NEUROLOGY the science of the nervous system General data The most important characteristic of living human is its capacity to move. Every living organism receives stimuli from its environments (external and internal) and responds to such stimuli by corresponding reactions which provide the contraction of muscular tissue, production of the glands secretion etc. In human organisms the area receiving the stimulus and the reacting organ are connected by the nervous system. Branching into all the organs and tissues, the nervous system binds and integrates all parts of the organism into a single, unified whole. Thus, the nervous system is an indescribably complex and fine instrument of relations involving the connection of numerous parts of the organism between one another and with the organism as a whole in a complex system with an infinite number of external influences (nature and society). The activity of the nervous system is based on the reflex (I.M. Sechenov). This means that a nervous receptor receives a signal from an agent of the external or internal environments of the organism. This signal is transformed into a nerve impulse, which passes along nerve fibers, as along wires, to the central nervous system and, from there, along established connections of other wires to the organ itself where it is transformed, into a specific process of the cells of this organ. The basic anatomical element of the nervous system is the nerve cell which, together with all the processes arising from it, is called the neuron. A long process, called the axon arises from the body of the cell in one direction. Short branched processes called dendrites lead in the other direction. Nervous impulse inside the neuron flows from the dendrites to the cell body and from there to the axon; the axons convey the nervous impulse away from the cell body. The nerve impulse from one neuron to another is provided by means of specially structures called the synapses. Can also be distinguished: axo-somatic connections of neurons in which the branches of one neuron approach the cell body of another neuron, axo-dendritic connections in which contact is accomplished by the dendrites of nerve cells. Also could be axo-axonic dendro- dendritic synapses. The adult human brain is estimated to contain from 1014 to 5 × 1014 (100– 500 trillion) synapses. The intermittent flow of nerve conduction throughout the body allows for a great 1 variety of connections. Thus, the nervous system is composed of a complex of neurons which come into contact with one another but never grow together. Consequently, the nervous impulse that arises in one part of the body is conveyed along the processes of nerve cells from one neuron to a second, from there to a third, and so on. The connection established between organs through the neurons is the reflex arc which forms the basis of the reflex, the simplest and at the same time most fundamental reaction of the nervous system. The simple reflex arc is composed of at least two neurons, one of which connects with a sensory aria (the skin, for example) the sensory neuron and the other, which, with its axon, ends in a muscle (or a gland) the motor (secretor) neuron. The nervous impulse is transferred centrifugally to the muscle or gland, when the sensory aria is stimulated, as a result the muscle contracts or the secretion of the gland changes. There is the simple reflex arc when reaction happened in limit of one segment of the central nervous system (bi- or tri-neuronal reflex arcs). Besides the simple reflex arc there are complex multineuronal reflex arcs passing through different hierarchical levels (segments) of the brain, including the top level - cerebral cortex. In man and other higher animals neurons also form temporary reflex connections of the highest order on the basis of simple and complex reflexes. These temporary reflex connections are known as conditioned reflexes. Therefore, the elements of the nervous system could be classified in 3 types, according to function. 1. Receptors transform the energy of the external stimulus into a nerve impulse; the receptors are connected with afferent (centripetal or receptor) neurons. This part informs the man about external or internal media. 2. Connecting neurons which transfer of the nerve impulse from the centripetal to the centrifugal neuron (the most - 99% of neurons). 3. Efferent (centrifugal) neurons realize the reactions (motor or secretory) by conducting the nervous impulse from the centre to the effector (the producer of the effect or the action) at the periphery, to the working organ (muscle, gland). This is why this neuron is also called the effector neuron. The receptors are stimulated by three sensory surfaces (receptor fields) of the organism: 1. the skin surface of the body (exteroceptive field) receive stimuli from the external environment; 2. the internal surfaces of the body (interoceptive field) stimulated mostly by chemical substances entering the internal cavities; 3. the locomotor apparatus of the body itself (proprioceptive fields) where the bones, muscles, and other organs produce stimuli received by special receptors. The receptors from such fields are connected with afferent neurons (bipolar or pseudounipolar) which reach the centers of central nervous system and transfer later to various efferent conductors by a very complicated system of conductors. These efferent conductors produce various effects in conjunction with the working organs. The physiology has established the common feedback principle of connections in the control and coordination of processes of living organisms. From this point of view a feedback connection can be distinguished in the nervous system between the working organ and the nerve centers. This phenomenon called feedback afferentation involves the transmission of impulses at any moment from the working organ to the central nervous system. When the centers of the nervous system send efferent impulses to the working organ, certain actions (movement, secretion) are activated in this organ. These actions, in turn, stimulate nervous (sensory) impulses which return along afferent ways to the central nervous system signaling that a special action just has been performed by the working organ (the result). Thus, the real meaning of feedback afferentation is to adjust the answer, to be more and more precise. Without the feedback mechanism living organisms would fail to adapt practically to the environment. The human nervous system is functionally divided into two parts: Somatic and Vegetative: 1) the somatic (animal) nervous system controls the striated musculature of the skeleton and some internal organs (tongue, larynx, pharynx). 2) the vegetative nervous system innervates the internal organs, the endocrine system, and the smooth muscles of the skin, heart, and vessels (the organs of vegetative life which create the internal environment of the organism), it is sometimes called autonomous or visceral nervous system; The vegetative part of the nervous system is subdivided into the sympathetic and parasympathetic systems. Both will have opposite action for certain organs. According by location (topography) the nervous system can be classified into central 3 and peripheral systems. 1. The central nervous system consists of the spinal cord and brain 2. The peripheral system includes all other components: the nerves from brain (cranial nerves) and spinal cord (spinal nerves), the ganglia, plexuses, nerves and peripheral nerve endings (receptors). The grey matter of the spinal cord and the brain is an accumulation of nerve cells together with the nearest branches and their processes called nerve centers. The gray matter could be organized in 2 types of structures: as nuclei or cortex. The white matter consists of nerve fibers (processes of nerve cells, axons) covered by a myelin sheath connecting the different centers together, i.e. the conducting pathways. Both the central and peripheral parts of the nervous system contain elements of its somatic and vegetative components, thus uniting the nervous system as a whole. GENERAL DEVELOPMENT OF THE NERVOUS SYSTEM The nervous system originates from the outer germinal layer of the embryo, or the ectoderm. The ectoderm forms a longitudinal thickening called the neural plate which is bounded on the sides by the remaining part of the ectoderm. The neural plate soon transforms into a neural groove whose margins (neural folds) are gradually raised, approach each other, and fuse so converting the groove into a tube (neural tube). After fusion of the margins the neural tube separates completely from the skin ectoderm and the mesoderm grows between them. The neural tube is the rudiment of the central part of the nervous system. The caudal end of the tube forms the origin of the spinal cord, while the expanded cephalic end is separated into 3 primary brain vesicles (4th week): rhombencephalon, mesencephalon and prosencephalon. After one week (5th) first and last primary vesicles will divide to obtain following next secondary vesicles: myelencephalon, metencephalon, mesencephalon, diencephalon and telencephalon from which develops the brain in all its complexity. At first the neural plate consists only of a single layer of epithelial cells. During its closure to form the neural tube the number of cells in the wall of the tube increases as a result of which three layers are formed: an inner layer (facing the cavity of the tube) from which the epithelial lining of the cerebral cavities is derived (the ependyma of the central canal of the spinal cord and the ventricles of the brain); a middle layer which gives rise to the grey matter of the brain (the nerve cells, neuroblasts); and, finally, an outer layer almost devoid of cell nuclei which develops into the white matter (processes of the nerve cells, axons or neurites). Bundles of the neuroblast axons spread either in the thickness of the neural tube to form the white matter or run off it to pass into the mesoderm and then become joined with the young muscle cells (myoblasts).