Sensory system

By: Dr.Fareeda Banu A.B Associate professor Dept of Physiology USM-KLE IMP OBJECTIVES  Regarding the components of sensory system and their function

 Define and classify sensory receptors and state their properties

 Explain the generation of receptor potential and its role in the stimulation of the afferent nerve

 Describe the ascending sensory pathways that sub serve perceived sensations and state the functions of collaterals from sensory pathway Introduction

 The is unique in the vast complexity of thought processes and control actions it can perform.

 It receives each minute literally millions of bits of information from the different sensory nerves and sensory organs and then integrates all these to determine responses to be made by the body.

 The central nervous system contains more than 100 billion . Which are the basic functional unit of CNS. 1) Definition: Conscious awareness of a particular feeling caused by stimulation of certain type of receptor by its adequate stimulus

2) Classifications:

Sensations

General/ Special Emotional Somesthetic Introduction

1. Special senses: Include 5 important sensations- Vision, smell, taste, and equilibrium.

2. Somaesthetic sense: Depending on point of origin are classified into three types: a. Cutaneous sensation: arising from surface of body; like touch, pressure, and temperature. b. Visceral sensations: arises from the deep viscera, i.e. from internal organs like liver , pancreas etc. c. Proprioceptive and Kinesthetic sensations: arising from muscles, tendons and joints.

3. Emotional sense: E.g. anxiety, fear, sadness

Classification of Sensations by Structural Complexity

Somatic (= general) Special senses senses 1. Vision 1. Touch 2. Hearing 2. Temperature 3. Taste 3. (pain) 4. Smell 4. Itch 5. Equilibrium 5. Proprioception 6. Stereognosis • Defn… Sensations arise from somatic structures of the body i.e. skin and deep tissues e.g. skin and muscles Types: 1. Mechanoceptive sensations: include a. Tactile: e.g. touch, pressure and vibration b. Proprioceptive e.g. sense of position and movement 2. Pain (nociceptive) sensation. 3. Thermal sensation; cold and warm. Somato sensory axis of the Nervous system  Sensory system transmits sensory information from the receptors of the entire body surface and from some deep structures.  This information enters the central nervous system through peripheral nerves and is conducted immediately to multiple sensory areas in  The at all levels  The reticular substance of the medulla,  and mesencephalon of the ;  The  The and  Sensory Areas of the . Somato sensory axis Components of Sensory System  The sensory division of the human nervous system includes following components: 1. Sensory receptors: These are specialized neurons that transduce stimulus energy into neural signals. 2. Afferent neurons: these carry sensory impulses to the sensory cortex and constitute the neural pathway, which consists of: • First-order neurons • Second-order neurons and • Third-order neurons 3. Sensory cortex: Includes the sensory areas of cortex; formed by the 4th order neurons. It is responsible for the conscious of the stimulus i.e. Sensation. Sensory Pathway

Stimulus

Sensory receptor (= transducer)

Afferent sensory neurons

Spinal cord

CNS (sensory cortex)

Integration, perception Sensory Receptors  Defn: These are specialized neuronal cells that receive stimuli from the external or internal environment and transduce these signals into nerve impulses.

 A stimulus is a change of environment of sufficient intensity to evoke a response in an organism; such as touch, sound, light, pain, cold, and warmth etc.

 The sensory receptors transduce sensory stimuli into nerve signals (Aps) that are then conveyed to and processed in the central nervous system (through ascending tracts). Types of Sensory Receptors…  Receptors can be grouped according to the stimulus source 1. Exteroceptors: These are close to the body surface and are specialized to detect sensory information from the external environment (such as visual, olfactory, gustatory, auditory, and tactile stimuli). These are further classified as: • Telereceptors (G. tele, “distant”), include receptors that respond to distant stimuli (Eg: Smell, vision and hearing), and do not require direct physical contact with the stimulus in order to be stimulated; • Contact receptors, which transmit tactile, pressure, pain, or thermal stimuli, require direct contact of the stimulus with the body. Receptors according to the stimulus source: 2. Proprioceptors: Proprioceptors transmit sensory information from muscles, tendons, and joints about the position of a body part, such as a limb in space and allow fine control of skeletal movements.

2. Interoceptors detect sensory information concerning the status of the body’s internal environment such as stretch, blood pressure, pH, oxygen or carbon dioxide concentration, and osmolarity.

Types of Sensory Receptors…

 Classification according to modality: Receptors are further classified into the following three categories according to the modality to which they respond: 1. Nociceptors (pain receptors), which detect damage occurring in the tissues, whether physical damage or chemical damage.

2. Thermoreceptors, which detect changes in temperature, with some receptors detecting cold and others warmth.

3. Mechanoreceptors, which detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor

Nociceptors  These are rapidly adapting receptors that are sensitive to noxious or painful stimuli. They are located at the free nerve endings of type Aδ or C fibers,. These are further classified into three types.

1. Mechanosensitive nociceptors (of Aδ fibers), which are sensitive to intense mechanical stimulation (such as pinching with pliers) or injury to tissues.

2. Temperature-sensitive (thermosensitive) nociceptors (of Aδ fibers), which are sensitive to intense heat and cold.

3. Polymodal nociceptors (of C fibers), which are sensitive to noxious stimuli that are mechanical, thermal, or chemical in nature. Although most nociceptors are sensitive to one particular type of painful stimulus, some may respond to two or more types. THERMORECEPTORS  These are sensitive to warmth or cold and are slowly adapting receptors which are further classified into three types: 1. Cold receptors, which consist of free nerve endings of lightly myelinated Aδ fibers.

2. Warmth receptors, which consist of the free nerve endings of unmyelinated C fibers that respond to increases in temperature.

3. Temperature-sensitive nociceptors that are sensitive to excessive heat or cold. Mechanoreceptor  Mechanoreceptors are activated following physical deformation of the skin, muscles, tendons, ligaments, and joint capsules in which they reside. These may be classified as non-encapsulated or encapsulated depending on whether a structural device encloses its peripheral nerve ending component.

. Meissner’s corpuscles are encapsulated and respond to changes in texture and slow vibrations. . Ruffini’s end organs are enlarged dendritic endings with elongated capsules, and they respond to sustained pressure. . Pacinian corpuscles are encapsulated by concentric lamellae of connective tissue that give the organ the appearance of a cocktail onion. Theses receptors respond to deep pressure and fast vibration.

The sensory nerves from these mechanoreceptors are large myelinated Aα and Aβ fibers whose conduction velocities range from ∼70–120 to ∼40–75 m/s, respectively.

Properties of Receptors

1. Specificity of response:

2. Effect of strength of Stimulus

3. Effect of velocity of stimulus

4. Projection.

5. Adaptation Properties of Receptors 1. Specificity of response: Each receptor is easily stimulated by only one type of appropriate adequate specific stimulus. This specificity is called law of adequate stimulus. Eg: Rods and cones – Light

2. Effect of strength of Stimulus: Receptor potential amplitude depends on the strength of stimulus. Greater the strength of the stimulus larger will be the amplitude of receptor potential. The magnitude of sensation felt is directly proportional to the log of intensity of stimulus which is known as Weber Fechner law. Properties of Receptors 3. Effect of velocity of stimulus: The magnitude of the receptor potential rises with rate of change of stimulus application. It also applies to removal of stimulus. Eg: off response

4. Projection: When any part of sensory path of a particular sense organ is stimulated, conscious sensation produced is referred to the location of the receptors, no matter how or where along the pathway the activity is originated. This principle is called law of projection. Eg- Phantom limb Properties of Receptors 5. Adaptation:  Another characteristic of all sensory receptors is that they adapt either partially or completely to any constant stimulus after a period of time.  That is, when a continuous sensory stimulus is applied, the receptor responds at a high impulse rate at first and then at a progressively slower rate until finally the rate of action potentials decreases to very few or often to none at all. This property is called adaptation.  Eg: the adapts very rapidly, hair receptors adapt within a second or so, and some joint capsule and receptors adapt slowly. 5. Adaptation:  Depending on the rate of adaptation the receptors are of two types: I. Tonic Receptors: These are slow and incompletely adapting receptors. These keep on firing action potentials continuously during stimulus application. These are important for life as they keep the brain constantly appraised of status of the body and its relation to its surroundings. Eg: Muscle spindles, Pain and cold receptors, Baro and chemo receptors II. Phasic receptors: These are rapidly adapting receptors, which fire APs at a progressively decreasing rate during stimulus application. These transmit signals only when the stimulus strength is changed. The receptor potential is short and decays rapidly. Eg: Meissner’s corpuscles and olfactory receptors. Tonic Receptors Phasic Receptors

 Slow or no adaptation  Rapid adaptation

 Continuous signal  Cease firing if strength of a transmission for duration of continuous stimulus stimulus remains constant

 Monitoring of parameters  Allow body to ignore that must be continually constant unimportant evaluated, e.g.: information, e.g.: baroreceptors  Smell Sensory Transduction… Question: How does a stimulus converted into a neural signal???

Steps of sensory transduction are: 1.Arrival of the stimulus to receptor: May be in the form of Mechanical force, light, chemicals like salt on tongue, cold or warm temperature, sound energy stimulating auditory receptors etc.

2. Production of generator or Receptor Potential.

3.Production of Action potential in the sensory or afferent nerves. Receptor Potential  When a stimulus excites the receptor, it changes the potential across the membrane of the receptor. This change in the potential is called Receptor or Generator potential.

 Mechanisms of Receptor Potentials:  The basic cause of the change in membrane potential is a change in membrane permeability of the receptor, which allows ions (Na+) to diffuse more or less readily through the membrane and thereby to change the transmembrane potential.

 Usually the current is inward which produces depolarization of the receptor. The exception is in the photoreceptors, where light causes hyperpolarization.

Properties of Receptor Potential

 It is a graded potential similar to EPP. Its amplitude increases with increasing velocity of stimulus application and increasing strength of stimulus.  The maximum amplitude of most sensory receptor potentials is about 100 millivolts, but this level occurs only at an extremely high intensity of sensory stimulus.

 When the receptor potential rises above the threshold for eliciting action potentials in the nerve fiber attached to the receptor, then action potentials occur.  It can be summated.  It is non-propagative and there is no refractory period.  Duration is greater than Action potential (5-10ms). Production of Action Potential in a

 The receptor potential developed in an unmyelinated nerve ending depolarizes the sensory nerve at the first node of Ranvier by electrotonic depolarization current sink action.  When the receptor potential rises above threshold level (10mV) it brings the membrane potential of the first node of Ranvier to the firing level causing production of AP.

 Which is propagated in the nerve fibre.  Thus the first node of Ranvier converts the graded potential of the receptor into AP.  Greater the magnitude of receptor potential greater is the AP discharge in nerve fibre.

SENSORY CODING  Converting a receptor stimulus to a recognizable sensation is termed Sensory coding.

 All sensory systems code for four elementary attributes of a stimulus: modality, location, intensity and duration.  Modality is the type of energy transmitted by the stimulus.  Location is the site on the body or space where the stimulus originated.  Intensity is signaled by the response amplitude or frequency of action potential generation.  Duration refers to the time from start to end of a response in the receptor SENSORY CODING SENSORY CODING..  When the nerve from a particular sensory receptor is stimulated, the sensation evoked is that for which the receptor is specialized no matter how or where along the nerve the activity is initiated.

 This principle, first enunciated by Johannes Müller in 1835, has been called the law of specific nerve energies.

 For example, if the sensory nerve from a Pacinian corpuscle in the hand is stimulated by pressure at the elbow or by irritation from a tumor in the brachial plexus, the sensation evoked is touch.

For example: if the sensory nerve from a Pacinian corpuscle in the hand is stimulated by pressure at the elbow, the sensation evoked is touch. LOCATION  The term sensory unit refers to a single sensory and all of its peripheral branches.

 The area of the skin or body surface that, when stimulated, leads to activity in the is called the receptive field for that neuron.

 Smaller the receptive field (finger tip and lips) the more precise the encoding of stimulus localization.

 Generally the areas supplied by one unit overlap and interdigitate with the areas supplied by others. LOCATION…  One of the most important mechanisms that enable localization of a stimulus site is lateral inhibition.

 Lateral inhibition enhances the contrast between the center and periphery of a stimulated area and increases the ability of the brain to localize a sensory input. Lateral inhibition underlies two-point discrimination. LOCATION…  Two-point discrimination: The ability to distinguish between two adjacent mechanical stimuli to the skin is greater on the thumb, fingers and lips, where the sensory units are small and overlap considerably, than on the back, where the sensory units are large and widely spaced. INTENSITY  The intensity of sensation is determined by the amplitude of the stimulus applied to the receptor.  As a greater pressure is applied to the skin, the receptor potential in the mechanoreceptor increases , and the frequency of the action potentials in a single axon transmitting information to the CNS is also increased.

 In addition the greater intensity of stimulation also will recruit more receptors into the receptive field.  As the strength of a stimulus is increased, it tends to spread over a large area and generally not only activates the sense organs immediately in contact with it but also “recruits” those in the surrounding area. DURATION  If a stimulus of constant strength is maintained on a sensory receptor, the frequency of the action potentials in its sensory nerve declines over time. This phenomenon is known as receptor adaptation or desensitization.

 The degree to which adaptation occurs varies from one sense to another.

 Receptors can be classified into: • Rapidly adapting receptors (phasic) : Meissner and Pacinian corpuscles • Slowly adapting receptors (tonic) : Merkel cells and Ruffini endings ASCENDING SENSORY PATHWAYS ASCENDING TRACTS • These are the Bundles or fasciculi of fibers that occupy definite positions in the . • They have the same Origin, Termination and carry the same Function.

• These are the long tracts which serve to join the brain to the spinal cord. • Carry impulses from pain, thermal, tactile, muscle and joint receptors to the brain.

• Some of this information eventually reaches a conscious level (the cerebral cortex), while some is destined for subconscious centers (e.g. the cerebellum).

Sensory Pathways for Transmitting Somatic Signals into the CNS  From the entry point into the cord through the dorsal roots of the spinal nerves and then to the brain, the sensory signals are carried through three major sensory pathways: 1. The Medial lemniscal system- Dorsal column Tracts (Gracile & Cuneate fasciculi) 2. The Anterolateral system a. Spinothalamic pathway b. Spinocerebellar pathway

The sensations carried by Ascending pathways

 Dorsal column pathway ( lemniscus medialis) “conscious” proprioception: joint position vibration deep pressure two point discrimination graphaesthesia ! stereoaesthesia !

 Spinothalamic system pathways: pain temperature light touch

 Spinocerebellar pathway : “unconscious” proprioception

• Sensory information is transmitted to higher brain centers, generally by a sequence of three neurons and interneurons: . A first order neuron (pseudounipolar neuron) whose cell body is located in a . It transmits sensory information from peripheral structures to the dorsal (posterior) horn of the spinal cord.

. A second order neuron whose cell body is located within the dorsal horn of the spinal cord, and whose axon usually decussates and ascends: • In the of the ALS () to in the contralateral thalamus, and sending some collaterals to the .

• 2nd order Neuron: • In the indirect pathway of the ALS (spinoreticular tract) to synapse in the reticular formation, and sending some collaterals to the thalamus; or as spinotectal, spinomesencephalic, or spinohypothalamic fibers to synapse in several nuclei

• A third order neuron whose cell body is located in the thalamus, and whose axon ascends ipsilaterally to terminate in the somatosensory cortex. Dorsal column Tracts  These pathways involve three orders of neurons in series.

 Sensory information from proprioceptors and pressure receptors is first carried by large, myelinated nerve fibers that ascend in the dorsal columns of the spinal cord on the same (ipsilateral) side.

 These fibers entering the dorsal columns do not synapse and pass uninterrupted up to the dorsal medulla, where they synapse in the (the cuneate and gracile nuclei).

 Forms two tracts:  Fasciculus Gracilis (tract of Gall).  Fasciculus Cuneatus (tract of Burdach).

Dorsal Column Tracts

 Origin: From the of 1st order neuron in posterior root of ganglia. Majority of fibers end in medulla in nucleus gracilis and nucleus cuneatus (1st relay station)

 Fasciculus Gracilis receives afferents from lower half of body (sacral, lumbar and lower thoracic levels)

 Fasciculus Cuneatus receives afferents from upper half of the body (cervical + upper thoracic segments).

• Second-order neurons arises from here whose axons are divided into 2 groups: a. External arcuate fibers, subdivide into dorsal and ventral group. Dorsal group end on same side in cerebellum and ventral group cross to opposite side and end in cerebellum. b. Internal arcuate fibers, decussate immediately to the opposite side of the brain stem and continue upward through the medial lemnisci to the thalamus.  In this pathway through the brain stem, each is joined by additional fibers from the sensory nuclei of the trigeminal nerve (head).

 From the ventrobasal complex of Thalamus (2nd relay station), third-order nerve fibers project mainly to the of the cerebral cortex, which is called somatic sensory area I. Dorsal column Tracts  The types of sensations transmitted by Dorsal column tracts are: 1. Fine Touch sensations requiring transmission of fine gradations of intensity/low threshold 2. Touch sensations requiring a high degree of localization of the stimulus-Tactile localization 3. Two-point discrimination 4. Vibratory sensations and sense of deep pressure. 5. Proprioceptive (position sensations from the joints) and Kinesthetic (joint movements) sensation. 6. Stereognosis. Dorsal column sensations  Vibratory sensations : Are felt when there are rhythmic vibrations in the force.

 Proprioception and Kinesthesia: Proprioception means the sense of the body’s position in space and Kinesthesia refers to the sensation associated with joint movement.

 Stereognosis is the perception of the form and nature of an object without looking at it . • This ability depends on relatively intact touch and pressure sensation and is compromised when the dorsal columns are damaged. • The inability to identify an object by touch is called tactile Agnosia. The anterolateral system  Sensations of touch, pressure, heat, cold, and pain are carried into the spinal cord by thin myelinated axons (A- delta fibers ) and thin unmyelinated axons ( C fibers ).

 Fibers that mediate pain and temperature cross over to the contralateral side and ascend to the brain in the lateral spinothalamic tract.

 Fibers that mediate touch and pressure ascend in the anterior spinothalamic tract.

 Because of crossing over, somatesthetic information from each side of the body is projected to the postcentral gyrus of the contralateral cerebral hemisphere. The anterolateral system

 The types of sensations transmitted by anterolateral system are: 1. Pain 2. Thermal sensations, including both warmth and cold sensations 3. Crude touch and pressure sensations capable only of crude localizing ability on the surface of the body 4. Tickle and itch sensations 5. Sexual sensations Tracts of anterolateral system  It includes the following major tracts: 1. Ventral/anterior spinothalamic tract 2. Lateral spinothalamic tract 3. Posterior/Dorsal 4. Anterior/VentralSpinocerebellar tract  Others: Spino reticular (arousal and alertness) Spinotectal (spinovisual reflexes) and Spino-olivary (Proprioception)

 Origin: All tracts originate from the axons of 1st order neuron in posterior root of ganglia. Ventral/anterior spinothalamic tract

 Receptors: Cutaneous receptors  1st order neurons end around the chief sensory cells in dorsal horn.

 Fibers of 2nd order neuron cross to opposite side and ascend in anterior column of spinal cord.  Then relay in VP nucleus of thalamus.

 Finally the 3rd order neurons from the thalamus ends in sensory cortex.  Conveys Crude touch and Pressure sensation. Lateral spinothalamic tract

 It carries all types of pain and temperature sensations (both hot and cold)

 After entering the spinal cord, the fibers of the 1st order neurons end around nociceptive neurons of the dorsal horn cells of spinal cord.

 Aδ group of fibers terminate in laminas I and V

 C group of fibers terminate in substantia gelatinosa and laminas IV and V. Lateral spinothalamic tract  Axons of second order neurons cross obliquely to opposite side of the same segment, and ascend as lateral spinothalamic tract to end in the VP nucleus of thalamus

 At higher brain stem level, this tract sends several collaterals into the reticular formation and before ending in the thalamus.

 The axons of third order neurons arising from the VP nucleus of thalamus end in the somato-sensory cortex. Posterior/Dorsal Spinocerebellar tract  Receptors: Muscle spindles, and other proprioceptors.

 These carry subconscious kinesthetic sensations from the upper part of body to the cerebellum (C7-T6).

 Unlike ventral spinocerebellar, the Axons of second order neurons of dorsal tract are uncrossed.

 Ascend upwards in the lateral funiculus of same side and reach medulla. Finally end in the cerebellum through inferior cerebellar peduncle. Ventral Spinocerebellar tract

 Receptors: Muscle spindles, Golgi tendon organ and other proprioceptors.  These carry subconscious kinesthetic sensations from the lower part of body to the cerebellum (L3-L5) and regulate body posture.  Fibers of the 1st order neuron end around the clark’s column of cells in the spinal cord  Axons of second order neurons cross to opposite side and ascend in the lateral funiculus.  Finally end in the cerebellum through superior cerebellar peduncle. Role of Thalamus in Somato sensory system  Ventral posterior nucleus of thalamus is concerned with .  It has two divisions: 1. Ventral posterior lateral nucleus and 2. Ventral posterior Medial nucleus

 Topographic representation of the body is seen in VPN of thalamus as follows: Fibers carrying sensation from • Face region- most medial part of nucleus • Arm region- middle part of nucleus • Leg region- lateral most part of nucleus Somato sensory function of Thalamus  Thalamus acts as: • Sensory relay centre • Centre for integration of sensory impulses and • Crude centre for perception of sensations. • Pain sensations are perceived in the thalamus itself

 All other sensations are transmitted to the cerebral cortex by third order neurons except pain sensation. Somato sensory cortex  VPL thalamic neurons carrying sensory information project in a highly specific way to the to the somatosensory (somesthetic) cortex.  The primary somatosensory cortex (S-I) consists of the postcentral gyrus of the parietal lobe, which corresponds to Brodmann’s areas 3a, 3b, 1, 2.  The secondary somatosensory cortex (S-II) consists of Brodmann’s area 43, located on the superior bank of the lateral fissure, at the inferior extent of the primary motor and sensory areas. Somato sensory area I/ Primary Somato sensory cortex

 Somatosensory area I has a high degree of localization of the different parts of the body but localization is poor in somatosensory area II. Somato sensory area I  Topographical organization: • SI receives sensory inputs from opposite half of body. • The arrangement of thalamic projections is such that the body is represented upside down with the legs on the top and head at the foot of the gyrus. • The extent of its representation depends on the parts of the body that are densely innervated (i.e number of sensory receptors) • Hence fingers, thumb, lips and tongue are represented by larger areas.

NEUROPHYSIOLOGY OF PAIN Physiology of Pain  Defn: An unpleasant sensory & emotional experience associated with actual or potential tissue damage.

 According to Sherrington pain is the psychical adjunct of an imperative protective reflex.

 Stimulus that elicits pain is called noxious or nociceptive stimulus.

 Its different at different tissues like pricking, cutting, burning (skin), inflammation or ischemia (GIT/muscle) etc. Physiology of Pain  Receptors: Nociceptor usually have larger receptive field and are three types:

1. Mechanosensitive nociceptors (of Aδ fibers), which are sensitive to intense mechanical stimulation (such as pinching with pliers) or injury to tissues.

2. Temperature-sensitive (thermosensitive) nociceptors (of Aδ fibers), which are sensitive to intense heat and cold.

3. Polymodal nociceptors (of C fibers), which are sensitive to noxious stimuli that are mechanical, thermal, or chemical in nature. Although most nociceptors are sensitive to one particular type of painful stimulus, some may respond to two or more types. Lateral Spinothalamic Tract • Axons of 1st order neurons terminate in the dorsal horn

• Axons of 2nd order neuron (laminae I,II,V)), decussate within same segment of their origin, by passing through the ventral white commissure & terminate on 3rd order neurons in ventral posterior nucleus of the thalamus

• Thalamic neurons project to the somatosensory cortex

Lateral Spinothalamic  The axons of the SecondTract order neurons course in either the direct (spinothalamic) Or indirect (spinoreticular) pathways of the ALS.

 Approximately 15% of nociceptive fibers project directly to the thalamus whereas 85% project to the thalamus via a relay in the reticular formation.

 Hence there are two types of pain pathway: 1. Direct pathway: also known as Neospinothalamic

2. Indirect Pathway : also known as Paleospinothalamic:

Pain pathways: 1. Neospinothalamic: • Direct pathway of ALS. • spinal cord →thalamus is phylogenetically a newer pathway, • Hence it is known as the Neospinothalamic pathway.

2. Paleospinothalamic: • Indirect pathway of the ALS: • Spinal cord →reticular formation →thalamus) • Phylogenetically an older pathway,& is referred as the Paleospinothalamic pathway Pain pathways: Cell bodies of third order neurons of the nociception- relaying pathway are located in: the ventral posterior lateral, the ventral posterior inferior, and the intralaminar thalamic nuclei.

 The fibers from VPLN terminate in the primary & secondary somatosensory cortex, S-I and S-II. Pain pathways divided into 2 types: Paleospinothalamic Neospinothalamic • Oldest • New • Carries slow pain • Carries fast pain • Fibers are C fibers • Aδ fibers • 1st order neuron ends in • 1st order neuron – lamina laminae II (SG) and III I & V • Fibers project to RF, • Few non-specific nuclei & projections hypothalamus (non- specific) Both Transmit Nociceptive, Thermal, and Nondiscriminatory (crude) touch sensations

DifferentDifferent types types of pain of Painsensation

• Sharp, • Fast pain & Slow pain • Stabbing types of pain • Dull, • Superficial pain & Deep pain • Burning • Aching • Somatic pain & Visceral pain • Throbbing, • Aching types. • Peripheral & Central pain • Lacerating, cutting, crushing etc • Physiologic & Pathologic pain Fast pain Slow Pain • Nerves are called A-delta • It is transmitted by very fibers. (30 meter/second) thin nerve fibers, called C-nerve fibers. • - ( 2meters/sec)

Glutamic acid • Neurotransmitter- SubstanceP • This is all to make the body withdraw • Body response - immediately from the immobilization painful and harmful (guarding, spasm or stimulus, in Order to rigidity), so that healing avoid further damage. can take place

Superficial pain Deep Pain • Fast pain • Slow pain

• Involves skin & • Muscles& hollow subcutaneous tissue viscera

• Dull &poorly localized • Sharp in character & well localized • Produces faintness, nausea, vomiting, • Leads to increase in HR, sweating, decrease in BP & respiration HR & BP

• Do not radiate • Radiates to distance site Visceral pain Ischemic/ Muscular pain  Pain originating from Visceral structures  Release of pain  Diffuse in nature & poorly producing substance localized called Lewis P- factor

 Associated with autonomic  Lewis P- factor: symptoms  Radiates/referred to other consists of K+, adenine, structures nucleotides & lactic acid

 Afferents: autonomic fibers eg.  Eg: Intermittant Pain from GIT carried by Vagus claudication pain, pain nerve of angina pectoris  Tract :lateral spinothalamic tract Substances related with pain…  The substances released from the traumatized tissue are: prostaglandins, bradykinin, serotonin, substance P, histamine.

 Non-steroidal anti-inflammatories, such as ibuprofen, are effective in minimizing pain because they minimize the effects of these substances released, especially prostaglandins.

 Pain Inhibitors: Serotonin, Endorphins, Enkephalins and Dynorphins.

Mechanisms associated with peripheral sensitization to pain