarian follicle (graafian vesicle). The ovarian follicle produces estrogen, which causes the uterine lining vaginal mucosa to proliferate. Estrogen promotes also the contraction of the uterus and fallopian tubes. At the midcycle, approximately day 14, there is an LH spike in response to a preceding estrogen surge, which stimulates ovulation. Ovulation is the process in the menstrual cycle, by which a mature ovarian follicle ruptures and discharges an ovum, which moves along the fallopian tube into the uterus. After ovulation the luteal phase begins. The remnants of the follicle left behind in the ovary, develop into

157 the corpus luteum, under stimulation by LH. It is responsible for the secretion of estrogen and significant quantities of progesterone, which causes the endometrium to become more glandular and secretory in preparation for implantation of a fertilized ovum. If fertilization occurs, the developing trophoblast synthesizes human chorionic gonadotropin (hCG), which maintains the corpus luteum so that it can continue production of estrogen and progesterone to support the endometrium until the placenta develops its synthetic function. If fertilization, with its concomitant rise in hCG, does not occur, the corpus luteum degenerates and progesterone level

Figure 28. Menstrual cycle. falls. Without progesterone, the endometrial lining is not maintained and sloughed off resulting in menstrual flow. This

158 phase is known as menstruation. At the same time, FSH level begins to rise slowly, in the absence of negative feedback and the follicular phase starts again.

Physiology of the pancreas

Pancreas is a mixed gland (Figure 29). It performs excretory and internal secretion functions. Due to excretory function it participates in digestive process by means of the digestive enzymes’ secretion. Besides, it comprises special clumps of cells, which are called the islets of Langerhans. The islets participate in secretion of hormones (direct secretion into the blood without any ducts). The islets consist of three types of cells: alpha, beta and gamma. Alpha cells produce glucagon, beta - insulin and gamma - somatostatin. The epithelium of small pancreatic ducts secrets lipocain, vagotonin and centropnein.

Figure 29. Pancres.

The targets of insulin are muscle and fatty tissues, in which insulin drastically increases the glucose permeability of

159 the cell membrane. Insulin promotes the utilization of glucose and formation of glycogen in liver cells. Insulin also increases the amino-acid permeability of cells. It stimulates the synthesis of RNA messenger and facilitates protein synthesis. The insulin large doses cause passage of the glucose large amounts from the blood plasma into the skeletal, heart muscles and the fatty tissues. As a result, the blood glucose level decreases, which leads to insufficiency of glucose in the nervous system cells. But insulin doesn’t act on the nervous cells’ membrane permeability to glucose. The accumulation of glucose in nerve cells is realized by concentration gradient. Therefore the brain and the spinal cord experience an acute lack of glucose, which is the main source of energy for the nerve cells. This state (hypoglycemic coma) is characterized by the acute disturbance of the cerebral activity, loss of consciousness, attacks of convulsions, decreased muscle tone and low body temperature. Insufficient function of beta-cells brings about sharp increase of glucose in blood (hyperglycemia). Diabetes mellitus develops, which is characterized by glucosuria, polyuria, polydipsia and polyphagia. Owing to glucosuria, the expenditure of proteins and fats providing energy metabolism sharply increases. Products of the fats’ incomplete oxidation are accumulated in the organism leading to severe functional disorders, e.g. acidosis. The secretion of insulin is regulated by the blood glucose level (feedback mechanism). Glucagon is an antagonist of insulin and provides the glucose level increase in the blood. It stimulates glycogenolysis

160 due to the activation of phosphorylation. But glucagon simultaneously activates glycogen synthesis from aminoacids (glyconeogenesis), inhibits fatty acid synthesis in the liver and facilitates the function of the myocardium. Its secretion is regulated by the blood glucose level. Somatatostatin inhibits the secretion of insulin and glucagon, lipocain participates in lipids’ metabolism, centropnein activates the respiratory centre and vagotonin increases the tone of vagus centre.

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CHAPTER 6. PHYSIOLOGY OF THE BLOOD SYSTEM

6.1. INTERNAL MEDIUM OF THE BODY

Blood system consists of four parts: blood proper, hemopoietic organs, organs of blood destruction, and regulatory neurohumoral mechanisms. Blood, lymph and intercellular liquid compose the internal medium of the organism. The blood performs the following functions: transport (including respiratory, nutritional, excretory functions, humoral regulation and creator connections), thermoregulation and defence function.

Composition and properties of blood

The total amount of blood in the body of an adult is normally 6 to 8 % of the body weight, i.e. 4.5-6 l. Blood consists of plasma (55-60%) and formed elements (40-45%). Plasma consists of water (90-92%) and dry residue (8-10%). The latter contains organic and inorganic substances. The main organic substances are: proteins (albumins – 4%, globulins – 2.8%, fibrinogen – 0.4%), carbohydrates (glucose – 0.08- 0.12%), lipids (cholesterine – 0.7%). The inorganic micro- and macro-elements are distinguished. The main inorganic substances are: NaCl (0.8%), Ca (9-11 mg%), P (3.00 – 4.00 - 2- mg%). There are also some anions: HCO3 , HPO4 . The microelements are Co, Ni, Cu, Mg, Mn, etc.

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The formed elements are: erythrocytes (4.5-6 mln/ mm3), leucocytes (4000-9000/mm3) and thrombocytes (blood platelets) (200000 - 400000/mm3). The composition of blood is relatively constant. The relative constancy of internal medium of the organism is called homeostasis. The mechanisms supporting the homeostasis are called homeokinesis. The main constants of blood are the followings: osmotic pressure (7.6 atm.), oncotic pressure (25-30 mm Hg, which is a 1/200 part of osmotic pressure), temperature (37oC), viscosity (5 in relation to distilled water), pH (7.35-7.4), hematocrit (percentage of the volume of a blood sample occupied by cells, 40-45%). Osmotic pressure is the pressure by which the molecules of dissolved substance act on the unit surface of the semi- permeable membrane, or the force, which provides the entering of dissolvent through semi permeable membrane from the medium with less concentration to the medium with the high concentration. The osmotic pressure is conditioned mainly by inorganic substances (salts). The maintenance of osmotic pressure is realized by a special mechanism, which includes the following phases: excitation of osmo-receptors in increased osmotic pressure; production of ADH from hypothalamus and its transport to the posterior pituitary; release of the ADH to the blood and its action on the kidney tubules providing the reabsorption of water to the blood with the normalization of osmotic pressure. Those solutions which have the osmotic pressure like blood are called isotonic. The 0.9% solution of

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NaCl is called physiological solution. Other physiological solutions are Ringer’s solution (for cold-blooded animals), Locke-Ringer’s solution and Tyrode’s solution (for warm- blooded animals). The solutions, which osmotic pressure is less than that of blood are called hypotonic once. The solutions, which osmotic pressure is more than that of the blood are called hypertonic once. Erythrocytes and other blood formed elements are destroyed in the hypotonic solution. This phenomenon is called osmotic hemolysis. In hypertonic solution the erythrocytes are squeezed, which is called plasmolysis. There are other types of hemolysis. Chemical hemolysis is caused by the action of chemical agents (alcohol, acids and bases). Physical hemolysis occurs in action of physical factors (temperature, mechanical agents, ultraviolet and other rays). Biological hemolysis is observed in transfusion of the incompatible blood, under the action of poisons of snakes or in presence of microbes and viruses. Physiological hemolysis is the result of the destroy of old erythrocytes (the span of erythrocytes is 120 days). The oncotic pressure is the part of osmotic pressure. It is conditioned by the protein molecules. Taking the viscosity of distilled water as unity, the viscosity of whole blood is about 5.0, the viscosity of plasma is 1.7-2.2. The pH of blood is weak alkaline. A shift of the normal pH in man even by 0.1-0.2 may be fatal. During metabolism blood is continuously supplied with carbon dioxide, lactic acid

164 and other metabolites which change the concentration of hydrogen ions. However, blood pH is supported at a constant level owing to the buffer systems. The buffer systems are: the hemoglobin (HHb/KHb) buffer system, the carbonate buffer system (H2CO3/NaHCO3), the phosphate buffer system (NaH2PO4/Na2HPO4), the plasma protein buffer system. The most active is the hemoglobin buffer system, which accounts for 75% of the buffer capacity of blood. Plasma proteins play the role of a buffer system due to their amphoteric properties (COOH and NH2 groups of aminoacids). Plasma proteins. The significance of plasma proteins is multiform: 1) they are responsible for oncotic pressure, the level of which is important for regulating water exchange between blood and tissue; 2) they maintain the acid-base balance of blood; 3) they ensure a definite viscosity; 4) they prevent sedimentation of erythrocytes; 5) proteins participate in blood coagulation; 6) they provide an immunity; 7) proteins are carriers of a number of hormones, mineral and organic substances; 8) they serve as a reserve for building up tissue proteins; 9) accomplish intercellular or creative interactions, i.e. transmission of information influencing the genetic cell apparatus and ensuring processes of growth, development, differentiation, and maintenance of the body structure (the nerve growth factor, erythropoietins, etc.). Blood plasma proteins are divided into three main groups: albumins, globulins (α1, α2, β and γ) and fibrinogen. These fractions are separated by electrophoresis, a method

165 based on different moving velocity of various proteins with different molecular mass in an electric field.

Composition and properties of lymph

In addition to the system of blood vessels the body contains a system of lymphatic vessels, which begins with a branching network of closed capillaries, which walls are extremely permeable and capable of absorbing colloidal solutions and suspensions. The lymphatic capillaries drain into lymphatic vessels along which a fluid, lymph, flows toward two large lymphatic ducts, the cervical and thoracic, which in turn empty into the subclavian veins. The lymphatic vessels serve to drain off lymph, i.e. to return the blood fluid exuded into the tissue. They are, thus, a type of drainage system that removes excess tissue, or interstitial fluid accumulating in the organs. The lymph flowing from the tissues passes to the veins via biological filters, the lymph nodes, performing defence function. The lymph collected from lymphatic ducts during fasting, or after a meal poor in fats, is a colorless, almost transparent. Lymph obtained from the thoracic duct and from the intestinal lymphatic tissues six to eight hours after a meal rich in fats is cloudy and milky because it contains emulsified fats. Lymph contains less protein, than blood, its viscosity is lower. Lymph has an alkaline reaction and contains fibrinogen, due to which it is capable of coagulation forming a loose, slightly yellowish clot. The lymph in the lymphatic vessels of the endocrine glands contains hormones. The lymph of the thoracic duct

166 contains a large number of lymphocytes, since they are produced in the lymph nodes and are carried to the blood with the lymph flow. Thus the main functions of lymph are: participation in regulation of water-salt, nutrient balance and performance of defence function. The production of lymph is connected with the passage of water and certain substances dissolved in blood plasma from the blood capillaries into the tissues and from there into the lymphatic capillaries. The filtration pressure is the result of difference of hydrostatic pressure (Ph) and oncotic pressure

(Ponc) of blood. In arterial part of capillary Ph= 40 mm Hg and

Ponc =30 mm Hg, i.e. Pf = 40 – 30 = 10 mm Hg. Pf provides the transport of water and diluted substances from the capillary into the tissue and then into the lymphatic capillary. In venous part the hydrostatic pressure is 20 mm Hg, therefore Pf= 20 -30 = -10 mm Hg. This negative pressure provides the transport of water and substances from the lymphatic capillary to the tissue and then into the blood capillary. Filtration process is facilitated by the action of 2 factors: 1) periodic pressure variations in tissues that occur due to pulsation of tissue arteries and regular contractions of skeletal and smooth muscles of the viscera causing periodic compression of the lymphatic vessels; 2) the presence of valves in the lymphatic vessels, owing to their periodic compression lymph is forced in the central direction, i.e. it is sucked off from the tissue. Mechanism of lymph movement. Under normal conditions there is equilibrium in the organism between the

167 rate of lymph formation and the rate of lymph flow from the tissues. Rhythmic contractions of the walls of certain lymphatic vessels play a definite role in the flow of lymph. Contraction of skeletal muscles and sucking property of the chest are also very important for lymph flow. Owing to the presence of valves in the lymphatic vessels the lymph flow takes place in one direction. The sympathetic nervous system evokes spasm of lymphatic vessels, which is accompanied by lymph flow velocity decrease.

6.2. BLOOD FORMED ELEMENTS

Erythrocytes

Erythrocytes or red blood cells (RBC) of humans and mammals are non-nucleated cells, which perform the following functions: 1) gas transport; 2) they are ideal carriers realizing intercellular or creator connections; 3) formation of blood groups and Rh-factor due to localisation of agglutinogenes on the erythrocyte membrane. Their number in blood is 4-5 million/mm3. Their number in blood can vary: erythrocytosis is increase in the number of erythrocytes and erythropenia is their decrease. These variations may be absolute and relative. Absolute erythrocytosis is encountered in hypoxia (low atmospheric pressure or chronic pulmonary and heart diseases). Relative erythrocytosis (increased erythrocyte number per unit of blood without its total increase) occurs in blood condensation (in profuse sweating, burns, cholera and

168 dysentery) and during hard muscular exertion when erythrocytes are discharged from blood depot. Absolute erythropenia develops due to decreased formation, augmented breakdown of erythrocytes or after blood loss. Relative erythropenia (decreased erythrocyte number per unit of blood without its total decrease) occurs in blood liquefaction at the expense of a rapidly increasing volume of liquid in the body. The diameter of erythrocyte is 7.2-7.5 µm, thickness is 2.2 µm. Erythrocyte has a shape of a biconcave disc and in cross section resembles dumb-bells. With this shape no point of the cell is located further than 0.85 µm from its surface, which promotes the easier transport of oxygen and its interaction with hemoglobin. Since erythrocytes are non-nucleated cells, they consume oxygen 200 times less, than their nucleated pre- stages. Erythrocytes supply oxygen to the whole body spending on themselves its negligible part during oxygen transport. Besides, loss of nucleus makes the space opening for hemoglobin possible. The erythrocyte membrane consists of a phospholipid bilayer covered by the monomolecular protein layers inside and outside. The erythrocyte membrane is poorly permeable to Na+ + - - + and K and readily permeable to HCO3 , Cl and to O2, CO2, H and OH-. Human erythrocytes contain more potassium than sodium.

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Formation of erythrocytes is activated by erythropoietins, which are produced mainly by the kidneys in hypoxia and blood loss. Vitamins B12, B6, B2 and C are also necessary for normal hemopoiesis. Hemopoiesis is activated by male sex hormones and inhibited by female hormones. That is why the number of erythrocytes in women is less than that in men. Erythrocyte sedimentation rate (ESR). Blood to which anticoagulant has been added will settle in a certain time, the erythrocytes will be deposited as a sediment. In norm ESR in males is 2-9 mm/h; in females 6-15 mm/h, in newborns 0.5-1.0 mm/h. The estimation of ESR has a prognostic significance. ESR can be increased in different inflammatory processes, cancer-genesis, during pregnancy, etc. The value of ESR depends on the properties of plasma, primarily on the proteins (albumins, globulins and fibrinogen). Globulins and fibrinogen decrease the negative charge on the surface of erythrocytes, providing the interaction of these formed elements and due to it the increase of ESR. In contrast, the albumins decrease ESR. So, ESR depends on albumin/globulin coefficient. Sometimes in anemia ESR also can be accelerated. Hemoglobin. Hemoglobin is located inside the erythrocytes and not in the plasma due to which 1) blood viscosity is reduced; 2) the oncotic pressure of blood is decreased; 3) loss of hemoglobin by filtration in the renal tubules is prevented. Hemoglobin is the chromoprotein consisting of the protein, globin and hem. One hemoglobin molecule contains one molecule of globin and four molecules of hem. The latter

170 contains bivalent iron. Human blood contains 14.5 g/100ml hemoglobin. The saturation of erythrocytes with hemoglobin is called color index. In norm, it is equal to 0.8-1.00. Erythrocytes that have this index are called normochromic; erythrocytes with an index more than 1.00 are called hyperchromic and those with a value below 0.8, hypochromic. This index has diagnostic significance, particularly in estimating anemia. According to color index three types of anemia are differed: hyperchromic (in insufficiency of B12), hypochromic (in deficit of iron) and normochromic (in loss of blood, hemorrhages). Hemoglobin is synthesized in the marrow. In degradation of erythrocytes in the liver the hemoglobin is converted into bile pigment bilirubin, which passes into the intestine and is transformed into stercobilin and urobilin, eliminated from the body by the faces and urine correspondingly. The human hemoglobin has various types: HbP (primitive, hemoglobin of embryo of 7-12 weeks); HbF (fetal, hemoglobin of embryo from the beginning of 9th week); HbA (adult, which appears in child’s blood prior to birth). During the first three years of life, HbF is almost fully replaced by HbA. HbP and HbF have a higher affinity to oxygen, than HbA. Different types of hemoglobin have identical hem; globins differ in their aminoacid composition and properties. Hemoglobin has two types of physiological compounds:

HbO2 (oxyhemoglobin) and HbCO2 (carbohemoglobin). Hemoglobin can also form pathologic compounds: HbCO (carboxyhemoglobin) and MetHb (methemoglobin). The affinity of hemoglobin to CO exceeds that to O2; therefore,

171 even 0.1% of CO in air leads to conversion of 80% of hemoglobin into HbCO which is unable to add oxygen. This causes hypoxia, intoxication and is dangerous for life. MetHb is formed under the influence of strong oxidizing agents (ferricyanide, potassium permanganate, aniline, atomic oxygen). In this compound, the bivalent iron is converted into the trivalent. Myoglobin. Mioglobin is present in muscles and myocardium. Its hem is identical to blood hemoglobin hem, but globin has a lower molecular mass. Myoglobin binds up to 14% of the total oxygen, which is important for the supply of oxygen to the working muscles. When the blood capillaries are compressed in contracted muscles, blood flow either decreases or ceases. However, owing to the presence of oxygen combined with myoglobin oxygen to the muscles is maintained for some time.

Leucocytes

Leucocytes or white blood corpuscles play an important role in body defence against micro-organisms, viruses, foreign substances, i.e. they insure immunity. In norm, their number is 4000-9000/mm3. The increase of number of leucocytes is called leucocytosis. Two types of leucocytosis are differed: reactive (in inflammation, several infectious, non-infectious diseases) and physiological (in emotional stress, physical exertion, etc.) The decrease of number of leucocytes is called leucopenia. It is observed under the action of X-rays and cell poisons.

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According to the presence of granules in the cytoplasm of leucocytes they are classified as granulocytes and agranulocytes. The granulocytes are neutrophils (45-70%), eosinophils (1-5%) and basophils (0-1%). The agranulocytes are monocytes (2-10%) and lymphocytes (20-40%). The percentage relation between different types of leucocytes is called differential count or leucogram. Neutrophils are stained readily with neutral dyes. Depending on the shape of their nucleus, the neurtrophils are classified into myelocytes (0%), juvenile or metamyelocytes (0- 1%), stab (1-5%) and segmented (45-70%). The content of the first three types of neutrophils rises in a number of diseases. Shift to the left of leucogram indicates the increase of myelocytes’, juveniles’ and stabs’ forms proportion to the number of segmented ones. This name implies that the neutrophils in the leucogram are arranged from the left to right by the degree of their maturity. This proportion is called regeneration index, which in norm is equal to 0.05-0.1 and can reach 1-2 in severe infections and inflammatory diseases. Neutrophils are the first to appear at the site of tissue injury and are, so to say, the leucocyte “vanguard”. Neutrophils put out their pseudopodia, pass through the capillary walls and actively move in the tissues to the site of bacteria penetration. The main function of neutrophils is phagocytosis. One neutrophil is capable to ingest 20-30 bacteria. The neutrophils are also called microphages. Eosinophils are stained with the acid dyes (eosin), hence, the name. They have a capacity to phagocytosis, but their role

173 in this process is insignificant because of small blood content. The main function of eosinophils is detoxication and destruction of toxins of protein origin. They produce enzyme histaminase which destroys histamine. The number of eosinophils increases in allergic reactions. Basophils are stained with basic dyes. Like the mast cells, they produce histamine and heparin. Heparin interferes with blood clotting at the site of the inflammation focus, and histamine causes vasodilation to promote resorption and healing of a lesion. The role of basophils is increased in various allergic reactions. Under the effect of the antigen-antibody complex basophils and mast cells release histamine that determines the clinical manifestations of urticaria, bronchial asthma, anaphylactic shock and other allergic reactions. Monocytes are capable of phagocytic activity. They ingest up to 100 bacteria and are called macrophages. At the inflammatory lesion monocytes ingest bacteria and abnormal cells of the inflamed tissue, clear the lesion and prepare it for regeneration. For that function monocytes are called “body scavengers”. Monocytes in contrast to neutrophils keep their activity in acidic medium. Lymphocytes unlike other leucocytes cannot only penetrate through the tissues, but also return into the blood. Their lifespan is 20 and more years. The lymphocytes are divided into three groups: T-lymphocytes (thymus-dependent), B-lymphocytes (bursa-dependent) and 0-lymphocytes. T- lymphocytes are formed in the marrow and differentiated in the thymus. They have a few forms. T-helpers help B lymphocytes

174 to convert them into plasmatic cells. T-suppressors block hyper reaction of B lymphocytes. T-killers interact with foreign (internal and external) cells and destroy them. They kill tumor cells, cells of foreign transplants and cell- mutants to preserve genetic homeostasis. T-killers release immunity mediators or lymphokins which kill foreign cells by activating their lysosomal enzymes. T-amplifiers activate T-killers. T- lymphocytes ensure specific cellular immunity. B-lymphocytes are produced in the marrow and differentiated in the bursa gland (in birds) and lymphoid tissue of the gut, appendix, palatine and pharyngeal tonsils (in mammals). B-lymphocytes play a major role in specific humoral immunity by producing antibodies. After the contact with antigens they recognize the given antigen due to the receptors located on their membrane. Then the B lymphocytes migrate into the lymphatic nodes, where they are transformed into the plasmatic cells, which produce antibodies. B- lymphocytes are very specific. Their each group reacts with one antigen and is responsible for antibody production only against it. 0-lymphocytes are not differentiated and can be transformed into B- as well as T-lymphocytes Immunity is the natural body resistance to microorganisms, viruses and genetically foreign cells. Various mechanisms that are classified as non-specific and specific are involved in its realization. The non-specific mechanisms are provided by the skin, mucous membranes, intestine, kidney, liver and lymph nodes.

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The non-specific mechanisms are also effected by defence substances (interferon, γ-globulins, lysozyme, properdin and the complement system) of blood plasma. The complement system consists of 11 enzymatic components, which are produced by monocytes and macrophages. Cellular mechanisms are also responsible for non-specific body defence. One of them is phagocytosis. In 1973 the World Health Organization (WHO) adopted a decision according to which all phagocyting mononuclear cells were united into the mononuclear phagocyte system (MPS). The specific mechanisms of immunity are ensured by lymphocytes that create specific immunity, i.e. production of defence proteins – antibodies and immunoglobulins, and cellular immunity, i.e. production of immune lymphocytes.

Thrombocytes (platelets)

Thrombocytes (platelets) are colorless biconvex structures 0.5 to 4 µm in diameter. Their content in blood is 200000-400000/mm3. Emotions, physical exertion and food intake change their number. In vessel damage platelets perform an important role in blood coagulation. They contain a number of enzymes, biologically active substances, including adrenaline, noradrenaline, serotonin, as well as coagulation factors. Blood platelets, along with their participation in hemostasis, are responsible for the transport of creative substances which are essential for the maintenance of the vessel wall structure.

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6.3. BLOOD COAGULATION

The liquid state of blood is the principle condition for the organism functioning. It is provided by the system of blood coagulation, which also restores the wholeness of the pathways of its circulation by forming thrombi (clots) in the damaged vessels. The process of coagulation or hemostasis involves three components: vessels, blood plasma and formed elements coagulation factors. Blood plasma coagulation factors are designated by Roman numerals in the chronological order of their discovery. Factor I, fibrinogen; Factor II, prothrombin; Factor III, thromboplastin; Factor IV, calcium ions; Factors V and VI, proaccelerin and accelerin; Factor VII, proconvertin, Factor VIII, antihemophilic globulin A; Factor IX, Christmas factor or antihemophilic globulin B; Factor X, Stuart-Prower factor; Factor XI, plasma thromboplastin antecedent; Factor XII, Hageman factor; Factor XIII, fibrin-stabilizing factor. Platelets’ coagulation factors are designated by Arabic numerals: Factor 3, thrombocytic thromboplastin; Factor 4, intracellular protein component (antiheparin); Factor 5, clotting factor (determines platelet adhesion and aggregation); Factor 6, thrombostenin; Factor 10, vasoconstrictive factor (serotonin); Factor 11, aggregation factor (ADP). Three processes are involved in blood clotting: primary or vascular-platelet hemostasis (pre-phase of hemostasis); coagulation hemostasis; after-phase of hemostasis.

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Vascular-platelet hemostasis

Bleeding from the microcirculatory vessels with low arterial pressure commonly subjected to damage can be arrested by the mechanisms underlying hemostasis which consists of a number of sequential processes: 1. Spasm of the damaged vessels by reflex is insured by vasoconstrictive substances released from the platelets (serotonin, adrenalin and nor-adrenalin). 2. Platelet adhesion to the site of trauma is associated with the fact that a negative electric charge of the vessel at the site of damage is altered to the positive one. Negatively charged platelets adhere to the bare collagen fibres of the basal membrane. 3. The reversal platelet aggregation begins nearly simultaneously with adhesion. ADP released from the damaged vessel and from the platelets and erythrocytes is the main stimulator of this process. 4. The irreversible platelet aggregation is accompanied by the loss of the structural pattern of platelets with the formation of homogenous mass. All the platelet factors and the new amounts of ADP are released to increase the size of a platelet thrombus. The release of the platelet factor 3 (thromboplastin) gives rise to the production of platelet prothrombinase promoting the mechanism of coagulation hemostasis. 5. Retraction of a platelet thrombus implies its thickening and fixation at the side of damage at the expense of

178 thrombostenin contraction. As a result of platelet clot formation, bleeding from microcirculatory vessels is arrested.

Coagulation hemostasis

Hemostasis is ensured by the vascular- platelet reaction only in microcirculatory vessels with low blood pressure. The same reaction initiates it in large vessels but the platelet thrombus cannot endure high pressure and is washed out. Hemostasis in such vessels can be achieved only if a fibrin thrombus (a more solid plug) is formed. Its formation is effected by enzymatic coagulation mechanisms which involve three phases. Phase I. Formation of prothrombinase from extrinsic (of damaged vessel tissue) and intrinsic (of platelet and erythrocyte) thromboplastin (phospolipid). Tissue prothrombinase takes 5-10 sec for its formation and blood prothrombinase - 5-10 min. Formation of prothrombinase is triggered by V-XII factors and calcium ions (factor IV) of blood plasma. Phase II. Formation of thrombin from prothrombin, which is initiated by prothrombinase, factors V, X and calcium ions. Duration of this phase is 2-5 sec. Phase III. Formation of fibrin from fibrinogen. At first fibrinogen is converted into the fibrin monomer under the action of thrombin. Then fibrin monomers are polymerized under the action of calcium ions with the formation of fibrin polymer (soluble fibrin S). And finally S fibrin is converted into fibrin I (insoluble) with the help of factor XIII.

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After-phase of hemostasis

Formation of a fibrin is followed by an after-phase of blood clotting, which consists of two processes: retraction and fibrinolysis. Retraction ensures thickening of a thrombus and its attachment to the walls of the damaged vessel. This process occurs due to contractile protein thrombostenin. Retraction is completed 2-3 hours after clot formation. Fibrinolysis or decomposition of fibrin is directed to restoration of vessel lumen (recanalization) that was stopped with a clot. Fibrin breakdown is effected by the proteolytic enzyme plasmin, which is present in the plasma as proenzyme plasminogen. Blood and tissue activators are required for the conversion of plasminogen into plasmin. The stimulants of fibrinolysis are lysokinase, urokinase, trypsin, streptokinase, kallikrein-kinin system and complement C1.

Anticoagulation mechanisms

Circulating blood contains all the necessary components for coagulation but remains fluid. The fluidity of blood is preserved due to the action of numerous mechanisms: 1) smooth surface of vascular endothelium, which prevents activation of Hageman factor; 2) the vessel walls and formed elements have negative charges owing to which blood cells are pushed away from the vascular walls; 3) vessel walls are covered with a thin layer of soluble fibrin that adsorbs active coagulation factors, especially thrombin; 4) high velocity of

180 blood flow which prevents the accumulation of coagulation factors in one place; 5) natural anticoagulants. The anticoagulants are divided into two groups: pre- existing anticoagulants (primary) and those which are produced during coagulation and fibrinolysis (secondary). The main primary anticoagulants are antithromboplastins, antithrombin III and antithrombin IV, heparin. Secondary anticoagulants are fibrin (adsorbs and neutralizes thrombin, hence fibrin is called antithrombin I), peptides which are cleaved from fibrinogen by thrombin, anticoagulants formed in fibrinolysis, which inhibit the action of thrombin. For blood conservation, synthetic anticoagulants are used. Among them pelentan, dicumarin are well known. Taking into consideration the important role of calcium ions in all phases of hemocoagulation, they are used as anticoagulant those substances which can combine calcium ions, e.g. sodium citrate.

Regulation of blood clotting

It can be realized by nervous and humoral pathways. Acceleration of the clotting time is known as hypercoagulaemia and its slowing down as hypocoagulaemia. Stimulation of pain, sensation of fright and anger, i.e. the states accompanied by excitation of the sympathetic part of the vegetative nervous system accelerate blood coagulation, from 5-10 min to 3-4 min. The development of hypercoagulaemia upon activation of the sympathetic system and stress-reactions are brought about by the action of adrenaline and

181 noradrenaline. Adrenaline promotes the release of thromboplastin from vessel walls, which is rapidly converted into tissue prothrombinase, as well as activates Hageman factor. Prostaglandins released by the kidneys or gastro- intestinal tract are the activators of hemocoagulation. Stimulation of the vagus nerve (or intravenous administration of acetylcholine) causes release from the vessel walls of the same substances that are released due to adrenalin action. Thus, only one defence-adaptive reaction has been formed during evolution in the blood coagulation system. I.e. hypercoagulation directed at urgent stoppage of bleeding. It confirms the fact that primary hypocoagulation does not exist. It is always secondary and develops after primary hypercoagulation at the expense of utilization of some blood coagulation factors. Acceleration of blood coagulation causes secondary stimulation of fibrinolysis. Fibrinolysis is activated in physical effort, emotions and pain. The blood coagulation is subjected to influence of the cerebral cortex and those endocrine glands that secrete vasoactive hormones. Owing to the dilation and narrowing of vessels, their walls release thromboplastin, natural anticoagulants and fibrinolysis activators. Blood coagulation system ensures maintenance of the fluid blood state and restoration of properties possessed by the vessel walls that undergo changes even during their normal functioning; it also keeps the content of blood coagulation factors at the optimal level in case of emergency: damage to the vessels, organs and tissues.

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6.4 BLOOD GROUPS AND RH-FACTOR BLOOD GROUPS

Medical practice is often faced with the need to replace the blood lost in hemorrhages, certain cases of poisoning, chronic infection as well as for other medical indications. In the past the attempts to transfuse blood not infrequently caused severe reactions, even lethal. In 1901 K.Landsteiner and in 1903 J. Jansky found that erythrocytes glue together when the blood of different persons is mixed. This phenomenon known as agglutination depends on the presence of erythrocytes of agglutinable factors or agglutinogens A and B on the membrane. In erythrocytes they can be found to occur either separately or together or be absent at all. It has simultaneously been established that plasma contains agglutinating agents, which agglutinate erythrocytes. They are called agglutinins α and β. Agglutination of erythrocytes occurs when agglutinogens of the donor are combined with the same agglutinins of the recipient (Table 2). Hence, the blood of each individual has dissimilar agglutinogen and agglutinin. The human groups are four (in the ABO system) according to four combinations of agglutinogens and agglutinins. They are designated as follows: I(0)-α,β; II(A)-A,β; III(B)-B,α; IV(AB)-AB. Agglutinogen A has more than 10 variants. The difference between them is that A1 is the strongest, while A2–A7 and other variants have weak agglutination properties. For this reason, blood of such individuals can be erroneously included in group I.

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Agglutinogen B also has a few variants which activity diminishes in the order of their numeration. Blood groups are inherited according to genetic principles and remain unchanged throughout life. Individuals belonging to group I can be transfused by the blood of the same group. Blood group I can be transfused to individuals of any blood group. Persons with blood group I are known as universal donors. But 10-20% of individuals with blood group I contain anti-A and anti-B agglutinins, therefore individuals with blood group I, containing anti-A and anti-B agglutinins are considered as “hazardous” universal donors. Individuals with blood group IV can be transfused by the blood of all groups and are known as universal recipients. Transfusion of this blood to individuals with other blood groups causes severe reactions (Table 2).

Table 2. The agglutination after mixing of serum and erythrocytes of different blood groups. Groups of agglutinins Groups of agglutinogens I (0) II A III B IV AB I (αβ) - + + + II (β) - - + + III (α) - + - + IV (0) - - - -

Blood group IV can be transfused to persons with the same blood group. Blood of persons belonging to groups II and III can be transfused to those with the identical group and to group IV individuals.

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Principles of hemotransfusion. Two main principles of blood transfusion have been formulated. 1. Blood should be crossmatched so as to prevent combination of identical agglutinogens of the donor with identical agglutinins of the recipient. 2. The donor agglutinins are disregarded; this is so- called the rule of dilution which is used in transfusion of small quantities of blood. They lose their activity in dilution. The agglutinins should be diluted 10-15 times. Transfusion of incompatible blood may cause hemotransfusion (clotting) shock which often ends death. The mechanism underlying its development is that the agglutinated erythrocytes on destruction secrete their coagulation factors including thromboplastin. The latter causes intravascular clotting and blockade of the microcirculatory vessels in all organs and tissues by fibrin and platelet thrombi. Study of the blood groups in different countries has shown their percentage distribution: group I – 40-50%; group II – 30-40%; group III – 10-20%; group IV – 5 %.

Rhesus-factor

Among the agglutinogens not included in the ABO system, rhesus-factor or rhesus agglutinogen was first discovered in macaca rhesus monkeys in 1940 by K. Lansteiner and A.Weiner. This agglutinogen is present in 85% of people (rhesus positive) and is absent in 15% (rhesus negative). Rh-factor - agglutinogen has several variants, D, C and E among which D is the most active.

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If Rh+ blood is transfused into the blood of a Rh- individual, immune anti-Rh agglutinins will form. Repeated transfusion of Rh+ blood may lead to the development of post- transfusion complications. If a Rh+ man marries a Rh- woman, there is a fair chance that their baby will be Rh+. The blood of the fetus penetrates (in case of damaged placental barrier) in the maternal body to cause production of anti-Rh agglutinins. They diffuse into the fetus blood causing erythrocyte destruction and intravascular blood clotting. If the concentration of anti-Rh- agglutinins is high, death of the fetus or miscarriage may ensure. In mild forms of Rh-incompatibility the baby will be born alive, but with hemolytic jaundice. The Rh-conflict occurs with a high concentration of anti-Rh agglutinins. The first baby is commonly born normal but during next pregnancies the hazard of the Rh-conflict increases due to the formation of new portions of anti-Rh agglutinins.

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CHAPTER 7. PHYSIOLOGY OF THE CARDIOVASCULAR SYSTEM

7.1. PHYSIOLOGY OF THE HEART

The heart is the central organ of the cardio-vascular system. It performs the following functions: pump, reserve and endocrine functions. It’s a muscular organ composed of three layers; internal endocardium, external epicardium and between these two layers the myocardium. The heart consists of four chambers: two atria and two ventricles (Figure 30).

Figure 30. Structure of the heart.

The left part contains only arterial blood and the right one only venous blood. Pursuing to the main anatomical classification the arteries are the vessels that begin from the heart and the veins are the vessels that enter the heart. There

187 are four one-way valves in the heart. Two of them lie between the ventricles and the great arteries and are called semilunar valves. The aortic semilunar valve prevents blood backward leakage from the aorta into the left ventricle. The pulmonary trunk valve performs a similar function between the pulmonary artery and the right ventricle. Two other valves, called atrioventricular (AV) valves, prevent blood backward leakage from the ventricles into the atria when the ventricles are contracting. The two-leaflet mitral valve is located between the left ventricle and left atrium and the three-leaflet tricuspid valve is located between the right ventricle and the right atrium. AV valves are represented by the endothelial structures that are attached to the papillary muscles by the tendinous cords. The semilunar valves are also derivatives of the endothelial tissue.

The heart conductive system and automatism

The functional element of the heart is the muscular fibre. Depending on the morphological and functional peculiarities there are two types of cells, myocytes and cardiomyocytes. Myocytes are atypical cells and resemble the embryonic muscle tissue, which are poor in myofibrils and mitochondria, rich in sarcoplasm and glycogen and glycogenic enzymes. These cells, contrary to the cardiomyocytes having a contraction function, are more stable towards O2 starvation. Myocytes don’t contract and are specialized on the impulse generation and its conduction, so they compose the conductive

188 system of the heart. Cardiomyocytes don’t have such ability and serve as a contractile system of the heart. Principal physiological properties of the cardiac muscle like other muscles are the following: excitability, contractility and conductivity. But the heart possesses also specific capacity, automatism. The capacity of the heart to respond to stimulus arisen within the heart without any exogenous influence by contraction is called automatism. It means the working ability under the influence of impulses originating directly in the myocyte system of the heart without the interference of external factors. To prove this given characteristics, the heart is removed from the organism and placed in a physiological solution, where the heart continues its contraction. The automatism of the heart is conditioned by the conductive system. The following nodes and bundles are differentiated in the conductive system (Figure 31): 1. The sinoatrial node (described by Keith and Flack and often called after them), which is found in the right atrium, at the opening of the vena cava superior. 2. The atrioventricular (AV) node or Aschoff - Tawara’s node is found at the AV border. 3. The bundle of His, beginning from the latter node, passes through the septum. Here it is divided into right and left roots (branches), which in turn passing through the respective ventricles give final branches. The terminal branches of the conductive system are represented by the network of Purkinje’s fibres. The impulse passes along all the branches of the conductive system, reaching the whole myocardium and causing its contraction.

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Figure 31. Heart conductive system.

Contraction of the cardiomyocytes (working myocardial cells) is realized due to impulses arising in the sinoatrial node. The cardiomyocites are not characterized by automatism. If the apex is separated from the heart, which does not contain elements of the conductive system and placed it in physiological solution, the contraction will be absent, while the other parts of the heart due to containing elements of the conducting system continue their contraction in physiological solution by different rates. This means that all the parts of the conductive system are characterized by automatism, but their ability to produce impulses is suppressed by the impulses originated from the sinus node. In normal physiological conditions the heart works under the influence of the impulses originating from the sinoatrial node, because of this the sinus

190 node is called the primary node or the pacemaker. The frequency of impulses in it is about 70-75 imp/min. For this reason the parts of this system, except the sinus node, have the capacity of latent automatism. These parts reveal their automatism in cases when the function of the sinus node is impaired. In this case the AV node becomes a primary one. The rate of the impulses originated in it is 40-50 imp/min. When its function is also impaired or obstructed the His’s bundle takes upon itself the function of the primary node, which produces 30-40 imp/min. The rate of the impulses produced in Purkinje`s fibres is 20 imp/min. The impulses produced by the last two parts are not enough to satisfy the pump-function of the heart and tissue supply by blood. Therefore Gaskell formulated the “automatism gradient” law according to which the further we move out from the sinoatrial node, the more the automatism of the conductive system decreases. It can be proved by application of Stannius`s ligatures on the frog’s heart (there are two nodes: Remak’s sinus node and the AV Bidder’s node). The first ligature is put on the site between the sinus venosus and the right atrium, in result of which asystolia comes up. Some time later the heart renovates its contractions, but slower twice. This restore is connected with the second node having automatism capacity two times lesser than that of the sinus node. If the second ligature is put on just after the first ligature between the atria and the ventricles, the heart starts contracting afterwards, as the second ligature is a mechanical stimulus for the second node. There can be 3 variants: 1) when the ligature passes through

191 the middle of the AV node both atria and ventricles are contracted; 2) when it passes on the upper border of the AV node, only the ventricles are contracted; 3) when it passes on the lower border of the AV node, only the atria are contracted. The third ligature is put on the apex of the contracting heart, and the apex does not contract due to the absence of the conducting elements. The nature of the heart’s automatism. The cells of the conductive system are characterized by a resting potential, which if compared with other muscular cells has a lower value, it is equal to -60 mV. The level of the critical depolarization is -50 mV. This small difference in the potentials (10 mV) assures high excitability. The resting potential is unstable because of the high permeability to the Na+ and Ca2+ ions. So, for these reasons the myocytes are able to produce action potential (AP) by themselves. We determine the following phases in AP (Figure 32, I): 1) slow diastolic depolarization (SDD); 2) depolarization; 3) repolarization. The first stage is based on the uniqueness of the myocyte membrane, which is conditioned by the fact, that its permeability towards Na+ and Ca2+ is very high being low towards K+ ions at the same time. Another reason is the high 2+ density of the fast activating T-type of, Ca -channels. By the inflow of Ca2+ ions the advance of the SDD is assured.

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1.slow diastolic 2 3 depolarization mv 1 (SDD); 2.depolarization; 3.repolarization.

2 20 3 1. rapid 0 depolarization; 2. initial (fast) -40 4 repolarization; mv 1 3. slow repolarization or plateau; -90 4. final rapid repolarisation.

msec

Figure 32. The AP- phases of myocytes (I) and cardiomyocytes (II).

The main characteristics of the myocyte AP are: 1) the small steepness; 2) the weak manifestation or the absence of overshoot. The maximal value of it is 0-10 mV, so the amplitude of AP is 60- 70 mV; 3) the absence of a constant resting potential. As soon as the potential reaches -60 mV, the SDD stage begins immediately. The rate of the heart contraction greatly depends on the appearing of the SDD, on which the sympathetic and parasympathetic nervous systems have an effect. Under the sympathetic mediator (noradrenaline) influence the Ca2+- permeability increases and K+-permeability decreases, bringing to the facilitation of the SDD steepness, so that, to increase the

193 heart rate. The opposite effect happens under the influence of parasympathetic mediator acetylcholine. Summarizing, the significance of the conductive system assures: 1. Heart automatism causing its contractions. 2. Coordinated contractions of the atria and the ventricles. This is realized because in the conductive system impulses spread at a speed of 1.0 m/sec and reaching the AV node continue at a speed of 0.2 m/sec, which is conditioned by the structural peculiarities of the AV node. This fact allows ventricles to be contracted afterwards the atria contraction. It is called AV retention or delay. 3. The synchronous contraction of myofibrils.

The phase analysis of the heart cycle

The function of the heart is based upon consequent replacing contraction and relaxation (systole and diastole). If the heart gives 75 contractions/min, the duration of 1 cycle is 0,8 sec. The heart cycle consists of: 1) atria systole, lasts 0.1 sec; 2) ventricular systole, lasts 0.33 sec; 3) general diastole, lasts 0.37 sec. The atria systole is the simplest one, during which the blood from the atria is pumped into the ventricles. Pumping of the blood is possible because of the open state of the AV valves. At that time the pressure in the left atrium is equal to 5- 8 mm Hg in the right atrium 2-4 mm Hg. In this phase the atria

194 don’t receive blood because the circular muscles of the veins are contracted and the blood is collected in the ventricles due to the closed semilunar valves. In the ventricular systole they differentiate 2 periods: period of tension and that of ejection. The tension period is divided into 2 phases: 1) asynchronous contraction phase lasts 0.05 sec and 2) isometric contraction phase lasts 0.03 sec. During the asynchronous contraction phase the wave of excitation is propagated along the ventricular myocardium causing contraction of the ventricles. At the end of this phase all myocardial fibres are involved into contraction and pressure in the ventricles rapidly rises, which leads to AV valves closure. The isometric contraction phase begins at the very moment, when the AV valves and the semilunar valves are closed. The ventricles represent as hermetically closed reservoir, which are filled with practically incompressible liquid, blood. This means, that during the contraction process the length of the muscle fibres do not change (hence is the “isometric” term), but the tension grows; in the left ventricle the pressure reaches 70-80 mm Hg and in the right, 15-20 mm Hg. This pressure is enough to open the semilunar valves, which occurs at the end of the isometric contraction phase and the ejection period begins. They differentiate two phases of ejection – rapid ejection, lasting 0.12 sec and the slow ejection lasting 0.13 sec. The pressure in the ventricles exceeds that in the aorta and in the pulmonary trunk, reaching in the left ventricle 130-140 mm Hg and in the right ventricle 25-30 mm Hg. At first the rapid

195 ejection occurs. Then it becomes slow while rushing out the blood. Then as blood ejection occurs and pressure in the aorta becomes more, the aortic and lung trunk valves begin to close and snaps shut, as soon as the ventricular pressure falls below that in the aorta and lung trunk. This phase is termed as protodiastole, which lasts 0.04 sec. Then the ventricular pressure continues to fall rapidly. This phase of the cardiac cycle is called isometric relaxation (0.08 sec.). In this phase there is no inflow to or outflow from the ventricles. As the pressure in the ventricles eventually becomes lower than that in the atria, the AV valves open and blood flows across AV commences. With the onset of the flow from the atria into the ventricles the period of ventricular filling commences. The flow from the atria into the ventricles at first is rapid (phase of rapid filling – 0.08 sec.), then it slows somewhat (phase of slow filling – 0.17 sec). Then a new cardiac cycle begins, in which the ventricles continue diastole, and the atria contract resulting in ventricular final filling and its further contraction.

Methods of investigation of the heart activity

Auscultation. The work of the heart is accompanied by sound manifestations known as heart sounds (tones) which can be heard by means of auscultation or can be recorded by phonograph. There are 4 tones. The first tone arises at the beginning of the ventricles’ isometric contraction, so it is a systolic tone. It can be heard rather well in the fifth intercostal span. It is dull and prolonged (0.12sec) and stands for a result

196 of ventricular contraction, the AV valves closure, vibration of the stretched cusps of the AV valves, their tendinous cords and ventricular walls, as well as of the blood fast ejection. The second tone is heard at the beginning of the ventricular diastole, so it is a diastolic tone. It is high, short (0.08 sec) and simple and is a result of the semilunar valves’ closure. It can be heard in the II intercostal span (in the left the valve of a. pulmonalis (pulmonary trunk) and in the right the valve of aorta). The third tone occurs after the second one as a result of vibrations of the ventricle walls during the maximal (rapid) filling. It can’t be heard by auscultation. It can be detected only by phonocardiography method. Throbbing of the ventricle walls caused by atrium contraction and the additional blood flow into the ventricle leads to the appearance of the fourth tone, which also can be recorded by phonocardiography method. In heart diseases with structural defects of the valves (insufficiency of valves or stenosis) the normal blood flow through the heart is impaired and murmurs are heard along with the tones. The murmur, which is connected with the AV valves, is named systolic; and the one which is because of the semilunar valves, is the diastolic murmur. It is the result of blood contrary turbulent movement. Electrocardiography (ECG) method is based on the heart bioelectrical phenomena arising. It is the registration method of electrical changes on the body surface as a result of cardiac activity. This method was introduced into practice by Einthoven.

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A B Figure 33. The electrocardiograms (B) registrated by 3 standard leads proposed by Einthoven (A).

In excitation certain parts of the heart obtain electronegative charge, i.e. the potential difference between the excited and non-excited areas arises, the electrical field is formed nearby the heart and lines of electrical force spread over the entire body. Consequently, typical tracings reflecting the oscillations of potentials can be registered by applying electrodes to certain points on the body. The record got by means of this method is named electrocardiogram. There are 3 standard leads proposed by Einthoven (Figure 33, A): I - right arm and left arm; II- right arm and left leg; III-left arm and left leg. As the pacemaker is placed in sinoatrial node, excitation arising here firstly spreads over the atria and is reflected on

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ECG as a P-wave (Figure 33, B). It’s a result of the atria excitation and represents the algebraic sum of the electric potentials arising on excitation of the both atria and lasts 0.08 sec. Then excitation is conducted by the conducting system upon the ventricles. Then QRST-ventricle complex comes. The Q, R and S waves characterize the initial period of the excitation (depolarization) of ventricles, and the T-wave, its end. The interval P-Q (0.12-0.18 sec) shows the time required for excitation to spread from the atria to the ventricles. Q-wave corresponds to the ventricle internal surface excitation, as well as the right papillary muscle and the heart apex. R–wave is the biggest and corresponds to the excitation of the external surface and base of both ventricles. By the end of the S-wave both ventricles are involved into excitation and there is no potential difference between their various areas. Therefore, the S-T interval is on the isoelectric line. As many scientists assert, T- wave is a result of the ventricle excitation finishing and it reflects the process of repolarisation. It is the most variable ingredient of an ECG. Its duration and amplitude have diagnostic significance. By veering it conversely we can evaluate the heart muscle trophic disorders (e.g. in the myocardial infarction). On the ECG we can determine arrhythmia, blockades of different degrees (prolongation of P- Q interval or lying on of P and QRST waves), hypertrophy of left (R1 S3) and right ventricles (R3 S1) etc. The minute volume and the systolic (stroke) volume. It is known that the main function of the heart is its pump function. For that reason, the amount of blood expelled from

199 the ventricle is an essential index of the functional state of the heart. We differentiate the blood circulation minute volume (BCMV) and the heart systolic volume (HSV). The minute volume or cardiac output – is the quantity of the blood expelled into the vascular system by left ventricle per minute. The mean cardiac output in man at rest is 4.5-5 l. It is the same in both ventricles. Dividing the BCMV by the number of heart beats per minute, we can determine the HSV (5 l : 75 = 65 –70 ml). The BCMV is measured in clinical conditions. 1. Fick’s method consists of an indirect estimation of BCMV from the following values: 1) the difference between the O2 content in the arterial and venous blood; 2) the volume of O2 (400 ml) utilized by the individual per minute. O 400ml 40000 BCMV = 2 / min ×100% = ×100% = = 5 l in rest O 2a − O2 v 20% −12% 8

2. A number of other methods have been elaborated to determine the BCMY and HSV. Some of them are based on the indicator dilution technique. The latter consists in the evaluation of the dilution and the rate of circulation of an indicator administered into the vein. 3. Integrative rheography technique is the method for registering the resistance of tissues. Resistance of blood is lower than that of tissues. Registration of electric resistance of the chest shows its periodic sharp decrease which occurs at the moment of expulsion by the heart of systolic blood volume. The value of the resistance decrease is proportional to the HSV value.

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The BCMV and HSV values are not constant and they can be altered under various conditions. In trained men the BCMV increases mainly due to the HSV increase; in untrained men, due to the heart rate increase. In muscular loading the BCMV may increase up to 20-30 l. There are many other clinical methods of investigating the heart’s functional activity: echocardiography, dynamocardiography, ballistocardiography, etc.

Cardiac muscle physiological peculiarities

These are those properties of the cardiac muscle by which it differs from the skeletal muscle. As the other muscles, the cardiac one also has excitability, conductivity and contractility. But compared with the skeletal muscle here are some differences: 1. Conductivity of the cardiac muscle is different in its different parts (e.g. in the atria the impulse transferring velocity makes up 1m/sec; near the AV node, it becomes 0.2 m/sec, i.e. there is an AV delay; along the His’s bundle and Purkinje`s fibres it composes 2 - 4 m /sec; and finally, in the ventricular myocardium it is 0.8-0.9 m/sec. The AV delay depends on the structural features of the AV node and contributes to the consecutive contractions of the atria formerly and of the ventricles lastly, so the AV delay provides the coordination of the consequence of the atrial and ventricular contractions. 2. “All–or-none” law. This law was formulated by Bowditch in 1871. It means, that to weak, subthreshold stimuli the heart doesn’t respond, but to the threshold and

201 suprathreshold stimuli the heart responds by a maximum force of contraction. While the skeletal muscle gives response according to the law of “strength’s relation”, i.e. as the stimuli strength increases, the response also increases. It is explained, that this comes from the anatomical structure of the cardiac and the skeletal muscles. In skeletal muscle excitation by weak stimuli, the most excitable fibres respond. As the excitation increases, the amount of the reactive fibres increases. The cardiac muscle has a syncytial structure, as each fibre is connected with other ones thereby nexuses, so the excitation threshold is similar. Further it was revealed, that this law is not absolute, but relative. In conditional changes, depending on the temperature, fatigue, the composition of the nutrient solution, hypoxia, etc. the cardiac muscle does not always respond with an equal force to stimuli of different strength (relativity of “all”). This assertion also is done by Bowditch and is known as ,,Bowditch staircase phenomenon’’. Besides, the heart can respond to rhythmic irritation by subthreshold stimuli, because the local potentials, arising in these conditions can be summed by revealing visual effect (relativity of ”none”). 3. Refractoriness. The heart absolute refractoriness is the absence of the response to the stimulus during its systole. It lasts 0.27 sec. If the heart is stimulated in diastole, an extraordinary premature contraction – extrasystole arises. It may be got only by superthreshold stimuli. This period coincides with the relative refractoriness period (0.03 sec). The whole refractoriness period is 0.27 + 0.03 = 0.3 sec. Refractoriness of the skeletal muscle and the nerve is less: in

202 the skeletal muscle 0.01 sec, in the nerve 0.008 sec. Such long- term refractoriness is connected with cardiomyocytes’ AP features (Figure 32, II). The cardiomyocyte AP has the following phases: 1) rapid depolarization; 2) initial (fast) repolarization; 3) slow repolarization or plateau; 4) final rapid repolarisation. Depolarization phase in the cardiomyocyte is a result of the Na+ and Ca2+ influx. The phase of initial repolarization occurs due to the increased K+ efflux. Plateau is conditioned by the equilibrium of Na+ and Ca2+ influx with K+ efflux. Then Na+ and Ca2+ channels gradually close and final repolarization occurs because of K+ outflow. The heart absolute refractoriness coincides with the 1, 2, 3 phases and nearly half duration of 4th phase. The next half duration of 4th phase corresponds to heart’s relative refractoriness period (0.03sec.) and super-excitatory (exaltation) period (0.03sec.). The amplitude of the cardiomyocytes AP is 120 mV (the resting potential is -90mV, reversal potential or overshoot is +30 mV). The presence of long refractoriness in the cardiac muscle explains the fact, that the heart never gives tetanus, meanwhile the skeletal muscle gives that contraction. Although in some poisonings the refractoriness of the cardiac muscle shortens and it can give tetanus leading to serious impairments of hemodynamics. 4. Extrasystole can be ventricular and supraventricular. In ventricular extrasystole extraordinary stimulus arises in the ventricles. After the extrasystole the compensatory pause comes and the following contraction becomes more expressive on its amplitude, which is explained by Starling’s law (the

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“Heart law”). The compensatory pause is a result of the fact, that coming from the sinus node stimulus during extrasystole finds the ventricles in refractoriness so the ventricles don’t respond to impulse and one cycle is lost. In supraventricular extrasystole the compensatory pause does not occur. 5. The Starling’s law. The more the diastole, the stronger the systole is, i. e. the more the fibre’s initial length, the stronger its contraction is (e. g. contraction after the compensatory pause at ventricular extrasystole). This law is known as the “Heart law”, since it underlies the heart work self-regulation.

Regulation of the heart functional activity

The regulatory mechanisms are divided into intracardiac and extracardiac mechanisms. Intracardiac regulatory mechanisms of cardiac activity. Intracardiac regulatory mechanisms of the heart activity are divided into intracellular, intercellular and mechanisms that are realized by peripheral reflexes. Intracellular mechanisms of regulation. As it is known the myocardium is a functional syncytium, i.e. consists of separate cells, connected between themselves by intercalated discs. In each cell there are mechanisms involved in the regulation of the proteins’ synthesis maintaining cell structure and function. The increased activity of the heart brings to enhanced synthesis of contractile proteins to ensure this activity. The so-called working (physiological) hypertrophy of myocardium appears.

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Myogenic self-regulation of the heart work also belongs to this type of regulation. There are two types of myogenic self-regulation: 1. Heterometric, i.e. regulation connected with the cardiomyocyte length (Starling’s law). 2. Homometric autoregulation is revealed without changes in cardiomyocyte length. It is Anrep’s phenomenon. If the blood pressure increases in the aorta, the heart contraction might be enough to overcome the increased pressure in the aorta. This phenomenon is based on improved nourishment of the heart by coronary vessels during the increased pressure in the aorta, as well as on Ca2+- mechanism. Intercellular mechanism of regulation. Some of the intercalated discs perform only mechanical function. Others ensure transport of necessary substances. They supply the myocardial contractile cells with certain complex high- molecular products that are necessary for maintenance of their structure and function (creative interactions); still others i.e. nexuses, provide the one-way transmission of excitation cell-to- cell. Impairment of this transmission causes discoordinated excitation of cardiac muscle cells and appearance of the heart arrhythmia. Peripheral reflexes. Observations of American physiologists Donald and Cooper concerning to the dogs with auto-transplanted hearts had shown that in muscular work, emotional tension, the auto-transplanted heart in all respects corresponded to the organism’s demands and did not differ from the intact dogs’ hearts. Kositsky electrophysiologically

205 and histologically asserted his own hypothesis about the intracardiac neuronal mechanisms that functioned out in a reflector way. After degeneration of extracardiac nerves in the auto- transplanted hearts the following neurons were established: afferent neurons (Dogiel’s II type neurons), reacting on the pressure changes within the cavities; interneurons (Dogiel’s III type neurons) and efferent ones (Dogiel’s I type neurons), endings of which terminate on the cardiac muscle and coronary vessels. And the reflex “from part to part” was asserted. The pressure increase in the right atrium leads to the strength contraction of the left ventricle. It is asserted, that the heart is rich in the receptors reacting on the pressure changes (pressoreceptors). Kositsky asserted also, that arising effects, when the heart pressoreceptors were being excited, are not equal. This reaction occurs only when the initial blood filling is low and the pressure in the aortic orifice and coronary vessels is not high. If the chambers of the heart are overfilled with blood and the pressure in the aortic orifice and coronary vessels is high, the stretch of mechanoreceptors of the right atrium causes suppression of myocardial contraction with the result that less blood is expelled into the aorta. Hence, it is clear that intracardial neuronal regulation provides adaptation of the heart activity to various hemodynamic situations and regulates not only the function of the heart, but also the hemodynamics. If the blood inflow into the heart increases the cardiac output also increases according to Starling’s law. So, intracardiac neuronal system is the additional and extremely subtle

206 regulator of the cardiac activity on extracardiac impulses’ background. Extracardiac regulatory mechanisms of cardiac activity. Like the other organs, the heart is submitted to the neuronal and humoral regulation. The neuronal regulation of cardiac activity. The heart receives an innervation by vagus and sympathetic nerves (Figure 34).

Figure 34. Innervation of the heart.

The vagus first neuron fibre, originating from the centre that is located in the medulla oblongata, terminates in the intramural cardiac ganglion. The second neuron is located there and its processes extend to the sinoatrial node (right- vagus), to the muscle fibres of the atria and to the atrioventricular node (left-vagus).The first neurons of the sympathetic nerves are located in the lateral horns of five upper thoracic spinal segments. Their processes terminate in the cervical and upper thoracic sympathetic ganglia, where the

207 second neurons lie in. Most of the sympathetic nerve fibres innervating the heart arise from the ganglion stellatum. In 1838 Folkman irritating the medulla oblongata (n. vagus) asserted deceleration and even stoppage of the cardiac activity. In 1846 Weber brothers were the first to demonstrate the vagus influence on the heart. They revealed that stimulation of these nerves inhibited the cardiac activity to the point of complete stoppage during a diastole (Figure 35, I).

A BC

D E Figure 35. Effects of parasympathetic (I) and sympathetic (II) nerves on the heart. A- negative chronotropic effect; B- negative inotropic effect; C- complete stoppage during a diastole; D- positive inotropic effect; E- positive chronotropic effect.

In 1902 Engelmann found out the vagus’s 5 negative effects on the heart: 1. Negative inotropic effect, consisting in diminishing the cardiac contraction amplitude. 2. Negative chronotropic effect, i.e. deceleration of the cardiac contraction rate.

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3. Negative dromotropic effect, weakening of the conductivity during stimulation of the vagus nerve. On the ECG the prolonged PQ interval is encountered. 4. Negative bathmotropic effect, reduction of excitability that causes increase of the stimulation threshold. 5. Negative tonotropic effect, decreasing the cardiac muscle tone. The latter effect is observed only in cold-blooded animals. The vagus effects are explained by the increase of membrane permeability to K+ and decrease to Na+ and Ca 2+. With prolonged stimulation of the vagus nerve, cardiac performance, at first arrested, begins again to inspire persisting stimulation. This phenomenon is known as “escape”, i.e. release of the heart from the influence of the vagus nerve. Some authors assume that this phenomenon is connected with: 1. Sympathetic nerve excitation, that is in the same stem with the vagus nerve. In the vago-sympathetic trunk stimulation the vagus-effect is observed first and the sympathetic effect afterwards. It can be explained by the fact that n. vagus has a short latent period and the vagus effect lasts for a very short period, but the sympathetic nerve has also an after-action. 2. A fatigue develops in the vagal endings. 3. The second nodal automatism appears, since the first one is inhibited by the vagus. The influence of the sympathetic nerve on the heart was first studied in 1867 by Zion brothers. They revealed that in stimulation of the sympathetic fibres innervating the heart

209 causes intensification of the cardiac activity. The nerve was named cardiac accelerator. Later, Engelmann asserted 5 positive effects of the sympathetic nerve on the heart: 1) positive inotropic; 2) positive chronotropic; 3) positive dromotropic; 4) positive bathmotropic; 5) positive tonotropic. The sympathetic nerve effects are explained by the increase of membrane permeability to Na+ and Ca2+ and decrease to K+. The mechanism of nerve impulse transmission in the heart. In 1921 Otto Loewi performed an experiment on the isolated hearts of frogs. He stimulated the vagus of the isolated heart and then transfered nutritious fluid (Ringer’s solution) from the first heart to another, which had not been subjected to nerve stimulation. The reaction of second isolated heart was identical to that produced in response to stimulation of the nerve (vagus effect). The same pattern was observed in the sympathetic nerve stimulation (sympathetic effect). Hence O. Loewi made a conclusion that under the stimulation of the cardiac nerves their endings produce chemically active substances causing inhibition or intensification of the cardiac activity. For that reason he named these substances as vagus- shtoff or sympathetic-shtoff substances. Later it was found out that the vagus mediator is acetylcholine, and the sympathetic mediator is noradrenaline, mediating the corresponding effects

210 on the heart. Loewi’s experiment serves as a base for the fact, that the heart regulation can be realized also by humoral way. Humoral regulation of heart activity. Similarly to n. vagus the K+ ions also evoke negative effects up to stoppage of the heart, as well as the bile acids (in jaundice bradycardia appears). These substances are called parasympathicotrop ones (parasympathomimetic). Sympathicotrop substances (sympathomimetics) are: Ca2+, adrenaline, thyroid hormones, glucagon, etc. Activation or depression of cardiac activity by these substances is realized by facilitation or suppression of SDD (positive or negative chronotropic effects), increase of concentration of cAMP and activation of cAMP-dependent phosphorylase with decomposing of glycogen and producing of glucose. The latter improves the nutrition of cardiac muscle and brings to positive inotropic effect. The positive or negative bathmotropic and dromotropic effects of these substances on the heart connect with changes of resting potential and critical level of depolarization. Interaction of intracardiac and extracardiac neural mechanisms. The intramural nodes and originating from them efferent neurons are the final link for the realization of the vagus effect as well as intracardiac reflexes. Thus they are the final common pathway through which influences of extra- and intracardiac origin are realized. There are cholinergic and adrenergic efferent neurons. It is known that adrenergic neurons are more excitable than cholinergic ones. As a result, the impulses of low intensity (both of extra- and intracardiac origin) lead to the excitation of adrenergic neurons, so to the

211 increase of force and rate of heart contraction. Augmentation of impulses brings about excitation of cholinergic neurons and correspondingly about inhibition of cardiac activity. That is why the same stimulation intensity of the vagus nerve as well as excitation of intracardiac stretch receptors can produce an opposite effect on the heart, depending on the degree of the heart and coronary vessels blood filling. As a result, the constancy of arterial blood filling is regulated not only by reflex reactions of the intracardiac nervous system, but by the vagus nerve as well. Here the doubling of the regulatory mechanisms takes place, which is essential to maintain the stability of arterial filling. Thus, the interaction of the vagus nerve with intracardiac mechanisms can both suppress and stimulate cardiac activity ensuring regulation of the required level of arterial filling. Tone of centres that control cardiac activity. The vagus influence is emphasized more than the sympathetic one. The proof is: after dissection of the n. vagus, the cardiac rate becomes twice as more, but after dissection of the n. sympathicus (or removal of both ganglia stellate) it becomes 15% as less. These phenomena are connected with the tone of the corresponding neuronal centres. The vagus centre tone exceeds the sympathetic one. The vagus tone is weak in newborns (120-150 heart beats per minute), and is increasing by the first year of age. The tone is a condition of continuous limited excitation that is not accompanied by fatigue. The vagus tone is supported by three pathways: 1) mechanical; 2) humoral; 3) reflector.

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The mechanical (pressure) pathway. As the nucleus n. vagus is situated in the medulla oblongata, the pressure of the craniocerebral liquid acts on the nucleus. This can be proved by the orthoclinostatic probe. In lying position the cardiac activity is decelerated because of the reduce of blood outflow from the head and increase of intra-cranial pressure. In standing position the blood outflow increases causing the pressure decrease and the cardiac activity intensification. The humoral way. The vagus centre tone is supported also in the humoral way. It was proved by Heimans on his experiment with cross blood circulation, which consists in joining the blood vessels of two dogs in such a manner that the blood of A dog flows into the head of B dog. After joining the vessels, B’s head is isolated from its body with only the vagus nerve left intact. Introduction of adrenaline into A’s blood causes cardiac activity stimulation but stoppage in B dog. Adrenaline through joint vessels reaches the B’s head resulting in excitation of the nucleus n. vagus and the stoppage of the cardiac activity thereafter. So, adrenaline evokes positive influence on the heart directly, while by central way it acts negatively. Identical to adrenaline the Ca2+ ions act. The reflector way. The nucleus n. vagus constantly gets signals from various reflexogenic zones of the organism, and at first of all the vascular reflexogenic zones. Particularly in this respect the afferent impulses that reach from the receptors lying in the aortic arch and carotid sinus are very important. The cardiac afferent nerves depending on their effects on stimulation are divided into the depressor and the presser ones.

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The depressor proceeds from the baro-receptors, reacting on the arterial pressure changes in the aortic arch and carotid sinus. Impulses are transmitted by the Ludwig-Zion’s nerve from the aorta and by the Herring’s nerve from the carotid sinus, and being reached the nucleus of n. vagus increase the latter’s tone. As a result the cardiac activity weakens and the arterial pressure drops. The analogical reflexogenic zone is located in the lung trunk, the reflex is beginning from which is called Parin’s reflex. But in arterial pressure decrease insufficient impulses come into vagus centre, and its tone is suppressed, which brings to the heart functional activity activation. The presser nerve begins from the pressoreceptors, situated in the vena cava and particularly in the right atrium. They are stimulated by an increase in blood pressure in the vena cava because of the heart insufficiency. Presser nerve originating from these receptors causes reflector decrease of the vagus centre tone and appearance of the sympathetic effect, as a result of which heart beats become more frequent and intensive. The heart pumps more blood into the arteries, due to which the venous return increases and pressure in the vena cava falls to normal. This phenomenon is known as Bainbridge reflex. The heart regulation is also completed by other mechanisms realizing by different parts of the CNS. Such structures as the cerebral cortex, hypothalamus and cerebellum also take part in the cardiac regulation. The cortex participation has been proved by the conditional reflexes’ formation. The

214 hypothalamus takes part as a higher centre of vegetative functions. Reflexes on the heart. Changes of the cardiac activity can be caused by reflector way. Depending on the nerve realizing changes of cardiac activity, the reflexes are divided into the vagal and the sympathetic ones. The vagal reflexes are: 1. Goltz’s classic reflex (Figure 36, A). Essence of this reflex is a light blow on frog’s gut causing cardiac arrest of long duration. Cardiac arrest resulting from a blow on the belly is also encountered in humans. The afferent pathways of this reflex extend from the intestine to the spinal cord along the splanchnic nerve and reach the nucleus of the vagus nerve in the medulla oblongata, where the reflex efferent pathway begins, formed by the branches of the vagus nerve passing to the heart. 2. Ortoclinostatic test: in lying position the cardiac activity weakens, and in standing position it intensifies. 3. Dagnini-Aschner’s oculocardiac reflex (Figure 36, B): slowing of the heart beats to ten per minute when pressure is exerted on the eyeball. 4. Chermak’s reflex: pushing on the medial part of m. sternocleidomastoideus can cause cardiac arrest in result of mechanical irritation of the receptors, from which the impulses pass to the vagus centre. 5. The divers’ reflex; in excitation of cold-sensitive skin receptors the cardiac activity slows. 6. Respiratory arrhythmia. N. vague is an extremely sensitive nerve for the lungs. So in breathing, during

215 inspiration and expiration the heart work changes: in the inspiration onset it becomes faster, and in the expiration slower. This reflex is expressed better in children and teenagers.

A. B. Figure 36. Goltz’s reflex (A ), Dagnini-Ashner’s oculocardiac reflex (B).

Cardiac activity can be altered by stimulation of receptors in the blood vessels of many internal organs (Cherningovsky’s reflex). Sympathetic reflexes are realized by sympathetic nerve. Reflex of acceleration and intensification of the cardiac activity is encountered in response to pain or in emotional states (anger, fright, joy) and with muscular exercise.

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The endocrine function of heart. In 1964 Jewisson and Palade revealed some granules in the right atrium. Then other scientist De’Bold proved that these granules possess natriuretic action and the substance, obtained from the granules was called natriuretic hormone or atriopeptide. It has properties, which are antagonist to aldosterone, i.e. it inhibits the sodium reabsorption in the renal distal tubules increasing the sodium concentration in the final urea and possesses vasodilator activity.

7.2. PHYSIOLOGY OF THE VASCULAR SYSTEM

General principles of the structure and functioning of the vascular system

The cardio-vascular system is represented by the big (systemic) and the small (pulmonary) circulations that are successively joint to each other. The systemic circulation starts from the left ventricle of the heart. The blood passes to the aorta and originating from its arteries also to all their branches, thence to the arterioles, capillaries and the veins of the whole body and finally to the two main veins (venae cavae), which enter the right atrium. The pulmonary circulation begins from the right ventricle, continues along the pulmonary artery and all its branches, then along the pulmonary arterioles, capillaries, and veins, and terminates in the pulmonary four veins which enter the left atrium. The blood circulation time is 23 - 25 sec. The pulmonary circulation is realized during 5 sec and the systemic

217 one during 20 sec. There is a difference between these two circulations. The systemic circulation supplies all organs and tissues with blood and is regulated by a number of mechanisms. The vessels of pulmonary circulation, i.e. the pulmonary vessels fulfil just the functions of gas exchange and thermoregulation. That’s why the regulation of this circulation is less complex if compared with the systemic circulation, where the powerful regulation is realized. The functional classification of the vascular system. In connection with the functional specificity all the vessels are divided into several groups: 1. Springing (amortizing) vessels are the aorta, pulmonary and other big arteries. In their walls a lot of elastic fibres are available, due to which they retain strong hits of the blood and during the systolic pressure increase. 2. Resistive vessels. It concerns in general to the peripheral arteries and arterioles. They have maximal resistance to the blood flow, i.e. have big length and relatively small diameter. Besides, they have a thick layer of smooth muscles in the walls capable to constrict and narrow the vessels’ lumen. 3. Vessels-sphincters. These are the terminal parts of the precapillary arterioles that contain circular layer of muscles, that makes the capillary lumen become smaller in contraction, which leads to the arterial pressure increase. In relaxation of the circular muscles the capillary lumen becomes larger improving a better blood supply in the working tissues. So this type of vessels performs a very important role in the regulation

218 of arterial pressure, as well as of the blood supply in the given organ. 4. Metabolic (exchange) vessels are the capillaries owing to some peculiarities: the capillary walls are thin with one endothelial layer and the blood flow is very slow there. They realize gas and substance exchange. 5. Volume vessels. These are the veins. Their walls contain muscular layer and that’s why can enlarge at a great extent. The big and middle veins have valves, which provide one-way blood flow. Big veins have a significance of blood depots. 6. Shunt-vessels. These are the artery-venous anastomoses. Along these vessels the blood from the arterioles enters directly into the venules, eluding the capillary network. They are located in the lungs, skin and participate in the thermoregulatory processes. The main principles of the blood flow along the vessels (hemodynamics). The main principles of hemodynamics are: 1) blood continuous flow and 2) one-way blood flow. Blood continuous flow. Despite the fact that the blood is pumped out only in systole, it flows by continuous stream both in systole and in diastole. It occurs due to the aorta’s and large arteries’ elasticity. When the blood is expelled from the ventricles into the aorta or pulmonary trunk, their elastic walls are distended, obtaining the blood pumped out from the heart. The heart energy partly is expended on the vascular wall distension, and partly on propelling the blood along the vessels.

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During diastole, when the blood is not expelled from the heart and the pressure in the aorta drops, the aorta’s and artery’s walls collapse and propel the blood along the vessels. One-way blood flow. It is contributed by intra- and extracardiac factors. The intracardiac factors are: 1) successive contractions of the atria and ventricles; 2) the valve apparatus of the heart; The extracardiac factors are: 1) the pressure difference between the initial and the terminal parts of the vascular system; 2) the contraction of the skeletal muscles, surrounding the vessels (veins); 3) the valve apparatus in the veins; 4) the sucking property of the atria in diastole; 5) the sucking property of the chest, promoting the blood flow in the vena cava, especially during inspiration.

The main indices of hemodynamics

These are: the volume (Q), linear (v) velocities, peripheral resistance (R) and blood pressure (P). The volume velocity (Q) is an amount of blood that passes through the total cross-sectional area of any part of the vascular network per unit of time. It’s a quantity of blood that passes through the aorta or all arteries, or all capillaries, or all veins equally per 1 minute. As much the blood flows out from the heart, as it returns to the heart. So this value is constant. It is determined by the formula:

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P − P Q = 1 2 , R where Q is the volume velocity;

P1-P2 is the difference between the pressure at the beginning and at the end of the vessels; R is the resistance to the flow. Since the beginning of the vascular network is the aorta and the end is vena cava, where the blood pressure is about 0, P the formula can be represented as: Q = . R The linear velocity (v) is a velocity of the motion of blood particles along the vessel, i.e. distance passed per unite time. It equals to the volume flow divided by the cross- sectional area Q of the blood vessel: v = . πr2 Hence, the larger the total cross-sectional area of the vessels, the lower linear velocity of blood flow is. The narrowest point in the cardiovascular system is the aorta (5 - 10 cm2) and the linear velocity is 50 cm/sec. The capillaries constitute the most extensive vascular area because the total of all capillary lumen is about 700-800 times more than the size of the aortic lumen. So, the linear velocity of blood flow in capillaries is the least (0.05 cm/sec). The cross-section of the veins is 1.5-2 times less than the size of the aorta, and so the linear velocity is 25 cm/sec.

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According to the laws of hydrodynamics, two forces determine the flow of fluid along the tube: 1) the difference between the pressures at the beginning and at the end of any part of the vascular network, that promotes the blood passage; 2) the peripheral resistance, which is conditioned by the blood viscosity, length and lumen of the vessel. In its turn the viscosity is determined by the friction of blood corpuscles with the vessel walls and blood turbulence. The total peripheral resistance is summed from a number of discrete resistances of vessels. Resistance in each discrete vessel can be calculated according to Poiseuille`s equation: 8η1 R = , πr 4 where η is the viscosity of the fluid; l is the length; r is the radius of tube. This formula is used in hydrodynamics for hard tubes and does not consider the vessels’ elastic properties, the changes in vascular diameters with changes in the blood pressure, etc. Nevertheless, this equation reflects the dependence of the resistance on the width and length of the vessels and on the viscosity of the blood. It shows that maximum resistance to blood flow is encountered in the narrowest blood vessels, capillaries. But, in reality, the total periphery resistance is the most in the arterioles, which depends on the way of their conjunction. The vessels are interconnected successively or

222 parallel. The arterioles are connected to each other by successive way and the total resistance of them will be:

Rtot = R1+R2+R3+ … Rn The capillaries are connected parallel, so their total length is less and the total resistance will be: 1 R = 1/ R 1 +1/ R 2 +1/ R 3 .....1/ R n

Hence, the total resistance is the most in the arterioles. It’s contributed also to the thick muscular layer inside their walls, which contraction brings to the resistance increase. This condition hinders the blood outflow from the arteries and the arterial pressure level increases. Just for this reason the arterioles are the arterial pressure regulators. According to Sechenov they also are called the “taps of the cardio-vascular system”. Opening of these taps improves the organ’s blood supply, but closure causes its disorder. The arterial pressure drop in different vessels is an evidence of the resistance inside them. Thus, the arterial pressure in the big and middle-sized arteries drops by 10%, but in the arterioles and capillaries – by 85%. The blood pressure is the force that blood influences on the vessel wall’s unit surface. The factors ensuring blood pressure are the follows: 1) the heart contraction force and rate; 2) the circulating blood volume; 3) the resistance or viscosity of the blood.

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There are some types of blood pressure: 1) systolic or maximal; 2) diastolic or minimal; 3) pulse or stroke blood pressure is the difference between systolic and diastolic pressures; 4) mean pressure, that ensures the blood flow without systolic and diastolic pressures’oscillations (i.e. the force of continuous blood flow) and produces the same haemodynamic effect as that observed in case of the natural fluctuating blood pressure. The maximal blood pressure is in the aorta and the arteries lying near to the heart 120-130 mm Hg c, the minimal pressure is 70-80 mm Hg c. The blood pressure in the arterioles is 70 mm Hg c; in the capillaries 30-40 mm Hg c.; in the venules 8-12 mm Hg c.; in the veins 2-4 mm Hg c.; in the vena cava about 0. In newborns the systolic pressure is 50 mm Hg c., but by the end of the first month it rises to 80 mm Hg c. Deviations from the normal arterial pressure (AP) value are called hypertension (increase) and hypotension (decrease). Methods of the blood pressure measuring. There are two methods – acute and chronic. Acute method is the direct one, when the AP is measured by inserting into an artery a needle connected to a manometer by tubule and recording apparatus. It is used in animals and was proposed by Ludwig in 1843 (Figure 37). The needle and connecting glass tubule are filled with anticoagulant to prevent the blood coagulation in them. Intra-arterial pressure is not constant, but it displays continuous fluctuations, rising above and dropping below a certain mean level, which produce three

224 types of waves on the blood pressure curve (plethysmogram). Waves of the first type are most frequent and arise from the heart contraction. A certain amount of blood enters the arteries with each systole and increases their elastic distension. During diastole the ejection of blood from the ventricles into the arterial system ceases and only the outflow of blood from the large arteries continues; their walls become less distended and the pressure falls.

Figure 37. Acute method of the blood pressure measuring. I - waves of the first type; II - waves of the second type (respiratory waves); III - waves of the third type.

Apart from these oscillations, the blood pressure curve shows the waves of the second type, which correspond to respiratory movements; because of that they are known as respiratory waves. Inspiration is accompanied by a decrease in blood pressure, and expiration by an increase. It is explained by the fact, that in inspiration due to the increase of the negative pressure in the pleural cleft, the walls of veins (vena cava) are

225 stretched, bringing to the blood pressure decrease in them and promoting the easy passing of the blood from arterial system to the venous one. Waves of the third type are also encountered as slower rises and falls in pressure, each comprises a few of respiratory waves. They are caused by a periodic increase and decrease in the tone of the vasomotor centre, and are most frequently associated with O2 deficiency in the brain, low atmospheric pressure or intoxication by certain poisons, etc. Other methods, namely indirect, are used to measure blood pressure in humans.

Arterial pulse

The rhythmical expansion of the arterial walls (aortal) caused by the systolic rise in pressure is called the arterial pulse. Arterial pulsation can easily be felt on any artery accessible to palpation, as the radial and temporal arteries, the dorsal artery of the foot, etc.

c a b

Figure 38. The sphygmograms of carotid artery (1), radial artery (2), finger artery (3).

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A pulse wave arises in the aorta at the moment of blood ejection from the ventricle, when the pressure in the aorta rises sharply and distends its walls. The wave of increased pressure and expansion of the arterial walls spreads from the aorta to the arterioles and capillaries, dying out in the capillaries. The velocity of blood flow does not influence on the rate at which the pulse wave spreads. The maximum linear velocity of arterial blood flow does not exceed 0.5 m/sec, while the pulse wave spreads with 5.5–8 m/sec velocity in the aorta. The pulse wave velocity increases with age, as the vessels lose their elasticity. Graphically the pulse wave can be represented in sphygmogram as a pulse curve. The curve recorded from the aorta or big vessels branching directly from it, is called the central pulse; that from the peripheral arteries - the peripheral pulse. On the peripheral pulse curve two main parts are distinguished: 1) an anacrotic, the ascending part of the curve (Figure 38, a); 2) a catacrotic, the descending part of the curve (Figure 38, b). The anacrotic rise results from the increase in arterial pressure and caused distension of the arterial walls at the beginning of the ejection phase. The catacrotic descent of the curve occurs at the end of the systole, as pressure in the ventricle begins to fall. At the start of ventricle relaxation the blood discharged into the arterial system rushes back toward

227 the ventricle. Pressure in the arteries falls sharply and a deep notch or incisura, appears on pulse curves recorded from the main arteries. The blood return to the heart is checked by the semilunar valves, which are pushed shut by the backward flow. The return stream of blood rebounds against the valves and gives rise to a secondary wave of increased pressure, which again distends the arterial walls. As a result, a secondary or dicrotic rise (Figure 38, c) appears on the sphygmogram. The central pulse sphygmogram besides these waves has two additional ones, connected with atrium systole and ventricle isometric contraction that lead to additional oscillation of aorta’s or big arteries’ walls. These waves are not registered in the vessels far from the heart. The pulse properties are: rate, velocity, amplitude, tone (hardness) and rhythm. Pulse rate is characterised by the rate of cardiac contractions. The pulse velocity is determined by the speed with which pressure rises in the arteries during the anacrotic ascend and declines during the catacrotic descend. On this base there are normal pulse, pulsus celer (a quick pulse) and pulsus tardus (abnormally slow pulse). Pulsus celer occurs in aortic valve insufficiency. Pulsus tardus occurs with stenosis of the aorta, when the blood is pushed into the aorta more slowly than normal. The pulse amplitude is characterized by the arterial wall expansion during pulse thrust. The pulse hardness is determined by the pressure required to compress the artery for the pulse disappearance. Pulse rhythm depends on the heart rhythm, but sometimes the heart’s normal rhythm can be accompanied by arrhythmic pulse. “Pulse deficit” is

228 sometimes encountered, when occasional wave of ventricular excitation is not attended with the blood discharge into the vascular system and by a pulse thrust. Some systoles are so weak, owing to a small systolic discharge, that they do not give rise to a pulse wave. Pulse rhythm then becomes irregular (arrhythmic pulse).

Regulation of the blood circulation

Activity of each organ is conditioned by adaptation of its blood supply to the given circumstances. It is possible due to change of the vessels’ lumen, which in turn proceeds by means of the central and the peripheral mechanisms. The central mechanisms regulate systemic hemodynamics, but the peripheral ones are responsible for the given organ’s blood supply. The central mechanisms can be realized by the neuronal and the humoral pathways. The neuronal regulation. It is realized by the nerves innervating the vessels. These nerves are divided into two groups: vasoconstrictor and vasodilator. Those causing spasm of the vessels are vasoconstrictor nerves and those causing dilatation – vasodilator nerves. The vasoconstrictor nerves exclusively are of sympathetic nature. Their existence was first revealed in 1842 by Walther in experiments on frogs. Walther observed the frog’s swimming membrane’s vessels’ dilatation in cutting of n. ischiadicus. Irritation of the cutting nerve causes the vessel constriction. In 1852 Claude Bernard once again got such a result in experiments on the ear of a rabbit. Removal of the

229 neck sympathetic ganglion or cutting of the sympathetic nerve on one side of the rabbit leads to its ear’s vasodilatation (turning red and temperature rise) on this side. Stimulation of the sympathetic nerve on the rabbit’s neck causes the corresponding ear to become pale, owing to constriction of its arterioles and arteries. The experiments described show that the blood vessels are under the continuous constrictive influence of sympathetic nerves, which maintain the arterial tone. The parasympathetic part of the nervous system is distinguished by realization of vasodilative effect. For instance, stimulation of chorda tympani causes vasodilatation in the submandibular salivary gland and the tongue. But some sympathetic nerves (cholinergic), innervating the skeletal muscle and the sweat gland vessels possess a vasodilative effect. Dilatation, mainly in the vessels of the skin, can be observed in stimulation of the peripheral endings of the posterior spinal roots. This fact was explained by Bayliss in the seventies of the 19th century. In accordance with Bayliss’s theory, in irritation of the afferent neuronal way the impulses are transmitted to the spinal cord, as well as to the skin through the collateral branches, leading to a vasodilation effect. According to another point of view, dilatation of the skin vessels at stimulation of the posterior roots results from the production of acetylcholine and histamine in the receptor endings. Vasomotor centre. By means of transversal sections at various levels of the brainstem Ovsyannikov revealed existence

230 of the tonically active vasomotor centre in the medulla oblongata. Cutting above the midbrain causes no changes in arterial pressure, but a section made between medulla oblongata and spinal cord leads to the fall of systolic pressure to 60-70 mm Hg c., but it has a temporary character because of the presence of the other vasomotor (vasoconstrictor) centres (e.g. sympathetic spinal centres). The main vasomotor centre of the medulla oblongata lies on the floor of the fourth ventricle and consists of two parts: the presser and the depressor. These parts are in reciprocal relations. The sympathetic vasoconstrictor centres are located in the lateral horns of the thoracic segments of the spinal cord. Besides, there are nervous centres in the hypothalamus, cerebral hemispheres that also regulate the vessel lumen, so the blood pressure. Regulation of vascular tone. The main vasomotor centre is activated by signals coming from the mechanoreceptors of the cardio-vascular system. The afferent system is divided into two receptive (reflexogenic) fields. Signals of one of them arise in the cardio-vascular system, forming proper cardio-vascular reflexes; signals of the other field come from all the other parts of the organism, forming coupled (conjugated) reflexes. The main reflexogenic zones that give rise of proper reflexes are the aortic arch, carotid sinus and a. pulmonaris. In arterial pressure increase the baroreceptors located in these areas are excited and the excitation is transferred through Ludwig-Zion’s (branch of n. vague, from the arch of aorta) and Herring’s (branch of n. glossopharingeus, from the carotid sinus) nerves activating the depressor part of vasomotor centre. The latter promotes

231 arterial pressure decrease by the inhibition in the tone of the presser centre (cardiac performance is inhibited, while vessels of the internal organs are dilated). The increase of blood pressure in the vena cava and right atrium leads to presser effect on the vessels (Beinbridge reflex). There is a feedback mechanism between vasomotor centre and reflexogenic fields. The tone of vasomotor centre is provided by impulses coming from the reflexogenic zones. Another flow of impulses influencing on hemodynamics comes from the chemoreceptors, located the same sites that the baroreceptors are in special corpuscles. They are excited by nicotine, cyanides; they are sensitive to hypoxia and hypercapnia. Impulses from these receptors coming to the vasomotor nervous centre evoke the inhibition of its depressor part, which causes a vasoconstrictive or pressing effect on the vessels and thus the arterial pressure increases. The coupled reflexes of the cardio-vascular system are provided by the impulses coming from the receptive fields out of the cardio-vascular system and are formed by the afferent nerves carrying tactile, temperature, pain, chemical and other sensitivities. Humoral regulation. It implies the regulation by means of different biologically active substances: hormones, ions and metabolites. They are divided into two groups: vasoconstrictors and vasodilators. Vasoconsrictors are: 1. Catecholamines (adrenaline, noradrenaline) act on the vessels of all organs, binding with the alpha-receptors; exclusively, binding with the beta-receptors (they are available

232 in the vessels of the heart, brain, etc.) they exert vasodilative effect. That’s why in physiological concentrations they cause blood distribution and as a result the heart, brain and other organs get more blood than the other regions, where vasocontriction occurs. 2. The more active from the known vasoconstrictors is the renin-angiotensine system. The arterial pressure decrease or the Na+ level decrease in the organism’s liquids causes the renin secretion in the kidneys, which acting on globulin angiotensinogen, converts it into angiotensine I. In its turn angiotensine I is converted into angiotensine II, which causes 1) significant constriction of the peripheral arterioles; 2) moderate constriction of the veins; 3) constriction of the renal arterioles, enforcing the kidney to retain the water and salts, resulting in increase of the blood volume and arterial pressure. 3. Vasopressin, which is secreted in the hypothalamus in high (pharmacological) dozes, causes significant constriction of the peripheral arterioles. 4. Serotonin, which is produced in the intestinal cells, thrombocytes, etc., promotes the vessel spasm and further bleeding stoppage. 5. Prostaglandins, which are produced in different tissues, have two types of action: prostaglandin F is a vasoconstrictor, but prostaglandins A and E are vasodilators. The vasodilators are: 1. Bradykinin that belongs to the kinins’ group and is produced by different tissues.

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2. Histamine evokes the same effect and is produced by basophiles, the digestive system. Its large concentrations lead to a shock with arterial pressure decrease. 3. Medullin is secreted by the renal medullar secretor cells. 4. Acetylcholine, the parasympathetic mediator, action of which is short and local due to rapid decomposition. Some ions also participate in regulation of vessel lumen, e.g. Ca2+ ions evoke vasoconstriction, K+, Mg2+, Na+ ions cause vasodilatation. The local mechanism of the vascular regulation. It occurs in a working organ thereby the accumulation of metabolites, NO, that causes vasodilatation and increased blood supply in the given organ. Besides, the endothelial cells of the vessel produce some biologically active factors, which also have vasoactive properties. The pacemaker elements of the vessel smooth muscle wall perform the most important role in local self-regulation, which is regulated by Ca2+ ions.

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CHAPTER 8. PHYSIOLOGY OF THE RESPIRATORY SYSTEM

Breathing is integrity of the processes ensuring continuous entrance of O2 into the organism, its utilization in oxidative reactions and the CO2 remove from the organism. The respiratory system ensures the gas content of the organism, as well as the indices connected with this system (pH, temperature, excretion, amount of erythrocytes, speaking, etc). Breathing consists of several interconnected processes (phases): 1. External respiration, that results in the lung ventilation (gas exchange). 2. Gas exchange between the alveolar air and the blood. 3. Gas transport by the blood. 4. Gas exchange between the blood and the tissues.

5. Inner breathing, the O2 uptake and utilization in biological oxidation by mitochondria of cells. This part is an issue for biochemical study.

External respiration

Ventilation is the process of gas delivery to the alveoli and vice versa. It is ensured by periodical changes of inspiration and expiration that are based on biomechanical processes. Biomechanics is a part of physiology that studies respiratory muscles’ movement, changes in the chest and

235 pulmonary volumes as well as the mechanisms of their realization. In rest an adult makes 14-18 respiratory movements per minute. In children it makes up 20-30 per minute, in newborns, 40-50. With growth breathing becomes rarer. Breathing rate can be changed because of different reasons: muscular work, emotions, heat and intensification of metabolism, which bring to respiratory intensification. In dreaming it becomes rarer and composes 4/5 from the normal breathing. The role of respiratory muscles. In breathing the lungs have a passive role. They can’t be extended and squeezed actively, because they have no muscles. Air delivering into the lung in inspiration and its removal in expiration take place due to the respiratory muscles’ contraction and relaxation, which leads to changes of the chest volumes. The lungs follow these changes. The junction link between the lungs and the chest is the pleural cleft. The contractions of respiratory muscles, and respectively the respiratory act are going on involuntary, without our consciousness. But they can be regulated voluntarily by impulses from the big hemisphere neurons. We can breathe frequently, slowly, or stop our breathing desirably consciously. Inspiration is an active phase of respiration, in which main and accessory respiratory muscles take part. The main muscles are the diaphragm and external oblique intercostal muscles; the accessory ones are the scalenus, the major and minor pectoralis, the serratus anterior, the trapezius, the

236 rhomboidei and the levator scapulae. The accessory muscles participate in forced respiration. Pleural cleft pressure. As it is known between the parietal and the visceral pleural sheets there is a cleft containing a serous liquid that decreases the friction of the sheets. But the important meaning of intrapleural space is in the negative pressure in it. Introducing a hollow needle in the space and attaching it for the manometer it is possible to measure this negative pressure. It is lower the atmosphere pressure by 3 mm Hg c., so they say it is negative in respect to the atmosphere pressure. Considering Patm=760 mm Hg c., the pleural cleft pressure will make up 757 mm Hg c. This pressure is conditioned by the elastic recoil of lungs, i.e. by their property to reduce their volume. The lung elastic recoil is equal to 3 mm Hg c.

Ppl = Patm – Pelast = 760 – 3=757 mm Hg c., or considering 760=0 mm Hg c.

Ppl = Patm – Pelast = 0 – 3= -3 mm Hg c. This value changes in inspiration and expiration. At the end of calm inspiration it is lower the Patm by 6-8 mm Hg c.; at the end of the maximal inspiration by 12-20 mm Hg c.; at the end of a calm expiration by 3-4 mm Hg c.; in deep expiration by 0-1 mm Hg c.

In newborns Ppl=0, so in pleural space there is no negativity. During the human’s individual development the Ppl value grows up, because the chest grows faster than the lungs. The elastic force of the lungs is conditioned by 3 factors:

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1. The force of the liquid superficial tension (provide 2/3 of the elastic traction). The substance, which covers the inner surface (by 20-100 nm layers) of alveoli, is called surfactant or superficial–active substance. It consists of phospholipids and proteins and is produced by the cells of alveolar epithelium (pneumocytes of type II). It prevents the alveoli from collapse that is named atelectasis because it decreases superficial tension. Besides, surfactant possesses bacteriostatic property. It also has an important significance in realization of the newborn’s first breathing. 2. Elastic fibres inside alveoli. 3. The bronchial muscle tone. The respiration mechanism. The inspiration begins with the diaphragm, as well as external oblique intercostal muscles’ contraction. In result the chest volume enhances in vertical, sagital and frontal directions. It proceeds owing to the oblique orientation of these intercostal muscle fibres, so the moment of force that determines level movement is greater for the lower rib (Figure 39). Because of the chest enlargement, the pleural cleft also increases, causing an increase of negative pressure and therefore the passive enlargement of the lungs. The air pressure falls inside the lungs, which leads to the air entrance from outside. With this process the inspiration is completed and the expiration begins. Expiration is a consequence of processes taking place during inspiration. During inspiration the inspiratory muscles have to overcome the following resistances:

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a) the abdominal organs’ and the abdominal muscles’ resistance; b) the weight of the chest being lifted; c) the elastic pulling of the lungs, as well as the elastic resistance of rib cartilages and other tissues of the chest. After inspiration all these forces cause the chest return to the initial state of rest, extruding air, i.e. expiration arises. So, for the quiet expiration there is no need in the special muscles’ contraction, and that is why the quiet (usual) expiration is considered to be a passive process. But in active forced expiration the muscles of the abdomen press (external and internal oblique, transversus, rectus abdominis), as well as the inner intercostal muscles participate. The contraction of the inner intercostal muscles causes the ribs to descend, their sides come closer to each other since the moment of force is greater for the upper than for the lower rib.

Figure 39. The mechanism of inspiration.

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So, inspiration and expiration are the results of consequent and periodic changes of the chest volumes, connected with filling in and emptying of the lungs. Interrelations between the chest volume alterations can be demonstrated by the model supposed by Dutch scientist Donders in 1880. It is a hermetically closed reservoir with an elastic rubber bottom. The lungs of animal are located inside and by the pipe passing through the cork are communicated with external environment. In drawing off the bottom downward the imitation of inspiration proceeds: volume of the reservoir, which imitates the chest, increases and the lungs passively, enlarge. Return of the rubber bottom to the initial state imitates expiration with the lung volume decrease. Pneumothorax. The entrance of air into the pleural cleft is called pneumothorax. It leads to the lung full collapse. Pneumothorax may be open and close. In open pneumothorax the pleural cavity has a direct communication with atmospheric air. This happens when the thorax is open by wound or during intrathoracic operations. Close pneumothorax may be external and internal. Internal pneumothorax is observed mainly in tuberculosis, in which formation of caverns tears the visceral pleural sheet and air enters the pleural cleft from alveoli. The introduced air into the cleft (close external) will restrict the lung movements temporarily. This type of pneumothorax may be used for curative purpose, in the cavernous tuberculosis treatment (in some forms, when the lung movements are not desirable due to the hurdling for the repairing of the lung). In

240 some time the air gradually is absorbed by pleural cleft sheets and the lung begins to breathe in the previous rhythm.

Lung volumes

The inspired air volume and the rate of respiratory movements determine extent of ventilation of the lungs. There are certain relations between the inspired air volume and the total air volume in the lungs. In purpose of quantitative evaluation of these interrelations the total volume of the lungs is divided into some composites. At first we have to give quantitative characteristics of the total volume of the lungs. It is the maximal air volume that can be contained in the lungs. This volume consists of 2 ones: 1) the lungs’ vital capacity (LVC) is 70-80%; 2) the residual volume is 20-30 %. LVC is an air volume that may be deeply expired after the previous maximal inspiration. It is a functional index of the lungs and the chest, and in its turn it consists of the following volumes (Figure 40): 1. Respiratory or tidal volume, which is the air volume that is inspired or expired in quiet breathing and composes 500 ml. 2. Reserve inspiratory air volume that may be additionally inspired over the respiratory volume, 2500-3000 ml. 3. Reserve expiratory air volume that may be expired after the quiet expiration, 1300-1500 ml. Summarized LVC = 500+3000+1300 = 4800ml.

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Figure 40. Lung volumes. TV- tidal volume; IRV - reserve inspiratory air volume; ERV - reserve expiratory air volume; RV - residual volume; FRC - functional residual capacity; FVC – forced vital capacity (LVC); TLC – total lung capacity.

Practically LVC is determined by the spirometer. LVC depends on sex (in men it is more, than in women by 25%), age (in children and old persons it is less, than in adults), constitution, presence of different illnesses of the respiratory and digestive systems. It’s noteworthy, that even after deepened expiration some air volume (residual volume equal to 1200ml) remains in the lungs. It exits only in pneumothorax. The sum of the reserve expiratory volume and the residual volume is called the functional residual capacity (FRC): 1300+1200=2500ml. The air remained in the air transferring ways (trachea, bronchi, bronchioles, up to alveoli) does not participate in the gas exchange, so this space is called as anatomically dead space. It makes up 1/3 of the respiratory volume (150 ml). Besides, definition of the functional dead

242 space also exists, that comprises all parts of the respiratory system, where the gas exchange does not occur. Actually it is the anatomical dead space plus two types of alveoli (that are not supplied by blood, but ventilated; and that are supplied by blood, but not ventilated).

Lung ventilation

Having described the mechanisms of inspiration and expiration, as well as volumes of the lungs, it is necessary to dwell on the process of ventilation. There are two types of lung ventilation: 1) minute volume of respiration (MVR); 2) alveolar ventilation. MVR is a product of the respiratory volume and the respiratory movements’ frequency. On average MVR makes up 16 x 500 ml = 8000 ml=8 l. MVR gives a certain information of the lung ventilation, but does not inform about the breathing efficiency. Efficiency of breathing is judged by that part of MVR that enters the alveoli and takes part in gas exchange, i.e. by the alveolar ventilation. So the alveolar ventilation is a part of MVR. Hence, MVR = (Valveolar + Vdead space) x f (frequency of breathing). Considering the respiratory volume consisting of the alveolar volume (350 ml) and the dead space volume (150 ml), and the alveolar volume, or functional residual capacity (FRV), which is 2500 ml, we can derive that in each inspiration only 1/7 of alveolar air is renovated, so

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RV − V 500 −150 1 Alveolar ventilation = dead space = = FRV 2500 7 The alveolar ventilation is being accounted as a product of each of these volumes on the breathing rate, so in normal deep respiration

Vdead space = 150x16=2400ml/min,

Valv=8000–2400=5600 ml/min. In superficial and frequent respiration

Vdead space=150x32=4800ml/min,

Valv=8000–4800=3200 ml/min. Since volume of the dead space is constant, the more the alveolar ventilation, the deeper the breathing is. The breathing effectiveness is determined by alveolar ventilation, and effectiveness of the latter is more in deep and rare breathing, than in frequent and superficial one.

Gas exchange in alveoli and tissues

The next process followed by is diffusion of gases into the blood, i.e. the alveolar gas exchange. Previous to this it is necessary to consider the contents of inspired, expired and the alveolar air. Inspired Expired Alveolar

O2 20.94% 16.3% 14.5%

CO2 0.03% 4.0% 5.5%

N2 79.03% 79.7% 80.0% Differences between the gas contents in the expired and the alveolar airs is explained by the fact that expired air is a mixture of the alveolar and the dead space airs. There is no

244 change of N2, which indicates N2 does not take part in the exchange. The alveolar air is an inner medium of the organism and determines the gas content of the arterial blood. Its content is relatively constant and does not depend on the phases of inspiration and expiration in calm breathing. It also is promoted by special regulatory mechanisms. Only in deep breathing the alveolar air content may be changed. It is clear, that for the exchange gases have to pass from the alveolar air into the blood and be dissolved in it or be included into certain compounds to be transferred to tissues and conversely. Gas passage into the capillaries of the small blood circulation and vice versa is realized by means of diffusion, herein in this process only molecules of the dissolved gas take part. On the other hand diffusion is possible only in the presence of partial pressures’ difference in the gases of alveolar air and their tension in the blood. Gas tension in liquid is the force with which molecules of the dissolved gas are driven into the gas medium. The gas partial pressure is a part of the atmosphere pressure formed by the given gas in the whole mixture. It is proportional to the content of the given gas in the mixture and to the total pressure of that; and does not depend on the gas nature. So the total alveolar air pressure is summed by the partial pressures of O2, CO2 and N2.

760mm Hg c.= Po2 + Pco2 + PN2

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Since water vapours are also present in the alveolar air (their partial pressure = 47 mm Hg c.), the total pressure of the alveolar gas will be: 760 – 47 = 713 mm Hg c. Having percentage of each gas in the alveolar air and considering the total pressure, it is possible to calculate the value of the partial pressure for the given gas by the following formula: 760 − 47 P = ×14.5% =102mmHgc. O2 100

PC02 =40 mm Hg c.

PN2= 571 mm Hg c. Determination of the dissolved (in the blood) gas amount may be done by the next formula: α × P g = ×100 , 760 where g – amount of the dissolved gas; α – coefficient of solubility for the given gas; P – its partial pressure in mixture; 100 – the blood volume in cm3 (ml). Coefficient of gas solubility is the amount of the given gas, which can be dissolved in 1 ml of solvent in normal conditions (00 C and 760 mm Hg c.).

Coefficients of solubility for O2 and CO2 are:

αO2 = 0.022 αCO2 = 0.51, α N2=0.011 So, the amount of the dissolved gas in the blood will be:

gO2 =0.3 %, gCO2 = 3%. g value depends on the: 1) liquid’s composition; 2) volume and pressure of the gas above the liquid; 3) liquid’s

246 temperature; 4) gas nature. The more is the gas partial pressure and the lower is the solvent’s temperature, the more gas is dissolved in the liquid. Dissolving process goes on until the dynamic equilibrium between the gas partial pressure above the liquid and its tension of it in the liquid is reached.

Figure 41. Diffusion of gases through the alveolus-capillary membrane (aerohematic barrier).

At the same time the gas passage through the alveolar membrane and its following dissolving in the blood depend on the: 1) diffusion membrane (alveolus-capillary membrane, containing alveolus epithelium, surfactant layer and capillary endothelium) thickness (1.2µm) (Figure 41); 2) diffusion

247 surface (90 m2); 3) partial pressure gradient; 4) gas nature (diffusion capacity). Lung diffusion capacity (DC) is the gas amount, passing through the membrane of alveoli in 1 min by 1 mm Hg of the pressure gradient. In norm it forms for O2 25 ml/min mm Hg. For CO2 it is 24 times higher than for O2 in connection with exclusively high solubility of CO2 in the pulmonary membranes. The importance of the latter for diffusion is followed from the comparison of values of the partial pressure and the tension for O2 and CO2 in the lungs, blood and tissues (Table 3). Table 3. Partial pressures or tentions (mm Hg c.) of gases. Gases Venous Alveolar Arterial Extracellular Cells blood air blood liquid

O2 40 102 100 20-40 0-1

CO2 46 40 40 46 60

Owing to these differences CO2 exits (46Æ 40) from the venous blood into the alveoli and the O2 entrance (102Æ40) from alveoli into the venous blood takes place with its following transformation into the arterial blood. In the same way the O2 and CO2 exchange between the blood and tissues takes place with transformation of the arterial blood into the venous one (O2 -100Æ20Æ0; CO2 - 60Æ46Æ40). On the other hand the time of the blood flow in capillaries of the small blood circulation is important for the O2 and CO2 diffusion process. It makes up 0.7 sec; and even in intensive physical work it is

248 enough for the O2 and CO2 diffusion. This diffusion theory was proposed by Krogh in 1909.

Gas transport by the blood

Oxygen transport. O2 is transported in the bound form of oxyhemoglobin (19.7%) and free form (0.3%). O2 passing into the blood (erythrocytes) binds to Hb and forms HbO2 with very high speed (time of a half saturation is 3 ms). So, transformation of Hb into HbO2 is conditioned by the O2 tension dissolved in the blood. There is a dependence of these processes that graphically is expressed by the HbO2 dissociation (S-shaped) curve (Figure 42).

%HbO2

pO2 mmHg c.

Figure 42. The HbO2 dissociation (S-shaped) curve.

Experimentally it has been established that in PO2= 0-10 mm Hg c. the HbO2 formation is going on very slowly because the Hb affinity to O2 is not significant. With the HbO2 following formation the affinity of Hb to O2 grows up abruptly

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(in PO2=20mm Hg c., 40% HbO2 and PO2=60mm Hg c., 90%

HbO2). From the beginning PO2 = 80mm Hg c. the curve becomes parallel to the abscissa axis, that testifies full saturation of Hb by oxygen.

Some factors, such as temperature, pH, CO2 concentration in the blood, content of diphosphoglycerate in erythrocytes influence on the duration of this process. Increase of temperature, of DPG and CO2 content, and decrease of pH shift the curve to the right that corresponds to the affinity of Hb to O2 decrease. These processes are common in tissues, where intensive metabolism occurs, in result of which CO2 concentration increases. The more CO2 concentration in the blood, the more intensive is dissociation of HbO2 and the lesser the HbO2 concentration becomes. This effect is called Bohr’s effect. The blood oxygen capacity is conditioned by Hb content in the given blood. This index is determined by the maximal quantity of O2 in 100 ml blood in condition of the haemoglobin complete saturation by O2. It is estimated by the following calculation: 1g Hb can bind to 1.34 ml O2. In case of Hb concentration 14.7% (100ml of blood contains 14.7g of Hb) the amount of O2 will be: 1.34 ×14.7=19.7 ml or 19.7v%. So, the

O2 total content in the arterial blood composes approximately

20 v%, in which 0.3 v % is simply solved O2 and 19.7 v% is O2 bound with Hb. The venous blood contains 12 v% O2. Difference in the gas contents in arterial and venous bloods is called artery-venous difference and shows the O2 utilization degree (UDO2) by tissues. Consequently, this value will be:

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(20 −12)×100 UDO = = 40%. 2 20 In rest this index varies in terms of 30-40%. In physical exertion it increases up to 50-60%.

Carbon dioxide transport. CO2 is transported in dissolved and bound forms. Partial tension of CO2 in tissues makes up 60 mm and in the arterial blood 40 mm Hg c. This difference leads to the CO2 diffusion into the blood. In erythrocytes PCO2=O, so CO2 passing into them binds to H2O and forms H2CO3. This reaction is catalysed by the enzyme carboanhydrase. CO2+H2O→H2CO3. Along with it KHbO2→

KHb+O2. O2 enters tissue, H2CO3 reacts with KHb in erythrocytes: H2CO3 + KHb = KHCO3+HHb. These reactions occur due to different acidic properties of the following compounds: HHb

HbCO2, and NaHCO3.

Formation of H2CO3 and its dissociation depend on the

O2 concentration in the blood. This dependence is named

Werigo`s or Holden’s effect. The more O2 concentration is, the more is H2CO3 dissociation.

The venous blood contains 55-58 v% CO2, from which:

4% HHbCO2; 3% dissolved form of CO2 and 51% carbonates

(KHCO3 and NaHCO3).

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Regulation of respiration

The physiological role of the respiration is the O2 supply provision for the organism and the CO2 removal from it, connected with its demands at the moment. So, respiration has to be controlled and regulated, that is realized by means of complex neuro-humoral pathways. Neuronal regulation is provided by respiratory centres, located in various parts of the CNS that are responsible for the coordinated rhythmical activity of the respiratory muscles, as well as for adaptation of respiration to the changes occurring in the internal and external media of the organism. Certain groups of the nerve cells are indispensable to the rhythmical activity of the respiratory muscles, and form a respiratory centre (RC). The RC is situated in the medulla oblongata in the 4-th ventricle, near the inferior angle of the rhomb-shaped pit. Localization of the RC first was found out and explained by Flourens by means of transversal sections of different parts of the CNS. The 1-st section was done between the cervical and thoracic segments of the spinal cord, which brought to stoppage of the intercostal muscles’ activity. The 2-nd section was done between the cervical spinal segment and the medulla oblongata. In that case the diaphragm activity also was inhibited. It was assumed that the RC must be located above section 2. The 3-rd section (in another animal) was done between the medulla oblongata and the pons varolii. But it didn’t stop breathing generally, but caused rhythmical impairments of the process. Consequently, the RC is located in the medulla oblongata. In destroying this

252 centre, breathing stops. The RC consists of the ventral and dorsal parts. In the dorsal part inspiratory neurons prevail, and in the ventral one, both inspiratory and expiratory neurons are available. Impulses from the dorsal part are mostly sent to the cervical segments (C3-C4), and thereby to the diaphragm. Impulses from the inspiratory neurons of ventral part are sent to the thoracic segments (Th2-Th6), where the motoneurons of external intercostal muscles are. Impulses from the expiratory neurons of the ventral part are sent to the Th8-Th10 segments that realize deep expiration. There are also groups of cells located in various other parts of the CNS that can take part in the respiration regulation: the reticular formation of the medulla oblongata, the pons varolii that serves as the pneumocontroling centre – “pneumotaxic centre”, the hypothalamus and cortex. So, the respiratory centre is an integrity of the interconnected neurons that ensures the coordinated rhythmic activity of the respiratory muscles and the breathing adaptation to the changes of the internal and external media. Respiratory centre function is supported by: I. automatism; II. neuro-humoral way; III. reflector way. I. The neurons of the RC are characterized by automatism, which evident is the fact that rhythmical oscillations of bioelectric potentials occurring in these neurons can be recorded. This automatism is much weaker than that of

253 the heart. Apart from this, it differs from the heart automatism by the follows: 1) not only excitatory, but also inhibitory elements are localized here; 2) it also submits to the cortex activity, so it is a limited voluntary process; 3) it is more sensitive to chemical substances; 4) for normal activity it needs getting of nerve impulses (information) coming from the peripheral reflexogenic zones. II. The fact, that the RC is controlled by the neuro- humoral way, was evident by Frederic’s experiment. Two dogs were anaesthetized and their carotid arteries and jugular veins were cut and joint, so that the head of each dog was supplied with the blood not from its own trunk, but from the other. Compression of the trachea in one of the dogs caused asphyxia, followed by respiration arrest (apnea) some time afterward, and was accompanied with severe hyperpnoea in the other dog. It was explained by the fact that closure of the trachea in the first dog led to accumulation of CO2 (hypercapnia) in the blood and to fall in its O2 content (hypoxemia). The blood passed to the head of the second dog and stimulated its respiratory centre. As a result, augmentation of respiration (hyperventilation) occurred in the second dog, leading to decrease in CO2 tension and increase in O2 tension in the blood. Blood rich in O2 and poor in CO2 flew from the trunk of this dog into the head of the first one, causing apnea. So it was shown, that the activity of the respiratory centre alters with changes in CO2 and O2 tensions in the blood. At

254 first it was assumed to be just in humoral way, but presently it is proved to be neuronal too. There are appropriate receptors for these substances – chemoreceptors. There are two types of them: central and peripheral. The central ones are localized in the medulla oblongata in the deepness of ≈ 0.2 mm. These are + sensitive to CO2 and H in the blood. The peripheral ones are localized in the aortal arch, at the site of ramification of a. carotid and are sensitive both to CO2 content increase and O2 content decrease in the blood. It has been revealed that the peripheral chemoreceptors are more prompt in their action, than the central ones. For instance, in CO2 content increase the excitation, coming from the peripheral chemoreceptors, appears already in 3-5 sec, but for the central receptors, it makes up 20-30 sec. It is explained that impulses from the peripheral chemoreceptors directly stimulate through the nerves the respiratory neurons, but for the central chemoreceptors some time for penetration of substances (CO2, H+) into the brain tissue is required. The mechanism of the first inspiration in newborns is connected with these chemoreceptors: fetus in its intra-uterus life is supplied by O2 from the mother’s blood and its own lungs don’t act. After the birth, when newborn is off the mother’s organism, the CO2 content increase in the blood stimulates the RC that causes the newborn’s first inspiration. For this process the influences of light, temperature change and other stimuli (mechanical stimulation from the skin receptors) are also important.

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III. The reflector regulation is commonly conditioned by impulses arising in different reflexogenic zones, where mechanoreceptors are available. By their localization, functions and their final effect they can be: 1) pulmonary stretch receptors; 2) irritant receptors; 3) J-receptors, which derive from the term “juxsta- capillaries”. The stretch receptors are the main receptors ensuring breathing. They are located throughout the pulmonary airways (in the muscular tissue of the bronchi, trachea) and their excitation is connected with the lung walls’ inflation. They adapt very slowly to a sustained stretch and provide information about the lung volume to the brain. From these receptors n. vagus derives, so they are considered to be the terminal parts of n. vagus. Impulses coming from these receptors provide periodical activity of the RC, i.e. the rhythmical sequence of inspiration and expiration. Just these receptors send impulses by the afferent branches of n. vagus to the RC and provide automatism. The reflexes considered as Herring – Breuer reflexes underlie breathing process. They were described in 1868 and formed the basis for the concept of the respiration reflector self-regulation. It is displayed as follows: with each inspiration impulses arise in the lungs, that inhibit inspiration and stimulate expiration, and with each expiration causes inspiration process. This point of view was the consequence of the following reflexes:

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1. In inspiration inflation of the lungs with additional air causes immediate stoppage of inspiration and the expiration stimulation. It is denoted as “inspiration - inhibitory” reflex. 2. In expiration introduction of additional air into the lungs brings to the expiration prolongation. It is denoted as “expiration - facilitating” reflex. 3. In case of a strong inflation of lungs, with excitation of irritant receptors and additional stimulation of the RC the convulsive inspiration, “sigh” is observed. It is denoted as “paradoxal breathing of Hedd”. By means of these reflexes the recurrent connection between the RC and the respiratory apparatus is realized. Inspiration referring neurons are located in the dorsal part of the RC. They are divided into two groups: inspiration providing (Iα) and inspiration inhibiting (Iβ). Being excited from chemoreceptors Iα send impulses to the motoneurons of the spinal cord cervical and thoracic segments. The motoneurons innervate the intercostal muscles and diaphragm and cause contraction of the latters, i.e. provide inspiration. During inspiration the stretch receptors are excited and impulses from them flow to Iβ and activate it (Figure 43). Iβ neuron inhibits Iα one and thereby cause cessation of contraction of the diaphragm and intercostal muscles, so that inspiration is arrested transforming into expiration. Herein the inhibitory influence of Iβ on Iα is mediated by specific inhibitory neurons. When expiration is already preceded, the stretch receptors of the lungs are not excited and the impulses

257 do not get to the Iβ, consequently they do not get to the inspiration inhibitory centre and Iα is rid of inhibition, so they are able to send impulses to the spinal cord. Cutting of the n. vagus causes inspiration to become much slower, deeper and longer and its transition into expiration hurdles. But nevertheless the inspiration is converted into expiration due to pneumotaxic centre, which controls activity of the inspiratory and expiratory centres lying below. It stimulates the expiratory centre during inspiration and thus ensures rhythmical alterations of inspiration and expiration by the following manner. The impulses from the Iα neurons are conducted to the pneumotaxic centre and pass from it to the expiratory neurons of the RC with stimulation of expiration.

PTC + EC + + IIN _ _ + Iβ Iα ChR +

n. vagus + + MN RM + LSR

Figure 43. Reflector regulation of respiration. Iα − inspiration providing neuron; Iβ − inspiration inhibiting neuron; ChR – chemoreceptors; MN- motor neurons of spinal cord; RM – respiratory muscles; LSR – lung stretchreceptors; EC – expiratory centre; IIN- inhibitory interneurons; PTC – pneumotaxic centre

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Similar respiratory changes are encountered when the vagus nerves are intact, but the brain stem is cut at the pons, disconnecting the pneumotaxic centre from the medulla. If both vagus nerves and the pons varolii are cut, no excitation of the expiratory centre occurs and breathing ceases at the peak of inspiration, and the animal dies. Irritant receptors are located along the trachea, bronchi in the epithelial and sub-epithelial layers. These receptors can be excited in great alterations of the lungs’ volume, as well as in chemical and mechanical irritation of mucous. J-receptors are located in the intracellular space of alveoli close to bronchial capillaries. They are excited in increase of the arterial pressure in the small blood circulation or when the liquid amount in the intracellular space increases (in the lung edema, pneumonia). Impulses pass through the unmyelinated nerves to the RC, distinctly from the stretch receptors, which send their impulses by the quick myelinated nerves. In this case breathing intensification takes place. Defence reflexes. They ensure the removal of various irritating agents and prevent their entry into the lungs. Dust, mucus, acids, bases or foreign bodies accumulated in the respiratory passages, cause spasmodic expiratory movements by stimulating the nerve endings. The glottis is closed at the beginning of expiration, but as soon as definite pressure is produced in the lungs and the respiratory passages, it opens suddenly and air is expelled with a force from the respiratory passages. Thus a cough occurs, which is the forcible thrust of

259 air and helps to clear the respiratory tract. If the receptors of the nasal mucus are stimulated sneezing reflex occurs. Respiration can be regulated not only by the main respiratory centre, but also by the other parts of the CNS. These are the hypothalamus, which is of importance in physical work, emotional stress, pain influence, and temperature increase. Herein, the intensification of respiration occurs. The cortex also influences on the process of respiration. It inhibits the RC. In case of its removal intensification of respiration is observed. Owing to the cortex influence respiration may be voluntary; e.g. divers before diving into water several times inspire air on purpose to supply tissues by O2.

Respiration under various circumstances

Respiration at reduced atmospheric pressure. The study of this problem is important, because some people deal with it by their specialty and work in this condition (alpinists, pilots, parachute jumpers). The main factor in high altitudes (reduced atmosphere pressure) is severe hypoxia and hypocapnia due to decrease of O2 and CO2 partial pressures. Hypoxia in general can be: 1. Hypoxic hypoxia (at high altitudes).

2. Anemic hypoxia (when the O2 partial pressure is normal, but the Hb content in erythrocytes is decreased). 3. Circular hypoxia is not connected with the above- mentioned factors, but circulation impairments are present: heart troubles, ischemic conditions, etc.

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4. Histological (tissue) hypoxia (when the O2 utilization by tissues is disturbed oxidative phosphorylation). 5. Physiological hypoxia (in hard physical work lack of

O2 may be observed). 6. Hyperoxic hypoxia (in case of damage to the respiratory centre or in intoxication of tissues). Hypoxia, observed in certain respiratory and circulatory changes has an adaptive character. A fall in blood O2 content stimulates the chemoreceptors of the vascular reflexogenic zones and produces an increase of the pulmonary ventilation, accelerates and increases the cardiac output, augments the amount of circulating blood owing to its discharge from the spleen and other blood reservoirs and opening the capillaries. The accumulation in the tissues of incompletely oxidized metabolites, that stimulate the respiratory nerve centres, also contributes to the development of these phenomena. If the factor responsible for hypoxia persists for a long time (prolonged residence at high altitudes, or certain types of cardiac disturbance), an adaptive increase in Hb content and in the number of erythrocytes occur. At altitudes (1-1.5 km) respiration intensification occurs. At 2.5-5 km the respiration acceleration is accompanied by the compensatory mechanisms: 1) increase of the heart rate; 2) activation of hemopoiesis; 3) formation of 2,3 – diphosphoglycerate, that reduces the O2 affinity to Hb and so stimulates the HbO2 dissociation. In this case hypocapnia will be observed, that will result in the RC inhibition.

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Starting from 5 km “mountain sickness” can develop, in which the organism is not capable to cope with the formed problem, and so fatigue, cyanosis, decreased heart rate, headache, vomiting and low arterial pressure are manifested. At the altitudes over 7 km unconsciousness is observed. In order to elevate at the altitudes from 7 km to 12 km man should use special balloons, where the gas mixture has to have a partial pressure more than the external air pressure. In this case sometimes the CO2 content in balloon is increased to activate the respiratory centre additionally.

Periodic respiration. A deficient supply of O2 to the respiratory centre sometimes results in abnormal respiration, which is called periodic or Cheyne-Stokes respiration (Figure 44). It is connected by decrease of the RC neurons’ functional activity and characterized by the recurrence of pauses between groups of respiratory movements. The pauses may last from 5 to 20 seconds; and each pause is followed by weak respiratory movements and then by rapid strengthened ones. The latter having reached a maximal volume, gradually, but quite quickly become weaker until they cease completely. A new pause occurs, after which the whole cycle is repeated. One cycle lasts 30-60 sec. The main reason for stoppage (pause) of respiration is hypocapnia, which reduces excitability of the respiratory centre. The accumulation of CO2 during the periodic pauses causes strengthening of respiration movements, which increase the CO2 elimination. Since the CO2 content in this condition drops below the level required to stimulate the respiratory centre.

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Figure 44. a) Cheyne-Stokes respiration;

b) CO2 tension in the blood.

This respiration is often observed in mountain sickness, as well as in newborns, prematurely-born babies and different disorders of CNS: ischemia, haemorrhages and other conditions of the RC decreased activity.

Respiration in increased atmospheric pressure conditions. At great depth under the water (in each 10 m the pressure enhances by 1 atm), where divers have to work, there could be two hazards: hyperoxic state of the organism that evokes a toxic effect on tissues; and a great amount of dissolved gases in the blood and tissue fluids. Work at depth requires special measure for gradual decompression. When a diver is brought to the surface slowly (gradually decompression), gases escape from the body with expired air, and he is out of danger. But in sudden decompression, for example, a diver is brought to the surface too quickly, the gases have no chance to escape and as their solubility in blood decreases with the fall of pressure to normal, gas bubbles appear. This can cause embolism of the

263 vessels, blockade of the vessels. As carbon dioxide and oxygen can form some compounds in the blood they are less hazard than nitrogen, which is dissolved in fats and lipoids and accumulated in a great amount in the brain. The condition caused by rapid decompression is called “caisson disease”. It is characterized by pains in joints and a number of cerebral symptoms (dizziness, vomiting, dyspnea and unconsciousness). A suffer should be treated by being exposed again to the effect of high pressures so that the gas bubbles can dissolve again. At present divers working at great depths are given a gas mixture containing helium instead of nitrogen, as the former is almost completely insoluble in water and blood. Respiration during physical effort. Since both respiration and circulation are concerned in satisfying the organism’s requirements of O2 and in removing from it the formed CO2, it is obvious that the intensity of respiration is closely associated with the intensity of oxidative processes; the depth and rate of respiratory movements diminish at rest and increase during work. Thus, the volume of pulmonary ventilation rises to 50 l per minute. O2 uptake of a resting individual is 250-350 ml per min, and may reach 4500-5000 ml in work. The O2 utilization degree increases markedly with intensive muscular work (in norm – 40%, in work – 60%). cardiac contractions may be doubled (from 70 to 140 - 150 per minute); systolic volume can increase from 70 to 220 ml, so the minute volume from 5-6 l to 20-25 l. Sequence of the processes may be described as follows: hypoxia→ hyperventilation→hypocapnia (due to the increased removal of

264 eliminated CO2). In this case we can expect that respiration has to be inhibited, but actually it doesn’t occur, since some metabolites (lactic acid, etc) formed during strenuous work accumulate in the blood. The latters are the stimulators of the RC activity. So, even after breaking of the muscular work the respiration remains intensified, since concentration of the lactic acid (H+ ions) is still preserved. Apart from this, a lot of compensatory mechanisms, like the liberation of erythrocytes from the blood depots, reduction of the water content through sweating, which result in a rise in Hb concentration and, consequently, in an increase blood oxygen capacity. All contribute to grater carriage of O2 and the O2 supply increase to the tissues of organism in physical effort.

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CHAPTER 9. PHYSIOLOGY OF THE DIGESTIVE SYSTEM

The human organism needs various substances and a considerable amount of energy for its normal functioning. The human body can uptake organic and inorganic substances, only after nutrients’ preliminary processing, which is realized by the digestive system. Digestion is a complex physiological process during which food undergoes physical (mechanical) and chemical changes. Physical changes of food comprise mechanical processing, fragmentation and swallowing and dissolution. Chemical changes consist of a sequence of reactions between the nutrients and the components of digestive gland secretion, hydrolytic enzymes. The main groups of these enzymes are: proteases, lipases and carbohydrases.

Functions of the digestive system and types of digestive processes

The functions of the digestive system are divided into digestive and indigestive ones. The digestive functions are: secretor, motor and absorption. The main stages of digestion are accomplished by enzymes secreted by the digestive glands, i.e. they are ensured by secretor function of the digestive tract. Motor function is performed by muscles of the alimentary tract and makes possible mastication, food and its

266 indigested parts movement through the digestive conveyer, microvillus contractions. The motor function also promotes passage of digestive secrets into the gastro-intestinal tract (opening and closure of the ducts’ sphincters), mixture of food or chyme with digestive juices. The simple products of hydrolyzed nutrients are absorbed in the digestive tract, which condition the absorption function. Organs of the gastro-intestinal tract take part in other, indigestive functions: participation in regulation of homeostasis (water-salt balance), excretory function (excretes the hard metals’ salts, bile pigments, exogenous substances including medical preparations), endocrine function (diffuse endocrine system produces biologically active substances: gastrin, bombesin, prosecretin, etc.), participation in the hemopoiesis regulation (it produces internal Castle’s factor regulating absorption of vit. B12 regulator of the hemopoiesis) and defence function (saliva and digestive juices contain antibacterial substances). According to the origin of digestive enzymes digestion can be proper (under the action of macroorganism’s digestive juices), autolytic (under the action of exogenous enzymes located in the food) and symbiotic (under the action of enzymes produced by macroorganism’s microflora). Processes of digestion are classified as intracellular and extracellular. Two types of digestion, distant (cavital) and membrane (contact, surface) are distinguished in extracellular digestion.

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Intracellular digestion is accomplished by nutrients which enter the cells by means of phagocytosis or pinocytosis. Nutrients are hydrolyzed by cellular (lysosomal) enzymes. In the human body intracellular digestion occurs in leucocytes and cells of the lympho- reticular system. The distant or cavital digestion is realized by enzymes contained in the juice of the gastro-intestinal tract glands at a considerable distance from the site of enzyme formation. Membrane digestion is carried out by membrane-bound enzymes and simultaneously the hydrolyzed nutrients are conveyed into the blood and lymph. Structures with bound enzymes are represented in the small intestine by glycocalyx. Here membrane digestion takes place to be preceded by gastric and small intestinal cavital digestion, i.e. the nutrients are hydrolyzed at first in the small intestine by the pancreatic enzymes. The formed oligomers undergo hydrolysis by the adsorbed pancreatic enzymes of glycocalyx. Due to this hydrolysis the formed dimers are hydrolyzed directly at the membrane by the membrane-bound intestinal intrinsic enzymes. The latters are synthesized in enterocytes and are carried into the membranes of their microvilli.

Digestion in the mouth

The first part of digestive conveyer is the mouth, where food is broken to small particles and moistened with saliva, its taste properties are analyzed, and initial hydrolysis of certain nutrients and formation of the bolus occur. In the mouth the taste, tactile, temperature and pain receptors are located. Test

268 receptors are divided into the following types: sour, sweet, salt and bitter receptors. Their location on the tongue is not proportional. Salt receptors are more at the top of the tongue, sweet and sour receptors on the sides and bitter, on the root of the tongue. The stimulation of receptors is important for the saliva secretion as well as for mastication and swallowing of bolus. Mastication. Mastication is a reflex act. Food in the mouth stimulates the receptors from which signals are conveyed along the afferent fibres of the trigeminal nerve to the mastication centre, and from this centre, along the efferent fibres of the same nerve, to the mastication muscles. Mastication has the following phases: the rest phase, intake of food into the mouth, the phase of orientation, the main phase, formation of the bolus and swallowing. Salivation. Saliva plays an important role in the initial stage of digestion. It is secreted by three pairs of large salivary glands: parotid, sub-maxillary and sublingual. A great number of small glands are situated on the surface of the tongue and the mucous membrane of the palate and cheeks. According to the consistency of saliva produced, three types of salivary glands are distinguished (Figure 45): serous (parotid gland and small glands on the sides of the tongue), mucous (small glands of palate and root of the tongue) and mixed (submaxillary, sublingual glands and small glands on the tip of the tongue, cheeks and lips). When food is not taken, saliva is secreted in man at an average rate of 0.24 ml/min to moisten the oral

269 cavity. During mastication 3-3.5 ml/min of saliva is secreted. From 0.5 to 2 l of saliva is produced daily. Composition and properties of saliva. Saliva is a viscous, slightly opalescent and cloudy fluid with a consistency of 1.001-1.017. Mixed saliva contains 99.4-99.5% of water; the remaining part is a dry residue. PH of mixed saliva is 5.8-7.4. Dry residue consists of organic and inorganic substances. The inorganic components of saliva are: chlorides, carbonates, phosphates and other salts of sodium, potassium, calcium, magnesium and others. The osmotic pressure of saliva is lower, than that of blood plasma. The organic components of saliva are: various proteins, free aminoacids, carbohydrates, urea, ammonia, creatinine and other substances. The significant viscosity of saliva is conditioned by mucin, included into the composition of saliva. Saliva is rich in enzymes: alpha-amylase (destroys polysaccharids with the formation of dextrins, which are further broken down to form maltose, disacharide) and maltase (destroys maltose into two particles). Saliva amylase begins acting in the mouth, but insignificantly because the food remains here for a short time. Hydrolysis of carbohydrates by saliva enzymes continues in the stomach only in the deep layers of bolus. Saliva possesses a bactericidal property due to enzyme lysozyme. Kallikrein found in saliva contributes to the formation of kinins which are vasodilators.

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Mucous gland gland Mucous (sublingual gland) gland) (sublingual Serous gland gland Serous (parotid gland) gland) (parotid

Mixed gland Mixed

(submundibul gland) gland) (submundibul Figure 45. Various secretor cells of the saliva glands.

Control of saliva secretion. The salivary secretion is realized by reflector mechanism. In intake of food and excitation of mouth receptors by it the stimuli are transmitted along the afferent fibres of the trigeminal, facial, glossopharyngeal and vagus nerves to the main salivation centre, located in the medulla oblongata. Efferent impulses come to salivary glands, to submaxillary and sublingual glands along chorda thympani and to parotid gland along glossopharyngeal nerve fibres. Parasympathetic innervation of the salivary glands arises from the nuclei of the medulla oblongata. Sympathetic innervation of the glands begins from the lateral horns of the second to fourth thoracic segments of the spinal cord. Stimulation of the parasympathetic nerve causes activation of salivary secretion. The sympathetic nerve

271 also activates salivary secretion, but in contrast to the parasympathetic nerve, when serous saliva is secreted, it increases secretion of viscous saliva due to high concentration of organic substances. Besides, salivation can be realized by conditional reflexes in response to the sight and smell of food. Methods of studying secretion of saliva. To study the salivary glands’ activity Pavlov suggested creating a fistula of the excretory duct. This was accomplished surgically as follows. The papilla of the duct of the parotid or submaxillary gland was separated from the surrounding tissues together with a part of the mucous, brought outside through a wound made in the cheek and sutured to the skin. Saliva secreted by the submaxillary or parotid gland can be obtained in man through a small metal funnel, known as a Lashley-Krasnogorsky capsule, fastened to the mucous membrane at the opening of the excretory duct of the examined gland. Deglutition. Deglutition (swallowing) is a reflex act, which begins from the tongue root receptors excited by bolus. The centre of swallowing is located in the medulla oblongata. The swallowing reflex consists of a series of successive links. Their strict coordination is effected by a complicated relationship between various parts of the CNS, from the medulla oblongata to the cerebral cortex. Stimulation of the tongue root receptors causes contraction of the muscles that elevate the soft palate, which prevents the food entering the nasal cavity. Tongue movements help to push the bolus into the pharynx; simultaneous contraction of muscles that move the hyoid bone and raise the larynx causes the epiglottis to close

272 the entrance to the larynx, so that food is prevented from entering the respiratory passages. As soon as food enters the pharynx, muscles that cause constriction of its lumen above the bolus contract, and the bolus passes into the esophagus. The act of deglutition is separated into three phases: 1) oral (voluntary); 2) pharyngeal (quick involuntary); 3) esophageal (slow involuntary).

Digestion in the stomach

The digestive functions of the stomach are as follows: storage of food, its mechanical and chemical processing and gradual evacuation of the food contents in portions into the intestine. In the stomach we differentiate 3 types of secretor cells: chief glandulocytes producing inactive enzymes, parietal glandulocytes, which secret hydrochloric acid and mucocytes (accessory cells) producing a mucoid secret. There are no pariental glandulocytes in the pyloric part of the stomach. From 2 to 2.5 l of gastric juice is secreted daily in the stomach. It is a colorless clear fluid containing hydrochloric acid (0.3- 0.5 %) and having, therefore, an acid reaction (pH 1.5-1.8). The chief glandulocytes synthesize and secrete two groups of pepsinogens. Those of the first group (5 in number) form in the fundus and pepsinogens of the second group (2 in numbers) are synthesized in the pyloric part of the stomach. Pepsinogens are activated in the acid medium by splitting off the pepsin-inhibiting polypeptide from them. The pepsins proper are enzymes which hydrolyze proteins at a maximum

273 rate at pH of 1.5-2. Another of their fractions hydrolyses proteins at maximum pH of 3.2-3.5 and is called gastricsin. The gastric juice of an adult is marked by mild lipolytic activity. This activity is important for a breast-fed infant (breakdown of milk fats that are already emulsified). The salivary carbohydrases continue to act upon the carbohydrates in the central part of bolus, where the acid gastric juice (that arrests the effect of carbohydrases of the saliva) has not penetrated yet. Hydrochloric acid of the gastric juice causes denaturation and swelling of proteins facilitating in this manner their subsequent breakdown by pepsin, activates pepsinogen and prosecretine, creates an acid medium which is necessary for the breakdown of food proteins by pepsins; it also contributes to an antibacterial effect of the gastric juice and regulation of the digestive tract motor activity. Mucoids are an important component of the gastric juice. They protect the gastric mucosa from mechanical and chemical irritations. The secretion of mucus is induced by local stimulation of the mucous membrane and is regulated by the vagus and splanchnic nerves. Gastromucoprotein (Castle’s intrinsic factor) is also a mucoid. Besides these above mentioned components, the gastric juice consists of other organic (urea, uric and lactic acids, aminoacids, polypeptides) and inorganic (chlorides, sulphates, phosphates, bicarbonates of sodium, potassium, calcium and magnesium and ammonia) substances. The osmotic pressure of the gastric juice is higher than that of blood plasma.

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Control of gastric juice secretion. The gland of the human stomach secretes a small amount of gastric juice when digestion is not taking place. The intake of food increases sharply its secretion by the glands due to their stimulation by neuronal and humoral mechanisms. The stimulating and inhibitory factors are responsible for the dependence of gastric juice secretion on the type of food. The character of the food determines not only the volume and duration of secretion, but also its acidity and the pepsin content, e.g. the values of secretion in response to three food stimuli (meat, bread, milk) are arranged as follows in a decreasing order: Volume of juice Meat Bread Milk Duration of secretion Bread Meat Milk Acidity of juice Meat Milk Bread Digesting power of juice Bread Meat Milk

The chief and parietal glandulocytes and the mucocytes of gastric glands are stimulated by vagus nerve. The sympathetic nerve has an inhibitory effect on the gastric glands. Gastrin is a strong stimulator of the gastric glands. It is released from the G cells. Most of them are located in the pyloric mucus. Gastric secretion is reduced sharply after surgical removal of the pylorus. The release of gastrin is increased by the vagus and by mechanical and chemical stimulation of this region. Products of protein digestion (peptides, aminoacids), meat and vegetable extracts are chemical stimuli of the G cells. The release of gastrin diminishes when pH of the pyloric part of the stomach reduces

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(in increased secretion of hydrochloric acid by the gastric glands) and ceases at pH 1.0. Gastrin, therefore, takes part in autoregulation of gastric secretion, depending on the pH of the pyloric contents. Gastrin stimulates the parietal glandulocytes to the greatest extent, the chief glandulocytes to a lesser extent and the mucocytes still lesser. Histamine is also a stimulator of the gastric glands and is formed in the mucous membrane of the stomach. It stimulates the parietal glandulocytes, and has a weaker effect on the chief glandulocytes. The secretion of histamine is activated by n. vagus. Products of protein digestion absorbed into the blood also stimulate gastric secretion. Bombesin stimulates secretion of the gastric gland through release of gastrin from the G cells. The hormone motilin produces a milder stimulating effect on the stomach secretion. Secretin and cholecystokinin-pancreozymin inhibit gastrin and histamine-stimulated hydrochloric acid secretion, but slightly intensify the secretion of pepsin. Other intestinal hormones (gastric inhibiting and vasoactive intestinal peptides, as well as neurotensin, somatostatin, enterogastrone, bulbogastrone) inhibit the secretion of hydrochloric acid in the stomach. Hyperacidity of the duodenal contents inhibits hydrochloric acid secretion by the gastric glands through reflex or through duodenal hormones, i.e. autoregulation occurs.

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Phases of gastric secretion. In gastric secretion three phases are differed: “cephalic”, gastric and intestinal. The first phase (“cephalic”) of secretion begins due to conditional and unconditional reflexes. Conditional reflexes originate from the receptors of the eyes, nose and ear by the sight and smell of food and by the sound and all the circumstances associated with its intake. Unconditional reflexes begin from the mouth and pharynx receptors. The existence of the first phase of gastric secretion has been proved in experiments with sham feeding esophagostomized dogs with a gastric fistula. Food given to the dog falls out of the esophagus and does not reach to the stomach, but secretion of gastric juice begins. The “cephalic” phase prepares the stomach in advance for the food. The gastrin mechanism also contributes to the first phase of secretion. This is proved by the increase of the blood gastrin content in dogs during sham feeding. In animals with removed pyloric part of the stomach in which gastrin is produced, secretion decreases in the first phase. Secretion occurs in the “cephalic” phase due to excitation of the feeding centre located in the medulla oblongata. The second phase (gastric phase) secretion is realized owing to irritation of the stomach mucous membrane. This phase is conditioned by neuro-humoral and local mechanisms. The neuronal mechanism has reflector nature. The impulses originated in excitation of mechanoreceptors of the stomach cause the stimulation of feeding centre and by the vagus the stomach glands. Simultaneously mechanical excitation of the

277 stomach, its pyloric part, through the intramural nervous pathways leads to the gastrin release from the G-cells. The release of gastrin in the gastric phase of secretion is also augmented by the products of protein hydrolysis, some aminoacids and by meat and vegetable extracts. Histamine is of a definite importance in accomplishing the gastric phase of secretion. It has a stimulating effect on the stomach glands. The third phase of gastric secretion (intestine phase) is conditioned by the excitation of intestine mechanoreceptors with the indigested chyme. The stimuli reaching the feeding centre by the afferent nerves provide the excitation of stomach secretion. The products of the nutrients’ hydrolysis, particularly proteins, which are absorbed into blood, also contribute to the stimulation of gastric secretion. But later the gastric secretion is inhibited by the substances of intestinal contents (products of fat hydrolysis, polypeptides and aminoacids). The release of the hormones, secretin and cholecystokinin-pancreazymin inhibits the secretion of hydrochloric acid. Other intestinal hormones from the group of gastrones also contribute to inhibition of gastric secretion. Evacuation of food from the stomach to the duodenum is due to contraction of muscles of the entire stomach, particularly to strong contractions of the pyloric muscles, rather than to opening of the sphincter. It is the contractions of the pylorus that create a high pressure gradient between the stomach and duodenum. The evacuation process depends on the consistency, chemical composition, pH, and volume of the gastric and intestinal contents.

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Methods for investigating functions of the stomach. The fundamentals of physiology of the digestion system were elaborated by Pavlov and his school. Before Pavlov, functions of the digestive organs were mainly studied in acute experiments in which the organism’s normal condition was disturbed by an inflicted trauma. At first, Russian surgeon Basov suggested collecting the gastric contents through a created “artificial entry into the stomach”, fistula. A fistula is an artificial communication between a hollow organ or the duct of a gland and the body surface. But the juice, obtained by such method was not pure, because it was mixed with the food. Pure gastric juice was collected from animals with a gastric fistula and esophagostoma in experiments with sham feeding (the operation was suggested by Pavlov and Shumova- Simanovskaya). This operation made it possible to study only the reflector (neuronal) regulation of gastric secretion, but not humoral regulation. That problem can be solved to some extend by experiments, involving the creation of an isolated miniature stomach or pouch as proposed by Klemensievich and Heidenhain. A small pouch is formed from a strip of gastric wall cut (triangle section) from the greater curvature and its opening is sutured to the skin wound. The intactness of the stomach is restored by means of sutures. Thus, two stomachs are formed: one large and normal, though made a little smaller by the operation, in which normal digestion occurs, and the other a small or isolated one into which food does not enter, but the latter secrets the juice, which can be collected with the help of fistula. The secretion of gastric juice from the isolated

279 pouch begins 30-40 min after the ingestion of food, while in an esophagotomized dog with a gastric fistula begins after 5-10 min. The regulation of gastric secretion by an isolated small stomach is realized through humoral pathway, because the nerve branches (n. vagus) supplying the pouch are cut when it is created. Pavlov developed the operation for forming an isolated miniature stomach or pouch from the greater curvature, leaving a seromuscular “bridge”. This bridge transmits small intact branches of the vagus nerve innervating the isolated miniature stomach, which reflects adequately the dynamics of changes in the secretory process, the initial reflex phase included. All these above mentioned methods are used in experimental conditions, but in the clinic other methods are used to examine patients: gastroenteroscopy, X-rays method, US-gastroscopy, radiotelemetric method by using radiopills.

Digestion in the small intestine

Digestion in the small intestine effects depolymerization of the nutrients to the stage (monomers for the most part) in which they are absorbed from the intestine into the blood and lymph. Two types of digestion (cavital and membrane) are represented in the small intestine, which are accomplished by the pancreatic and intestinal juices’ enzymes; bile plays an important role in intestinal digestion. Secretor activity of the pancreas. Composition and properties of pancreatic juice. The pancreas (Figure 46)

280 secrets daily 1.5-2.0 l juice. It is a colorless clear fluid with a pH of 7.8-8.4. The juice contains bicarbonates, sodium and potassium chlorides. The juice is rich in enzymes (proteases, lipases, carbohydrases, nucleases, etc.). Proteases are produced by the pancreas in the form of zymogens that are activated by other enzymes.

Figure 46. The opening of the common bile duct in the duodenum.

Trypsinogen (protease) is converted to trypsin in the duodenum by its enzyme enterokinase. The formed trypsin also activates trypsinogen. The second proteolytic enzyme, chymortypsin is formed from chymotrypsinogen by trypsin. Trypsin and chymotrypsin (as well as pancreatopeptidase E or elastase) split the internal peptide bonds of the proteins. They also exert an action on the high-molecular polypeptides, due to which low-molecular peptides and aminoacids form.

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The pancreas secrets procarboxypeptidase A and B, proelastase and prophospholipase A. They are activated by trypsin with the formation of the corresponding enzymes: carboxypeptidase A and B, elastase and phospholipase. Carboxypeptidase splits the C-terminal bonds in proteins and peptides. Pancreatic juice is rich in a α−amylase, which breaks down polysaccharides to oligo-, di- and monosaccharides. Ribo- and desoxyribonucleases of the pancreatic juice act upon the nucleic acids. Pancreatic lipase breaks down fats to monoglycerides and fatty acids. Phospholipase A and esterase also exert an action on the lipids. Influence of various foodstuffs on the pancreatic juice secretion. When digestion is not taking place, pancreatic juice is secreted in small amounts due to the periodic activity of the digestive tract. Pancreatic secretion increases sharply two to three minutes after food is taken and lasts from 6-14 hours depending on the composition of the meal. The higher is the acidity of the gastric contents entering the duodenum, the more pancreatic juice is secreted and the greater is the content of bicarbonates in it. The dynamic of changes in pancreatic secretion mainly repeats the pattern of gastric secretion. The ingestion of food induces increased secretion of all enzymes in the juice, which nature depends on the type of food. The secretion of amylase increases to the greatest extent in a carbohydrate diet, more trypsin and chymotrypsin are secreted in response to a protein diet, while juice with a high lipolytic activity is secreted when food is rich in fats.

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Control of pancreatic secretion. Secretion of the pancreatic gland is controlled by neuronal and humoral mechanisms. The initial pancreatic secretion is induced by the sight and smell of food and by other stimuli (conditional reflexes), as well as unconditional reflexes, formed from the mouth and stomach receptors. All these signals reach the medulla oblongata and then the efferent impulses pass along the fibres of the vagus nerve to the gland and excite its secretion. In excitation of sympathetic fibres innervating the pancreas inhibits its secretor activity, but increases organic substances’ level in it. Inhibition of pancreatic secretion is uncounted in stimulation of many centripetal nerves, in pain reaction, during sleep and in intensive physical and mental work. Gastro-intestinal hormones possess the principal significance in the humoral control of pancreatic secretion. It was shown in Pavlov’s laboratory that administration of hydrochloric acid into the duodenum excited secretion of pancreatic juice. In 1902 Bayliss and Starling demonstrated stimulation of the pancreatic secretion by intravenous infusion of a hydrochloric acid extract of the duodenal mucosa. The substance formed in the duodenum under the effect of hydrochloric acid was named the hormone secretin. Secretin induces production of a great amount of pancreatic juice which is rich in bicarbonates, but poor in enzymes because it hardly acts on the enzyme-secreting acinar cells. The second hormone that augments pancreatic secretion is cholecystokinin- pancreozymin. Products of the initial hydrolysis of protein and fat contained in the food, as well as certain aminoacids,

283 hydrochloric acid and carbohydrates stimulate to a greatest extent the release of this hormone. Cholecystokinin- pancreozymin induces secretion of enzyme–rich juice. Gastrin, serotonin, insulin, bombesin, substance P are also activators of pancreatic secretion. Glucagon, calcitonin, gastric inhibiting peptide and somatostatin inhibit it. Vasoactive intestinal peptide may both stimulate and inhibit pancreatic secretion. In ingestion of food the neuronal signals merely exert trigger effects on the gland, whereas the humoral mechanisms play an important role in the correction of pancreatic secretion. The phases of pancreatic secretion stimulated by the intake of food are the same as those of gastric secretion, but the humoral influences on the pancreas are more marked, especially in the intestinal phase. Bile, its composition and participation in digestion. About 500-1500 ml of bile is produced daily. Its production occurs continuously, while its ejection or secretion into the duodenum takes place periodically, mainly in association with the intake of food. Produced bile flows into the gall bladder where it is concentrated, therefore hepatic and cystic forms of bile are distinguished. Bile contains water, mineral salts, proteins, aminoacids, vitamins, bile acids, bile pigments, cholesterol, mucin, etc. The bile pigments are the final products of the breakdown of hemoglobin. The bile pigments are bilirubin (reddish-yellow) and biliverdin (green). Primary cholic and xenodeoxycholic acids form in the human liver, which are converted to several secondary bile acids in the intestine under the action of enzymes. Most of the

284 bile acids and their salts are combined in the bile with glycocoll and taurine. The human bile contains approximately 80% of glycocholic and 20 % of taurocholic acids. This ratio changes under the effect of certain factors. For instance, the content of glycocholic acid increases in ingestion of food rich in carbohydrates, while taurocholic acid level grows with a protein-rich diet. About 85-90 % of bile acids (glycocholic and taurocholic) ejected into the small intestine are absorbed into the blood. They again return into the liver and participate in the formation of the bile. The bile has an important role in digestion process. Bile emulsifies fats increasing the surface on which their hydrolysis by lipase takes place; it dissolves the products of fat hydrolysis to facilitate their absorption; bile increases the activity of pancreatic and intestinal enzymes; it neutralizes the acidic chyme passed from the stomach. Bile salts take part in the formation of fat particles, which are dispersed, so finely that can be absorbed from the small intestine in small amounts without preliminary hydrolysis. Bile also fulfils a controlling role since it stimulates bile production and ejection, the motor and secretor activity of the small intestine. Bile also possesses bacteriostatic properties. Bile plays a very important role in absorption of fat-soluble vitamins, cholesterol and aminoacids. Control of bile ejection. Conditional and unconditional reflex influences on the biliary apparatus are affected with the participation of numerous reflexogenic zones among which are the receptors in the mouth, stomach and duodenum. The regulation of bile production and gall bladder contraction are

285 realized by humoral mechanisms: cholecystokinin- pancreozymin, gastrin, secretin and bombesin are stimulators; glucagon, calcitonin, anticholecystokinin, vasoactive intestinal peptide and pancreatic polypeptide are inhibitors. The secretion of bile and pancreatic juice can be investigated by operations suggested by Pavlov. He gave methods of operations for drawing out the common bile duct and pancreatic duct into the skin wound with the surrounding them tissue small fragments of duodenum. The intactness of duodenum was restored by means of sutures. Intestinal secretion. The intestinal juice is a cloudy and quite viscous fluid. In centrifugation the intestinal juice separates into liquid and solid parts. The liquid part consists of water, inorganic (chlorides, bicarbonates, phosphates of sodium, potassium and calcium) and organic (mucus, protein, aminoacids urea, etc.) substances. The solid part of the juice is a yellow-gray mass of mucous clots made up of undestroyed epithelial cells. The surface layer of epithelial cells in the small intestinal mucus is continuously replaced. More than 20 different enzymes which take part in digestion are contained in the intestinal juice. The main ones among them are: enterokinase, several peptidases, alkaline phosphatase, nuclease, lipase, phospholipase, amylase, lactase and saccharase. Under natural conditions they are attached in the zone of the brush border and effect surface digestion. The secretion of the intestinal glands increases during a meal, in localized mechanical or chemical stimulation of the intestine, under the effect of some intestinal hormones. Local

286 mechanisms are of principal importance, which are realized by peripheral reflexes. Mechanical stimulation of the small intestinal mucosa sharply increases the secretion of the liquid part of the juice. The products of protein and fat digestion, pancreatic juice, hydrochloric acid (and other acids) are the chemical stimulators of the small intestine. The local effect of the products of nutrient digestion induces the secretion of juice rich in enzymes. In contrast to the stomach secretion, which is a morphostatic type, the intestinal secretion is a morphonecrotic one. Cavital and membrane hydrolysis of nutrients in the small intestine. Cavital and membrane or surface digestion takes place in the small intestine. Digestion in the cavity of the small intestine is accomplished by pancreatic and intestinal juices and bile. Macromolecular substances are hydrolyzed after the cavital digestion. The main products of this process are oligomers which hydrolysis is completed in the zone of the striated border of the intestinal epitheliocytes by enzymes adsorbed on the microvilli (Figure 47) and glycocalyx. The microvilli increase the effective surface of intestine 300-500- fold. The final products of oligomer hydrolysis, monomers, are absorbed into the blood and lymph. The main intestinal enzymes taking part in surface hydrolysis of carbohydrates are: maltase, trehalase, lactase amylase, invertase, etc. Oligo- and dipeptides are hydrolyzed by several peptidases, phosphoric ethers by alkaline phosphatase and lipids by lipases.

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Figure 47. The microvilli of the small intestine.

Intestinal secretion is studied on the isolated segments of the small intestine. One end (in Thiry’s method) or both ends (in Thiry-Vella’s method) of the isolated segment are drawn out into the skin wound.

Digestion in the large intestine

A portion of chyme passes from the small intestine into the large intestine through the ileocecal sphincter, which acts as a valve permitting the passage of content only in one direction. The large intestine plays a small role in the digestion process, because food is almost completely broken down and absorbed in the small intestine, except for certain substances, e.g. plant cellulose. A small amount of food and digestive

288 juices are hydrolyzed in the large intestine under the effect of enzymes, which have entered from the small intestine, as well as the juice of the large intestine itself. In large intestine juice the liquid and solid parts are distinguished. This juice has alkaline reaction (8.5-9.0). The solid part consists of mucous clumps formed of rejected epithelial cells. Most of enzymes are contained in the solid part of the juice. Neither enterokinase nor saccharase is found in it. Alkaline phosphatase is present in a concentration 15-20 times less than in the small intestine. It also contains small amounts of cathepsin, peptidases, lipase, amylase and nucleases. The secretion of juice in the large intestine is induced by local mechanisms. Mechanical stimulation increases it 8-10- fold. Approximately 400 g of chyme passes daily from the small intestine into the large intestine. Certain substances are digested in its proximal part. Water is intensively absorbed in the large intestine. The chyme is gradually converted to feces, 150-250 g of which is produced and excreted daily. Food rich in fibres (cellulose, pectin and lignin) increases the amount of feces due to the presence of undigested fibres in it and accelerates passage of the chyme, acting like a purgative. Microflora of large intestine. Bacterial flora of the gastro-intestinal tract is indispensable for normal body functioning. The number of microorganisms is minimal in the stomach, much more in the small intestine and optionally great in the large intestine, up to tens of billions per kg of contents.

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The importance of intestinal microflora consists in the final breakdown of the remnants of undigested food and components of the digestive juices, the creation of an immune barrier, suppression of pathogenic microbes, and synthesis of some vitamins (vit. K and vitamins of B complex), enzymes and other biologically active substances, participation in metabolism. The bacterial enzymes break down cellulose fibres which were not digested in the small intestine. Microflora participates in the process of carbohydrate fermentation and protein decay. Unabsorbed aminoacids and other products of protein digestion are destroyed in the colon by the action of putrefactive bacteria. In this process a number of toxic compounds (indole, putrescin, skatole, phenol, etc.) are formed that are capable to cause intoxication of the organism. They are detoxified in the liver by formation of coupled complexes with sulfuric and glucuronic acids. This barrier function of the liver has been shown by Pavlov-Eck operation by means of liver portal vein and vena cava anastomosis. Defecation. Defecation, i.e. evacuation of the large intestine content, occurs as a result of stimulation of the rectal receptors by the faecal material, accumulated in it. The urge to pass stool appears when the pressure in the rectum increases to the 40-50 cm H2O. The reflex arc of the act of defecation closes in the lumbosacral segment of the spinal cord. It causes involuntary defecation. Voluntary defecation occurs with the participation of centres in the brain, particularly in the cerebral cortex. As a result of impulses coming from the defecation centre through motor nerve fibres (parasympathetic pelvic

290 nerve) is sphincter ani internus (consisting of smooth muscle fibres) and sphincter ani externus (made up of striated muscles) relaxation. Due to peristaltic contraction of the intestine the feces are pushed out. This is facilitated by contractions of the abdominal muscles and diaphragm affected by straining. Contraction of the abdominal muscles causes a marked increase in intra-abdominal pressure. When defecation is not taking place, both sphincters are in the state of tonic contraction which prevents the fecal masses from falling out. The sympathetic neuronal stimuli raise the tone of sphincters’ muscles and inhibit rectal motor activity.

Absorption

Absorption is a process in which various substances are transported into the blood and lymph from the surface, cavities, or from hollow organs of the body through cells, their membrane or intercellular passages. Transport of macro- and micromolecules are distinguished. Macromolecules and their aggregates are transported by means of phagocytosis and pinocytosis and the process is called endocytosis. A certain amount of substances may be transported along the intercellular spaces; this is called persorption. This mechanism explains the penetration of small amount of proteins (antibodies, allergens, enzymes, etc.) and other substances (dyes) and even bacteria into the internal media from the intestinal cavity. Micromolecules (monomers of nutrients and ions) are transported mainly into the internal environment from the

291 cavity of the gastrointestinal tract. This transport is divided into passive transport, facilitated diffusion and active transport. Passive transport includes diffusion, filtration and osmosis. It takes place in the direction of concentration, osmotic and electrochemical gradients of the transported substances. Facilitated diffusion occurs with the aid of special membrane carriers. Active transport is the transport of substances through membranes in the direction opposite to the concentration, osmotic and electrochemical gradient with expenditure of energy and the participation of special transport systems: mobile carriers, conformation carriers, etc. Absorption in various parts of the digestive tract. Absorption takes place along the entire length of the digestive tract, but varies in intensity in its different parts. Absorption does not actually occur in the mouth, because food remains in it for a very short time. Besides, the monomer products of nutrient hydrolysis are still not formed here. Only some medical preparations can be absorbed in the mouth (cardiac preparations). Absorption in the stomach is also limited. Water and mineral salts, alcohol and glucose are absorbed here to a somewhat greater extent; aminoacids are absorbed in very small amounts. The main process of absorption takes place in the small intestine. Here the absorption of substances depends on the contraction of its microvilli. When they contract, the cavity of their lymph vessels is constricted and the lymph is pressed out, which creates a sucking action of the central lymph vessels. In

292 the small intestine all products (monomers) of hydrolyzed substances as well as water and electrolytes are absorbed. The absorption of nutrients in the large intestine is insignificant under normal physiological conditions, because they are absorbed mostly in the small intestine. Much water is absorbed in the large intestine, which is essential in the formation of feces.

Motor activity of the gastro-intestinal tract

Motor activity of the stomach. Contraction of the smooth muscles forming the wall of the stomach conditions its motor activity. The motor function of the stomach consists in storing the ingested food, mixing it with the gastric juice in the zone adjacent to the gastric mucosa, moving the gastric contents to the exit into the intestine and, finally, evacuating them in portions into the duodenum. During the intake of food and for some period after the stomach relaxes. This is known as food receptive relaxation. Some time later, depending on the type of ingested food, the contractions increase noticeably, the cardiac part contracting with a lesser force and the pyloric ones with the greatest force. There are two types of gastric contractions: phasic and tonic. The former contractions have a peristaltic character and maintain the tone of the stomach and promote mixing of the food with the gastric juice. The latters are propulsive and characteristic of the pyloric part of the stomach. They contribute to pushing the contents into the duodenum.

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The motor activity of the stomach is controlled by neuronal and humoral mechanisms. Stimuli arriving along the efferent fibres of the vagus nerve intensify gastric motor activity (rhythm and force of contractions). Stimuli passing along the sympathetic nerve reduce the stomach motor activity. Gastro-intestinal hormones are very important in the control of gastric motor activity. Gastrin, motilin, serotonin and insulin intensify motor activity of the stomach. Secretin, cholecystokinin-pancreozymin, gastric inhibiting peptide, vasoactive intestinal peptide, bulbogastrone and enterogastrone inhibit it. Vomiting. Vomiting is a complex reflex motor act, which starts with contractions of the small intestine. These contractions push some of the intestinal contents into the stomach. In 10-20 sec the stomach contracts, the entrance into it opens and violent contractions of the muscles of the abdominal wall and diaphragm occur, as a result of which the gastric contents are ejected through the esophagus into the mouth during expiration. Vomiting has a protective importance and occurs by reflex due to stimulation of the receptors of the tongue root, pharynx, mucous membrane of the stomach and intestine, peritoneum and the vestibular apparatus. Vomiting may be induced by smell and taste stimuli. Signals from the receptors of the above indicated areas reach the vomiting centre located in the medulla oblongata along the afferent fibres of the vagus, glossopharyngeal and some other nerves. The efferent stimuli, which excite vomiting, pass along the fibres of the vagus and splanchnic nerves to the esophagus, stomach and

294 intestine and along the motor fibres to the muscles of the abdominal wall and diaphragm. Motor activity of the small intestine. The motor activity of the small intestine is responsible for mixing the food contained in it with the digestive juices, moving the chyme along the intestine, raising the intra-intestinal pressure to promote absorption of some of the chyme components from the intestinal cavity into the blood and lymph. Contraction of the small intestine results from the coordinated movements of the longitudinal and circular smooth muscle fibres. These contractions may be of several types. According to function all contractions are divided into two groups: 1) local that mix and grind the contents of the small intestine; 2) contractions propelling the contents. The types of contractions are as follows: rhythmic segmentation and peristaltic (Figure 48, A, B), pendular, and microvilli contraction. Rhythmic segmentation is mainly produced by contraction of the circular muscles, which separate the intestinal contents into parts. The next contraction forms a new intestinal segment which contents consist of parts of the former segment. As a result, the chyme is mixed, and pressure is raised in every intestinal formed segment. The pendular movements are produced by contractions of the longitudinal muscles. These contractions move the chyme forwards and backwards and propel it slightly.

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A. B. Figure 48. Motor activity of small intestine. A- Rhythmic segmentation; B- peristalsis

Peristaltic consists in the formation of a constriction above the chyme owing to contraction of the circular layer of muscles and dilation of the intestine below the chyme due to contraction of the longitudinal muscles. The constriction and dilation move along the intestine and propel the portion of chyme in front of the constriction. Several peristaltic waves move simultaneously along the length of the intestine. Antiperistaltic contractions of the small intestine do not occur in norm. Tonic contractions may be marked by a very low rate and sometimes may not spread at all causing considerable constriction of the intestinal lumen for a great distance. Motor activity of the small intestine is controlled by neuronal and humoral mechanisms; the role of myogenic mechanisms, which are based on the automatism of smooth muscles, is quite important.

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The control of the motor activity of the small intestine is effected by the intramural nervous system and signals arriving from the CNS, as well as humoral factors. The parasympathetic nerve fibres stimulate, while the sympathetic nerve fibres inhibit the contractions of the small intestine. Motor activity is also inhibited in anger, fear and pain. Vigorous peristalsis of the intestine is sometimes encountered in some strong emotions, e.g. in fear (“nervous diarrhea”). The intestinal motor activity depends on the physical and chemical properties of the chyme. The local, mechanical regulation of the motor activity of the small intestine is the most important, which is realized by peripheral reflexes. The humoral substances alter the intestinal motor activity by acting on the muscle fibres directly and on the neurons of the intramural ganglia. Vasopressin, oxytocin, bradykinin, serotonin, histamine, gastrin, motilin, cholecystokinin-pancreozymin, substance P, etc increase the motor activity of the small intestine. Motor activity of the large intestine. The digestive process in humans lasts one to three days, the propulsion of the food remnants via the large intestine taking most of the time. The motor activity of the large intestine is responsible for its reservoir function: accumulation of the intestinal contents, absorption of certain substances, mostly water, from it, the formation of fecal masses and their discharge from the intestine. The large intestine performs several movements. Small and large pendular movements mix the contents which are thickened through absorption of water. Peristaltic and antiperistaltic movements fulfil the same functions; strong

297 propulsive contractions that push the contents caudally occur three or four times daily. The large intestine possesses automatism which is, however, weaker, than that of the small intestine. The large intestine is supplied with intramural and external innervation by the sympathetic and parasympathetic parts of vegetative nervous system. The sympathetic nerve fibres, which inhibit motor activity, arise from the superior and inferior mesenteric plexuses, while the parasympathetic fibres stimulating motor activity run as components of the vagus and pelvic splanchnic nerves. Local mechanical and chemical stimuli are very important in exciting contractions of the large intestine. Serotonin, adrenaline and glucagon inhibit contractions.

Periodic activity of the digestive organs

The motor and secretor activity of the digestive organs increase in certain periods independent from the food entrance. This is called periodic activity. A cycle of contractions of an empty stomach (period of work) and secretion of pepsinogen rich juice (but not free hydrochloric acid) occurs approximately 20-50 min, the period of rest 45-90 min. The periodic activity of the digestive tract is also manifested by contractions of the esophageal wall, increased salivation, production of bile and its ejection, pancreatic secretion and contractions of the small and large intestines. Periodic activity of the digestive tract is attended by changes in the functions of other systems of the organism: the heart and respiration rates increase, more blood is supplied to

298 the digestive organs, the content of glucose, acetylcholine, catecholamines and certain enzymes in the blood and the blood red cell and leukocyte counts are changed. Marked changes are found on the electroencephalogram. This is an evidence of the influence of periodic activity on many aspects of metabolism, on the organism as a whole. On the other hand, periodic activity of the digestive organs depends on the metabolism in the body. The principal role in ensuring periodic activity of the digestive organs belongs to the central nervous system which stimulates and inhibits their activity by means of parasympathetic and sympathetic influences. In turn, the central nervous system is regulated by changes of the content of certain substances, glucose, aminoacids, etc. in the blood. Humoral factors (acetylcholine, adrenaline, gastrointestinal and adrenocortical hormones) have a definite part in the formation of periodic activity of the digestive organs. Several hypotheses have been advanced on the physiological significance of periodic activity of the digestive organs. According to one of them, periodic activity in the active phase causes a feeling of hunger and induces the individual to seek food. Periodic activity is, therefore, called “hunger periodicity”. Periodicity inhibiting factors reduce the appetite. According to another view point, digestive juices contain a great amount of energetically and plastically valuable substances, proteins among others. They are hydrolyzed in the digestive tract, absorbed and utilized by the organism’s tissue. In physiological starvation the organism can utilize its own substances for nutrition.

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The feeding centre is located in the hypothalamus, the lateral nuclei of which are responsible for the sense of hunger and ventromedial ones for satiety. Stimulation or inhibition of the feeding centre depends on the chemical composition of the blood and arrival of various signals from different peripheral receptors.

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CHAPTER 10. METABOLISM OF ENERGY AND THERMOREGULATION

10.1. METABOLISM OF ENERGY

Alive organism is an open system, which needs in continuous energy influx. Energy is spent for the body temperature maintaining, work performance and plastic processes. For the humans (heterotrophic organisms) the energy source is nutrients. Transformation of energy takes place continuously in the process of metabolism: the potential energy of the complex organic compounds in food is transformed into thermal, mechanical and electrical energy: 75% into thermal; 25% into mechanical and an insignificant part, into electrical. But all the energy types eventually turn into thermal energy and leave the organism. Energetic expenses of the organism are to be measured in units of heat calories or joules. It is detectable by means of calorimetry. Let’s suppose the potential energy of nutrients is A, and the energy leaving the body is B: so the ratio A/B is the energetic balance of the organism. In a healthy grown–up person with constant weight it is equal 1. When one of these conditions is disturbed, the balance shifts. It is more than 1, if the cumulative processes prevail over the expenditure; and is less than 1, in case of exhausterd organism. For the nutrients’ potential energy (A) determination we need to detect the caloric indices of nutrients. That is the

301 heat quantity, forming in 1 g nutrient’s burning process. The caloric index of carbohydrates is 4.1 kcal; for fats 9.3 kcal; for proteins 5.8 kcal. Because of the fact, that proteins of the organism don’t undergo oxidation completely up to the final products, their caloric index in the organism is less than 4.1 kcal. For the A determination we also need the examined person’s diet ratio. It is calculated, that an average loads an organism takes daily approximately 400-500g of carbohydrates, 100-120 g of proteins and 90-100g of fats. The value of A will be: 400× 4.1+ 100× 4.1+90×9.3. For determinating the energy leaving the body (B), there are 2 methods: direct calorimetry and indirect calorimetry. Direct calorimetry is based on immediate measurement of the heat amount liberated by the organism in a biological calorimeter chamber. The apparatus is an airtight chamber with thermal insulation from the external environment. At the beginning and the end of the tubes, filled with water, thermometers are installed. Water circulating in the tubes is warmed by the heat liberated by a person or animal placed in the chamber. The amount of heat liberated by the body is calculated from the amount of circulating water and change in its temperature. This method is expensive. That is why more often they use the method of indirect calorimetry. Indirect calorimetry accounts not the liberated heat by the body immediately, but the amount of O2 consumed in the heat production, and that of CO2 liberated with the following calculation of the heat expensed. For this method 3 values are necessary to know:

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1) O2 amount consumed during a day, or a definite period of time;

2) caloric equivalent of O2; 3) the respiratory index.

O2 consumed is determined by the comparative analysis of the inhaled and exhaled air content (% O2).

The caloric equivalent of O2 is the heat amount generated after utilization of 1 l O2. For this determination they use the caloric index in order to calculate the O2 amount consumed. Oxidation of 1 g of proteins, fats and carbohydrates does not bring to the same heat amount generation, so is required different O2 supply. For example, 1g of carbohydrates gives 4.1 kcal of heat (caloric index) and uses for that 0.8 l of O2. The heat amount when 1 l of O2 is used will be 5.05 kcal (caloric equivalent of O2).

Caloric equivalent of O2 for proteins is 4.60 kcal, for fats, 4.69 kcal. Taking into consideration the amount of consumed oxygen and O2 caloric equivalent of different substances, which undergo oxidation, it is possible to determine B. For the identification of these substances (prevalence of fat, protein or carbohydrate), and so their caloric equivalents, it is necessary to know the respiratory index (RI).

The respiratory index, that is ratio of the exhaled CO2

[CO 2 ] from the organism and the O2 consumed: RI = []O 2

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For the CO2 and O2 volumes’ determination the comparative analysis of the inhaled and exhaled air contents is needed. In the exhaled air the CO2 is 4.1 %; in the inhaled air, 0.03 %.

[CO2] = 4.1 %-0.03%=4.1%

The O2 consumed is being determined by the % difference in the inhaled and exhaled airs

[O2] = 20.94 % - 16.4% = 4.5% RI = 4.1 / 4.5 = 0.85-09 If organism intakes mixed food, the respiratory index oscillates from 0.85-0.9. The respiratory index differs in fats, proteins and carbohydrates. In carbohydrates’ prevailing oxidization it is equal to:

C6H12O6+ 6O2=6H2O+6CO2

RI = 6CO2/6O2 = 1

The amount of the O2 moles consumed is equal to the

CO2 moles. According to Avogadro-Gerard’s law at the same temperature and the same atmospheric pressure different gases take the same volume. So the respiratory index for the carbohydrates is equal to 1. The respiratory index in the fats’ oxidation is equal to 0.7. We can exemplify the mentioned with tripalmitin:

2C3 H5 (C15H31COO)3+145 O2=102 CO2+98 H2O

RI = 102 CO2/145 O2 = 0.7 For proteins it is 0.8. The respiratory index enables the determination of caloric equivalent of the corresponding nutrients. When the respiratory index is equal to 0.7 they use the corresponding caloric equivalent of O2 (4.69 kcal). By

304 multiplying that by the consumed O2 volume we determine the organism’s energetic expenses. There is significance as well. It shows what metabolic pathway prevails, assimilation or dissimilation. If the assimilation prevails, the respiratory index is more 1. When fat is getting cumulated at the glucose expense in the organism, 1 molecule of fat is synthesized from 8-9molecules of glucose.

C6H12O6 (8-9)

C51H98O6 (1)

In this case approximately 22 shares of O2 leave free, so less oxygen is required and the respiratory index becomes more than 1. This value is important also for the blood chemical content reflection. Before physical work this index is 0.85-0.9, during work it tends to 1 since the main source of energy in muscles is glucose. Just after work it becomes more than 1, then decreases lesser than norm and in 50-60 minutes that comes to norm. All these changes could be explained by accumulation of the lactic acid which expels carbonic acid from bicarbonates of the buffer system.

RCOOH + NaHCO3 = RCOONa + H2CO3

The formed CO2 volume increases and the respiratory index becomes more 1. When carbon dioxide amount decreases because of the metabolic conversions in the organism the respiratory index suddenly drops and just after that it comes to norm. According to the isodynamic law of Rubner all the nutrients can substitute each other in the energetic aspect. For

305 example energy got from 1 g of fat is equal to that of 2 g of glucose. But this law is somehow limited since it does not consider the nutrients’ plastic properties (irreplaceable aminoacids). Compulsory ratio of different nutrients having not just nutritional significance is to be considered as well. Basal metabolism. Energy produced by the organism cannot be constant because of the functional state of the organism. In physical work the energy produced daily is 5000- 6000 kcal, in mental work 3000 kcal. Nevertheless, there is some energy amount which is standard for all the types of the organism. The energy expenses in standard conditions (t = 18-200C, at fast (in 12-15 hours after the meal intake), in physical and mental rest state) is a basal exchange. In men (70kg 165 cm, 35 years) it is equal to 1700 kcal per day. In women the basal exchange is lesser by 10%. Due to this the constant temperature is maintained, as well the vital activity. The basal exchange can be changed in connection with: 1) meal, especially proteins; 2) medium temperature, if it is low, the metabolism increases; 3) sex, in men it is more intensive; 4) age, in children that is higher; 5) daily activity, in sleep that decreases by 10%; 6) health state, that increases in hyper-function of thyroid glands; 7) the state of the neuronal system; in actors, teachers that is significantly higher.

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10.2. THERMOREGULATION

In low-organized vertebrates the body temperature is not constant and can be changed regarding to the environment temperature. These animals are poikilothermic ones. In birds and mammalians temperature is constant, and they are homoiothermic animals. Constancy of the temperature is isothermia. Nevertheless, the notion “constant temperature” is relative, because it varies during a day. At day time (at 4:00 PM) the highest temperature is 36.5-36.90 C, but at night or early morning (at 4:00 AM) it becomes by 0.50C lower. Different parts of the body have different temperature, e.g. the blood temperature is 370C, in the fossa axillaris that is 36.5-36.90C, in rectum 37.2-37.5, on the nose 320C. Constancy of the temperature is maintained by two counteracting and predetermining processes: heat production and heat loss. Heat production takes place in mitochondria of the cells. So, the most heat production occurs in the organs, which are rich in them, the liver, skeletal muscles, kidneys. This process is a result of continuous biochemical reactions. The chemical and physical thermoregulatory mechanisms are distinguished. The chemical one takes place by means of changes in the cell metabolism. The physical thermoregulation encounters by means of changes in the heat emission intensity. The chemical one takes place continuously and is most significant, when the temperature of medium becomes lower than 18-20oC. It is provided in the neuronal and humoral ways.

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The reflexogenic zone for the neuronal regulation is the skin, where the cold-sensitive and the heat-sensitive receptors are located. When the body surface temperature decreases the cold- sensitive receptors are excited, and the impulses go through the sensory nerves towards the posterior part of hypothalamus, where the centre of chemical thermoregulation (heat production) is located. Thereafter, the impulses reach the muscles, bringing them to chaotic involuntary contractions, in a type of shivering. Herein, the metabolic processes intensify markedly, so the heat production increases. Beyond the muscles, the kidneys and the liver participate in thermoregulation as well. In cold-perception the heat production in them increases in reflector way. There is also a humoral way of thermoregulation. The latter is provided by the hormones, increasing the metabolic processes and hence the heat production: the hormones of thyroid and adrenal glands. The physical thermoregulation becomes of a special significance in increase of the environmental temperature and is realized by the heat emission changes by the organism. The heat emission takes place in the following ways: 1. Radiation heat loss, the body gives off heat into the medium. 2. Convection, movement and mixing of air heated by the body. 3. Heat conduction, the body gives off heat to objects that are in direct contact with the body surface.

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4. Evaporation of water from the skin and the lung surface. In a human being at rest (the air temperature is equal to +200 C) radiation accounts for 66%, evaporation for 19% and convection for 15% of the total heat loss of the body. When the environmental temperature rises to 350 C heats can be lost mainly by evaporation of water from the surface of the skin and lungs. The latter share can increase up to 75% in intensive muscular work. It is calculated that in 1 ml of sweat evaporation 0.56 kcal of heat is released. Evaporation depends on the relative humidity of the air. In the air saturated by water vapours, e. g. in the bath there is excessive sweat production, but it does not undergo evaporation. This sweat production does not contribute to the heat loss. The centre of physical thermoregulation (heat loss) is located in the anterior part of the hypothalamus. Heat loss process also is regulated by the neuronal and humoral pathways. In the environmental temperature increase the heat-sensitive receptors of the skin are excited, and the impulses are directed to the heat loss centre, due to which the skin vessels are dilated in reflector way and the circulating blood in them increases. This contributes to the heat loss by radiation and convection. Here, blood will be taken by the sweat glands as well, and sweat is going to be produced more. In this case the heat production is inhibited. At cold the cold-sensitive receptors are excited bringing to vasoconstriction of the skin arterioles, so the blood enters

309 the abdominal vessels. In result the cutaneous vessels will get less blood, so the heat loss will decrease. Endocrine glands, mainly adrenals and thyroid gland also take part in the regulation of the body temperature. Beyond the neuronal and humoral regulation the blood temperature itself has a significant regulatory meaning. The blood, having higher temperature than normal, excites the heat loss centre and evokes heat loss. The blood with decreased temperature excites the heat production centre and brings to increase of metabolic processes in the organism and the heat production consequently. The above-described thermoregulatory mechanisms have an important significance in the organisms’ adaptation to the altering circumstances of the external medium.

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CHAPTER 11. PHYSIOLOGY OF THE EXCRETORY SYSTEM

All organs, the functions of which are directed to the excretion of the metabolic products, toxic substances, as well as the excess of water and salts from the organism, belong to the excretory system. The excretory organs are the lungs (eliminate the carbon dioxide, water and alcohol vapours, metabolic products of surfactant), the sweat glands (eliminate water, salts and urea), the organs of the gastrointestinal tract (excrete heavy metals, products of haemoglobin metabolism, excess of hormones) and the kidneys.

Morpho-functional characteristics of the kidneys

1. The basic function of the kidney is the excretory function, the formation of urine, by which the waste products of nitrogen exchange like urea, uric acid, creatinine and ammonia (NH3) are excreted from the organism. Urea is a waste product of catabolism of proteins, uric acid of purine bases, creatinin of creatin-phosphoric acid, and NH3 results from deamination of aminoacids. 2. The kidneys participate in regulation of the homeostatic constants: a) the water balance of the organism, b) the ionic composition of the inner medium, c) the pH of blood, d) the osmotic pressure of blood, e) the content of the organic substances in blood. 3. The kidneys perform the endocrine function. By producing a set of biologically active substances (renin,

311 prostaglandins, bradykinin, hemopoietins, calciferol) the kidneys also take part in the regulation of the blood pressure, hemopoiesis, calcium metabolism, etc. 4. They provide the synthesis (synthetic function) of some substances (glucose, NH3). 5. The kidneys also take part in the hemostasis process, synthesizing prostacyclin, thromboxane and urokinase. The morpho-functional unit of the kidney is the nephron (Figure 49), because all processes that bring to the formation of urine are realized in the nephron. Each kidney of humans contains 1.2 million nephrons. But all nephrons do not function simultaneously. Some of them serve as a reserve. But altogether the functional reduction of the nephrons until 30% causes kidney failure. The main part of nephron is Shumlyansky-Bowman’s capsule, which consists of double walls. The inner wall is covered by the layer of podocytes, the external wall by one-layer of epithelial cells.

Figure 49. Nephron - the morpho-functional unit of the kidney.

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Between the inner and external membranes of the capsule there is a cavity, which continues as the lumen of the tubule. The first part of the tubule is the proximal convoluted tubules, which wall is formed by brush border epithelial cells with microvilli. Thereafter the tubule straightens and enters the medullar region of the kidney. In the medulla the tubule turns by 180O ascends back and up, and at the boundary of the cortex and the medulla it again forms the convolutions, and this part is called distal convoluted tubule (Figure 49). The part of tubule, which is located in the medulla, is called Henle’s loop. In the loop we differentiate the descending and ascending limbs. The distal convoluted tube opens in the collecting tubule. The collecting tubules by origin don’t belong to the nephron but because they actively take part in the formation of urine, are considered as a part of it. The collecting tubules of different nephrons join together and form big extracting channels, which open into the renal pelvis. Malpighian tuft (capillary glomerulus) is the most important element of the capsule. According to their form and location the nephrons are divided into 3 types: superficial nephrons (malpigian glomerulus is on the surface of the cortex, Henle’s loop is very short) and makes up 20-30%; intracortical nephrons (malpigian glomerulus is in the cortex), 60-70%, they participate in the filtration process; juxtamedullary nephrons (malpigian glomerulus is on the boundary between the cortex and medulla), 10-15%, they have a long Henle’s loop, so they perform an important significance in the concentration of the urine.

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Kidney blood supply

The kidney receives blood from the renal artery, which originates from the abdominal aorta under 900, and after a shortcut enters the kidney and separates into many branches: interlobar, arcuate, interlobular arteries and afferent arteriols (Figure 50). The latters form afferent vessels (vas afference), from which a capillary network, nephron’s malpigian glomerulus originates. The vessel originating from the glomerulus is not a venous vessel, but is an arteriole which is called efferent arteriole (vas efference). The diameter of the efferent vessel is significantly smaller than that of the afferent vessel. The efferent vessel again separates into secondary capillary network, which surrounds the tubules. The peritubular capillaries empty into the vessels of the venous system and progressively form the interlobular vein, arcuate vein, interlobar vein and renal vein, which leaves the kidney. Peculiarities of the kidneys’ blood supply 1. The kidneys, being the 0.43% body’s mass, receive 1/4 of the blood pumped by the heart, therefore they are the most blood supplied organs (4-5 ml/min/g of tissue). 2. In the capillaries, particularly in the glomerulus of the kidneys, the hydrostatic pressure is significantly high, which is equal to 70 mm Hg, while in the other organs this makes up 40 mm Hg. A high hydrostatic pressure is conditioned by two factors: the renal artery originates from the abdominal aorta under 90o; the difference between the diameters of the afferent and efferent vessels.

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Figure 50. Kidneys’ blood supply.

3. The capillary pressure is relatively independent from the common arterial pressure of the organism. The variations in the arterial pressure by 90-190 mm Hg c. don’t reflect on the pressure of the kidney capillary pressure. This is explained by the self-regulating mechanisms. 4. The existence of double capillary network. 5. The difference in volumes between the kidney inflowing and out flowing blood. 6. In the juxstamedullary nephrons, the secondary capillary network and the difference between the diameters of the afferent and efferent arterioles are not available.

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Formation of urine

According to the recent data the formation of urine is a complex of processes and it consists of filtration, reabsorption, secretion and synthesis. Filtration. It occurs in Shumlyansky-Bowman’s capsule, into the cavity of which water and different substances that are dissolved in the blood are filtered. The filtration membrane (barrier) (Figure 51), through which the filtered substances pass, consists of 3 sheets. They are: the endothelial and basement membranes in the glomerulus capillaries and the capsule podocyte layer. Endothelial and basement membrane pores’ diameter correspondingly is 100 nm and 10 nm. But the basement membrane pores have also negative charge and proteoglycans’ layer, which provides additional restriction to filtration. The third barrier is composed by podocytes, which encircle the outer surface of the glomerulus capillaries and form the specific structure with slit pores (d=3 nm). These scales limit the passage of substances by high molecular mass (proteins, blood formed elements). Thus, the ultra-filtrate or the primary urine, which is formed in the cavity of the capsule, by its composition, differs from blood plasma only by the absence of proteins and blood cells. The ultra-filtrate is formed in a result of 3 different pressures: 1) the hydrostatic pressure of the renal capillary (70 mm Hg c.), which promotes filtration; 2) the blood oncotic pressure (30 mm Hg c.), it is the obstructing to filtration

316 pressure; 3) the intracapsular pressure (20 mm Hg c.), it is also an obstructing pressure.

Figure 51. Structure of the filtration barrier.

Pfilt. = Phyd. – (Ponc. + Pcap.) = 70– (30+20) = 20 mm Hg c.

Changes in these pressures (decrease of Phyd. or increase of Ponc.) result in urine formation disturbances.

Filtration

mm 20 g

20 20

Figure 52. Filtration mechanism. I – Hydrostatic pressure, II-oncotic pressure, III- intracapsular pressure, IV- filtration pressure

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Daily 1800 l of blood passes through the kidney, from which 180 l of primary urine is formed, and from which 1.5-2 l of the final urine is excreted. In clinical practice, in order to assess the kidney’s filtration ability, they use an inulin- cleaning coefficient. For this purpose inulin (polymer of fructose) is injected into the blood. Inulin has the following characteristics: 1) it’s not toxic for the organism; 2) it is completely filtered by the kidneys; 3) during its passage through tubules it doesn’t undergo any changes; 4) it is not reabsorbed, secreted or synthesized. So its quantity in the primary urine is the same as its quantity in the final urine. Taking this into consideration we can derive the following equations:

Pin. ×F = Uin. ×V,

where Pin. is the concentration of inulin in the blood, which is equal to its concentration in the primary urine; F is the volume of primary urine, formed in a unit of time;

Uin. is the concentration of inulin in the final urine; V is the volume of final urine, formed in a unit of time. U × V F = in Pin The value of F in male is 125 ml/min, in female 110 ml/min. Reabsorption. 180 l of primary urine is formed daily and 1.5 l of final urine is excreted daily from the organism.

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Therefore the most amount of the primary urine is reabsorbed in the tubules. In the composition of primary urine there are not only waste products, but also substances that are essential for the organism, like glucose, aminoacids, salts, vitamins, microelements, hormones, etc. The reabsorption of 2/3 of these substances occurs in the proximal convoluted tubules. But all the substances are not reabsorbed. Those substances that can be reabsorbed are called threshold substances (glucose), and those that can’t be reabsorbed are called non-threshold (sulfates, inulin). The non-threshold substances can be used to calculate the quantity of the filtration. There is a conception of elimination threshold for the threshold substances. The elimination threshold is the concentration of a substance in the blood at which it cannot be reabsorbed completely and appears in the final urine. The normal concentration of glucose in blood is 80-120 mg%, and glucose will not appear in the final urine until its concentration in the blood does not reach to 160-180 mg%, i.e. its elimination threshold value is 160-180 mg%. There are 2 types of reabsorption: obligatory and facultative. The obligatory reabsorption of the substance is the reabsorption, which is independent from the blood concentration of the given substance. This type of reabsorption mainly occurs in the proximal tubules. The reabsorption occurring in the distal and collecting tubules of nephron depends on blood concentration of substances and is called facultative, which is regulated.

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We can differentiate 2 mechanisms of reabsorption, passive and active. Passive reabsorption is the reabsorption, when the substance is reabsorbed into the blood in the direction of the concentration, electrochemical and osmotic gradients. - Water, CO2, some ions (Cl ), and urea are reabsorbed by this mechanism. By active reabsorption, substances are reabsorbed against the concentration gradient. There are 2 types of active transport (reabsorption): primary active and secondary active transport. During the primary active reabsorbtion energy obtained from the cell metabolism is used. The reabsortion of the Na ions is realized by the primary active transport. This type of reabsorption is divided into two phases: 1. Na ions pass from the cavity of the tubule towards the epithelial cell owing to electrochemical gradient, because sodium is not cumulated by this cell, and the concentration of Na ions is less than that in the primary urine; 2. Na ions pass from the epithelial cell into the intercellular liquid and then into the blood against the concentration gradient by the energy expenditure. It occurs due to Na-K-ATP-ase. This enzyme is responsible for transport of sodium from the cell and simultaneously for entry of potassium into it. For the secondary active reabsorption energy is not used for the given substance, e.g. glucose or aminoacid transport. At first glucose combines with the carrier and Na ions and this complex passes from the lumen of the tubules into the epithelial cells according to the sodium electrochemical gradient without energy expenditure. Inside the cell the

320 complex is decomposed. The carrier passes back into the tubule to combine with new portions of glucose. Sodium penetrates through the basement membrane in a mentioned manner (active transport) providing the glucose transport (co-transport), so energy is not needed for the passage of glucose. Nevertheless, a small amount of proteins (Hb, plasma albumin) appear in the ultra-filtrate but they are also subjected to reabsorption. The protein reabsorption mechanism is active (energy dependent) and is called pinocytosis. The proteins are engulfed by the epithelial cells forming the vacuole, and are decomposed by the lysosomal enzymes forming low-molecular fragments, which are then transported to the blood. The reabsorption of all the substances necessary for the organism occurs in the proximal convoluted tubules. In the distal parts, basically water and salts are reabsorbed. The reabsorption following percentage relativity exists in different parts of the tubule. 45% of water and 65% of Na+ ions are reabsorbed in the proximal convoluted tubules. In Henley’s loop it is 25% for both and in the distal convoluted tubules it is 10% for water and 9% for Na+ ions. In the collecting tubules it is 20% for water and 1% for Na+. Depending on the organism’s water balance, the kidneys excrete either dense or dilute urine. The diluting and concentrating urine occurs in Henley’s loop and also in the collecting tubules. The medullar tissue is supplied with less blood than the cortex, so the osmotic density of urine in the medullar region is higher. When the primary urine flows through the descending portion of Henley’s loop, which walls

321 are transparent only for water, it loses water and gradually becomes denser. The density of urine in the descending part of Henley’s loop is 300 mOsm/l and it is isotonic to plasma, while in the apex of the loop its density reaches the highest value (1400 mOsm/l). But if we compare the urine density in two neighbouring Henle’s loop limbs, a big difference will not be noticed. Water, by leaving the descending limb dilutes the liquid of the medullar interstitial tissue, and thus it helps the passage of Na+ and Cl- from the ascending limb of Henley’s loop. An increase in the interstitial liquid density results in the exit of water from the descending limb of Henley’s loop. So urine advancing through the ascending limb of Henley’s loop becomes less dense and enters the distal convoluted tube as hypotonic urine. Its density is 100mOsm/ l. The final concentrating of urine takes place in the collecting tubules. The mechanism, which works in Henley’s loop, is called counter- current multiplier mechanism. By this mechanism the water- salt balance in the organism is regulated. The mechanism is called so because in the parallel arranged tubules the urine flows in opposite direction, and the process which occurs in one tubule provides the process occurring in the neighbouring tube and vice versa. In the entrance and exit of each limb of Henley’s loop the solitary multiplication effect occurs. Reabsorption is mainly an active process (energy- dependent), and it might be reduced in cases when energy synthesis is disturbed, which results in polyuria. In clinical practice they induce polyuria (when there is excess of water in the organism) with the help of special pharmacological

322 substances, which have a high osmotic activity and are filtered, but are not reabsorbed. In clinical practice to measure the kidney reabsorption function, they calculate the so-called maximum value of a substance transportation that can be done by the glucose. For this purpose they inject enough glucose in the blood so that it excels the threshold. In this case the most part of the glucose is reabsorbed according to its threshold; the rest is excreted in the composition of final urine. The amount that has undergone reabsorption will be equal to the difference between the filtered amount and the amount found in the final urine.

Tmaxgl. = Pgl×F– Ugl. ×V,

where Tmaxgl. is the maximal transport of glucose from the primary urine towards the blood, that is the reabsorbed amount;

Pgl. is the concentration of glucose in the primary urine which is equal to its concentration in plasma; F is the volume of the primary urine, produced in a unit of time;

Ugl. is the concentration of glucose in the final urine; V is the volume of the final urine in a unit of time.

Here we have 2 unknowns: Tmaxgl. and F. To calculate F, besides glucose they also inject inulin in the blood calculating its clearance coefficient. In the norm the maximal transport value of glucose is 375 mg/min in male and 303 mg/min in female. Secretion. The secretion function has a special significance for the excretory function of the kidney, which is

323 also an active process and occurs in the opposite direction of reabsorption. The substances useless for the organism, passing to the interstitial tissue from the blood, are picked up by the carriers in the epithelial cells of the tubule’s wall. Then the combination breaks down and the substance is transported to the cavity of the tubule. Now the carrier returns to the epithelial cell for a new transportation of the given substance. This process needs expense of energy. Secretion occurs in the proximal and also in the distal convoluted tubules. The substances that are secreted are the following: K+, organic acids, bases, pigments, different medical preparations, especially antibiotics (penicillin). Synthesis. Synthesis appears that the epithelial cells forming the wall of the tubules are capable of picking up two different sources of substances and synthesizing a new substance that doesn’t exist in the blood. For example from glycocoll and benzoic acid found in blood a hypuric acid is synthesized. NH3 is also the result of a synthetic process, which is formed from the deamination of aminoacids. Thus the substances found in the final urine can be grouped into 4 categories according to the ways of their excretion: 1) substances that are only filtered, e.g. inulin, sulfates, creatinine; 2) substances that are filtered and are reabsorped, e.g. glucose; 3) substances that are filtered and undergone the secretion, e.g. paraamino-hypuric acid; 4) substances that are excreted from the organism by synthetic ways, e.g. NH3.

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Urine excretion and micturition

Composition of urine. During a day 1.5 l of final urine is produced, in which are found urea (25-35g), creatinine (1-

2g), uric acid (till 1g), NH3 (till 1g). There are also uropigments (urobilin, urochrom), which are formed in the intestine from bile pigments. In pathologic cases, acetone, cetonic bodies, proteins, glucose, fatty acids and formed blood elements also appear in the urine composition. Urine formed in the renal tubules passes into the renal pelvis and fills it gradually. When the filling threshold is reached the signal of which is accepted by the baroreceptors, the pelvis muscles contract and the uteral lumen opens, and the urine flows into the urinary bladder. When the volume of the urine reaches a definite level the mechanoreceptors are excited and the impulses pass along the afferent nerve to the sacral part of the spinal cord where the micturition centre is located. The impulses coming by efferent nerve cause contraction of the urinary bladder and relaxation of sphincter which promotes the flow of urine into the urethra and its distension. The spinal micturition centre is controlled by the cerebral cortex, midbrain, hypothalamus, etc.

Endocrine function of the kidney

In the kidneys different biologically active substances are produced, by which different homeostatic constants are regulated. 1. Renin is produced by the secretor cells that are found in the walls of the afferent arterioles, in that part where it

325 approaches the distal convoluted tubules, forming the juxtaglomerular complex. Renin is a proteolytic enzyme, which by passing to the blood acts on the angiotensinogen of the blood, detaching from it a peptide composed of 10 aminoacids, which is called angiotensin I, the latter is transformed into angiotensin II (a peptide composed of 8 aminoacids) due to other enzyme carboxipeptidase. Angiotensin II is one of the most powerful vasoconstrictor agents, by pressing the vessels; it helps to increase blood pressure. At the same time angiotensin II stimulates the production of aldosterone in the cortical part of adrenal glands. The aldosterone in the kidneys increases the reabsorption of Na+ which provides the conservation of water with the increasing of blood volume, and blood pressure. The production of renin is increased in case the kidneys are not being supplied well by blood, when the overall blood pressure is low, when the overall blood volume is decreased, and in case when the sympathetic nervous system is excited. Renal pathologies that are accompanied by the kidney’s blood hypo- supply, bring to hypertension (increase in blood pressure), because of the renin - angiotensin - aldosterone system activation. 2. Urokinase by passing to the blood activates the plasminogen protein, transforming it into plasmine, which takes part in the hemostasis process, it simulates fibrinolysis. 3. Hemopoietins (erythro-, leuco- thrombopoietins) are produced in the form of hemopoietinogenes, which after the activation stimulate hemopoiesis in the bone marrow.

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4. Prostaglandins. The representative of this group is medullin. It regulates the medullar blood supply. It is a vasodilator agent. 5. Kinins. Bradikinin is a vasodilator agent.

6. Calcipherole. The kidney uptakes Vit. D3 (non active form) from the blood and activates it, which has a significant role in Ca2+ metabolism regulating processes.

Regulation of kidney function

The function of the kidneys is regulated by neuronal and humoral ways. The kidneys receive sympathetic and parasympathetic nerves; nevertheless the neuronal regulation does not have a serious significance. This can be proved by depriving the kidneys from their nerves and monitoring the formation of urine, even in case of water overloading the kidneys function properly.

Hormonal regulation. 1. Antidiuretic hormone (ADH) is produced by the neurosecretory cells forming supraoptic and paraventricular nuclei of the hypothalamus. ADH stimulates the reabsorption of water in the collecting and distal convoluted tubules by the adenylatecyclase-dependent mechanism, so the volume of final urine decreases. The production of this hormone is stimulated when the blood’s osmotic pressure is high, also by nervous impulses, especially in pain feeling. The pain is accompanied by anuria.

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2. Aldosteron acts on the distal tubules of the nephron and stimulates the reabsorption of Na+ and the secretion of K+ ions. 3. Parathyroid hormone results in the reabsorption of Ca2+ in the kidneys, increasing its concentration in the blood, and also it brings to the excretion of phosphates. 4. Calcitonine is produced in the C-cells of the thyroid gland, inhibits the reabsorption of Ca2+ and phosphates in the kidneys. 5. Adrenaline is the hormone of the medulla of the adrenal glands. In small doses it presses the efferent vessels increasing the hydrostatic blood pressure in the capillary glomerulus and activating the filtration process. In large doses it also presses the afferent vessels, ensuring the opposite effect, even until anuria.

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LITERATURE 1. Human Physiology, ed. by G.I.Kositsky, Moscow, Mir, 1990, I volume, 286 p. 2. Human Physiology, ed. by G.I.Kositsky, Moscow, Mir,1990, II volume, 447 p. 3. A.C. Guyton and J.E. Hall - Text book of Medical Physiology, 10th Edition, Harcourt Asia PTE LTD; W.B. Saunders Company, 2001, 1064p. 4. W.F.Ganong – Review of Medical Physiology, 20th Edition, McGraw-Hill Companies, 2001, 870 p. 5. L. Praksam Reddy – Fundamentals of Medical Physiology, second edition, Paras Publishing, 2001, 514 p. 6. Physiology (Editors – Robert M. Berne, Mattew N. Levi, Bruce M. Koeppen, Bruce A. Stanton), 5th Edition, Elsevier, Inc., 2004, 1014 p. 7. Robert G.Carroll – Physiology (Elsevier’s Integrated), Mosby INC., Elsevier Inc., 2007, 239 p.

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Authors: 1. Anna Ter-Markosyan – Professor of the Department of Physiology of the Yerevan State Medical University after M. Heratsi 2. Knarik Harutunyan – Docent of the Department of Physiology of the Yerevan State Medical University after M. Heratsi 3. Karen Arakelyan - Docent of the Department of Physiology of the Yerevan State Medical University after M. Heratsi 4. Karine Avetisyan - Assistant of the Department of Physiology of the Yerevan State Medical University after M. Heratsi

Editor: Drastamat Khudaverdyan – Head of the Department of Physiology of the Yerevan State Medical University after M. Heratsi, professor

English language editor: Meline Bisharyan – Assistant of the Department of Foreign Languages of the Yerevan State Medical University after M. Heratsi

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