Neurophysiology PDF Lecture Notes

Neurophysiology Dr. Adel Hussien Prof. of Medical Physiology Physiology dept.-Faculty of Medicine – Minia University Learning objectives: By the end of this lecture student will be able to 1- List different types of sensations 2- Classify somatic sensations 3- Discuss mechanical sensations (t...

Neurophysiology Dr. Adel Hussien Prof. of Medical Physiology Physiology dept.-Faculty of Medicine – Minia University Learning objectives: By the end of this lecture student will be able to 1- List different types of sensations 2- Classify somatic sensations 3- Discuss mechanical sensations (types, characters) 4- Discus thermal sensation (types, mechanism and characters) 5- Discus pain sensation (mechanism, types, body reactions to pain)  Sensations Sensation is the appreciation (understanding) of the meaning of a stimulus Sensations in the body include (1) Somatic sensations: sensations from skin and deep structures like muscles, joints and bones (1) Visceral sensations: from viscera e.g. GIT, heart, kidney… (1) Special sensations: vision, hearing, smell and taste (1) Organic sensations: hunger, thirst and sexual desire  Classification of somatic sensations (A) According to the site of sensation 1- exteroceptive sensations (superficial sensations) These are the skin sensations e.g. pain, touch and temperature sensation 2- interoceptive sensations (deep sensations) These are sensations from skeletal muscles, tendons, joints, bones, ligaments… They include the following types a- proprioceptive sensations (kinesthetic sensation): include sense of position and sense of movement b- pressure sense c- muscle tension sense d- muscle sense (pressure pain): resulting from squeezing the muscle with hands 3- combined sensations: vibration sense (B) According to the modality of sensation 1- Mechanoreceptive sensations Tactile sensations (touch, pressure, vibration and tickle) Position and movement sensations (proprioceptive sensations) 2- Thermoreceptive sensations Heat and cold sensations 3- Pain sensation Cutaneous, deep, visceral or neuropathic pain sensation  1- Mechanoreceptive sensations Mechanoreceptive sensations include the following I- Touch sensation II- pressure sense III- Vibration sense IV- Tickling and itching V- proprioceptive (Kinesthetic ) sensation: includes - Sense of position (static proprioception) - Sense of movement (dynamic proprioception)  I- Touch sensation A- Crude touch Poorly localized touch sensation (Feeling of clothes and hair comb) Receptors: free nerve endings – hair end organs Afferent nerves: Aδ fibers Central pathway: Ventral spinothalamic tract Test: By stroking the skin lightly by a piece of cotton B- Fine touch This include 1- tactile localisation 2- tactile discrimination 3- stereognosis and 4- texture of material Receptors: Meissner’s corpuscles and Merkel’s disks Afferent nerves: Aᵦ fibers Central pathway: Gracile and cuneate tract (dorsal column) 1- tactile localisation: localising the point of touch with both eyes closed 2- tactile disrimination: 2 point discrimination Feeling 2 points touched simultaneously as 2 separate points with both eyes closed The distance between the 2 touched stimuli should be above the threshold distance (very narrow at the finger tips and lips 2mm and most wider in the back 5-20mm) Tactile localization and discrimination depend on - Number of receptors in the area - Receptive field of the receptors - Presence of convergence in the pathway - Area of cortical representation 3- Stereognosis Recognizing familiar objects ( key or coin) put in the hands with both eyes closed It depends on a- Cutaneous and deep sensations b- previous knowledge about the object 4- Texture of material Knowing the texture of materials whether it is made of silk, wool, cotton.. by touching the materials with both eyes closed  II- Pressure sense Knowing the weights of objects and discrimination between different weights by pressure on hands Receptors: Mainly Pacinian corpuscles Afferent nerves: Aᵦ fibers Central pathway: Gracile and cuneate tract (dorsal column)  III- Vibration sense Rhythmic repetitive pressure sensation Feeling the vibration of the tuning fork put on bony prominencies (although bone insenstive to vibration but it magnifies the vibration waves) vibration sense of the body ranges (30-800Hz) Receptors: Meissner’s corpusles. Respond to low frequency vibration (80Hz) Pacinian corpuscles. Respond to high frequency vibration (500Hz) Afferent nerves: Aᵦ fibers Central pathway: Gracile and cuneate tract (dorsal column)  IV- Tickling and itching Tickle: pleasurable sensation (often causing laugh) that results from mild tactile stimulation of the skin Itch: annoying sensation that results from skin irritation by - Moving tactile stimuli e.g. crawling flea - Chemical substances released in the skin e.g. histamine in allergic conditions Receptors: free nerve endings Afferent nerves: unmyelinated C fibers Central pathway: Ventral spinothalamic tract  V- Proprioceptive sensations These sensations arise from deep structures specially muscles and joints Divided into the following 2 types 1- Sense of position (Static proprioception ) Position of different parts of the body in space 2- Sense of movement (dynamic proprioception) Sensation of movement and rate of movement of different parts of the body Receptors: present in joint capsule, ligaments, tendons, tissue around joints Slowly adapting Ruffini’s end organs for sense of position Rapidly adapting Pacinian corpuscles for sense of movement Afferents: Aα and Aᵦ fibers Central pathway: Gracile and cuneate tract (dorsal column)  2- Thermoreceptive sensations It is the perception of different gradations of cold and heat Types of thermal receptors 1- Warm receptors: free nerve endings attached to C fibers 2- Cold receptors: free nerve endings attached to C and Aδ fibers 3- Two subtypes of pain receptors - Cold pain receptors: give freezing cold sensation - Heat pain receptors: give burning hot sensation Mechanism of stimulation: Thermoreceptors are stimulated chemically by the changes in the concentration of metabolites in the receptors Increase temp…..increase metabolic rate of receptor Decrease temp…decrease metabolic rate of receptor Characters of thermal receptors: 1- Located immediately under the skin..detect temperature of subcutaneous tissues not the environment 2- They have small receptive field and widely separated (spatial summation from stimulation of large skin area) 3- Cold receptors are 4-10 times more numerous than warm receptors 4- Thermoreceptors are moderately adapting receptors but cold receptors adapt more slowly than warm receptors 5- Distribution: more in lips>fingers>trunk Discharge from Thermoreceptors: 1- Cold receptors Stimulated from 10-43oC maximum At 25oC 2- Cold pain receptors Stimulated from 5-15oC maximum At 5oC 3- Warm receptors Stimulated from 30-50oC maximum At 45oC 4- Heat pain receptors Stimulated at 45oC and above At 0oC all receptors stop discharge. It is used as a method for local anaesthesia  3- Pain sensation Pain is an unpleasant sensation despite that it protect the body when there is tissue damage Pain sensation cause the individual to react to remove the pain stimulus or seek medical advice Mechanism of pain sensation: Damaged tissues & the surrounding blood vessels release chemical mediators of pain (pain producing substances- pain sensitizers) as Histamine, serotonin, prostaglandins, bradykinins, K+ ions, proteolytic enzymes These pain producing substances increase sensitivity of pain receptors and decrease their pain threshold Transduction of pain signals: 1- Stimulation of nociceptors by specific stimulus..open specific transduction channels as a- Degenerin channels: Opened by injurious mechanical stimuli b- Vanilloid channels: Opened by extreme temp. and capsaicin c- Acid sensing channels: Opened by chemical stimuli 2- Opening of these channels…Na+ and Ca2+ inflow..depolarization 3- Activation of pain afferent neurons…release of neurotransmitter substances to spinal cord (Aδ fibers release glutamate while C fibers release substance P) Pain receptors: Free nerve endings are slowly adapting and has 4 types 1- Mechanical pain receptors. Stimulated by mechanical injurious stimuli 2- Thermal pain receptors. Stimulated by extremes of temp. (as cold pain and heat pain receptors) 3- Chemical pain receptors. Stimulated by chemical injurious stimuli as pain of gastric ulcer due to excess HCl 4- Polymodal pain receptors..respond to all types of stimuli Pain receptors present more in skin, periosteum, arterial wall, joint surfaces, falx cerebri and tentorium cerebelli and absent in liver parenchyma, lung alveoli, Brain tissue, Bones Afferent fibers: Aδ & C fibers Central pathway: Lateral spinothalamic tract Types of pain sensation: pain is classified according to A- Site of pain: into 1- Cutaneous pain 2- Deep pain 3- Visceral pain 4- Neuropathic pain B- Quality of pain: into 1- Fast (acute sharp pricking pain) 2- Slow (chronic burning dull aching pain)  (1)Cutaneous pain It is the pain produced by stimulation of pain receptors in the skin It is accurately localised (unlike other types) due to 1- Large number of pain receptors in the skin 2- The fast pain reaches the sensory cortex 3- Touch and vision help in localisation It occurs in 2 phases: fast pricking followed by slow burning pain  Differences between fast and slow pain Fast pain Slow pain Site (origin) Skin mainly, may occurs in Skin, deep tissues & viscera pleura, pericardium and peritoneum Stimulus Mainly the mechanical and All noxious stimuli specially thermal noxious stimuli the chemical ones Quality Pricking (sharp-acute) well Burning (aching-chronic) localised poorly localised Perception 0.1 sec after stimulation 1 sec or more after stimulation Duration Short duration Long duration Afferent nerve Aδ fibers..glutamate C fibers..substance p Tract neospinothalamic paleospinothalamic Center Mainly Cerebral cortex Mainly thalamus Blocked by Pressure and hypoxia Local anaesthetics Reaction to pain Somatic protective reflexes Autonomic & somatic reflex Threshold of pain: minimal intensity of stimulus (heat) that will produce pain when applied for a prolonged period of time pain threshold is the same for all individuals (when skin temp. reaches 45oC or more) The individuals differ only in their reactions to pain Reactions to pain: Reactions to pain take the following forms (1) Somatic reflexes: protective reflexes, their center in spinal cord A- Withdrawal reflex e.g. flexion withdrawal reflex B- immobilisation reflex e.g. rigidity of overlying muscles (guarding) (2) Autonomic reflexes: - Moderate pain..sympathetic effects (tachycardia-hypertension) - Severe pain..parasympathetic effects (bradycardia-hypotension) (3) Emotional reactions: crying, anxiety, depression (4) Cutaneous hyperalgesia (tenderness): Pathological hypersensitivity to pain that commonly accompanies skin injuries, inflammation and burns Pain is aroused by non noxious stimuli which are normally not painful e.g. touch elicited pain sensation Hyperalgesia is of 2 types A- Primary hyperalgesia: at the site of injury B- Secondary hyperalgesia: healthy area around injury Differences between primary and secondary hyperalgesia  (2) Deep pain Pain produced from muscles, joints, ligaments & periosteum of bones It is a slow pain conducted by C fibers Characters: Diffuse, Dull aching, associated with depressor effects (bradycardia-hypotension) Causes: inflammation, ischemia, muscle spasm, bone fractures Ischemic pain Type of deep pain felt in muscles when their blood supply is decreased Blood supply may decrease due to: 1- Intravascular thrombosis (clot formation) 2- Narrowing of lumen of blood vessels due to inflammation or compression Mechanism of ischemic pain - accumulation of lactic acid - release of proteolytic enzymes from ischemic tissue Examples of ischemic pain 1- Cardiac muscle ischemia…Angina pectoris 2- Skeletal muscle ischemia….intermittent claudications Ischemic pain is aggravated by exercise and relived or decreased by rest  (3) Visceral pain Pain from internal viscera of the thorax, abdomen & pelvis Visceral pain receptors are few in number so its stimulation requires diffuse type of stimulation not sharp localised one (sharp cut in viscera does not cause pain) Afferent: - From viscera along C fibers - From peritoneum, pleura or pericardium along Aδ fibers Causes of visceral pain: 1- Ischemia..accumulation of metabolites 2- Inflammation..of peritoneal covering 3- Irritation by chemical agents..HCl in peptic ulcer 4- Compression or infilteration by tumor 5- Overdistention of viscus e.g. Urinary bladder 6- Spasm of a hollow viscus e.g. colon, gall bladder, ureters, uterus…. Characters of visceral pain: 1- Dull aching or rhythmic cramps (colics) 2- Diffuse (poorly localised) 3- Depressor autonomic changes (bradycardia, hypotension) 4- Rigidity of overlaying muscles 5- Referred to body surface (causing referred pain sensation)  Referred (radiating) pain This is pain that is felt away from its original site It is most frequent with visceral pain and also may occur with deep pain Examples: 1- Cardiac pain is referred to left shoulder and inner side of left arm 2- Gall bladder pain is referred to tip of right shoulder 3- Renal pain is referred to the Loin and testicular region 4- Gastric pain is referred to epigastrium 5- Appendicular pain is referred to umbilical region 6- Headache is a type of referred pain  Mechanism of referred pain: convergence projection theory - Afferent nerves from both somatic and diseased viscus that develop from the same embryonic segment converge on the same neurons of spinothalamic tract - The cortex is accustomed to receive pain sensation from the skin and not from the viscera (less sensitive), so pain from the viscera is projected to the skin  Headache It is a type of referred pain from deep structures to the surface of the head Brain itself is insensitive to pain Pain sensitive intracranial structures include arteries, venous sinuses, Dura matter Causes of headache: A- Intracranial causes 1- meningeal irritation: by meningitis, tumors or operative trauma 2- brain tumors: traction on meninges 3- decreased CSF pressure: after lumbar puncture..traction on meninges 4- Hypertension: marked expansion of cerebral blood vessels 5- Migraine headache: unknown cause.spasm followed by dilatation 6- Alcoholic headache: toxic meningeal irritation 7- constipation: due to absorption of toxins B- extracranial headache: 1- Head and neck…spasm of neck muscles 2- Eye causes: Errors of refraction 3- nasal causes: sinusitis 4- Ear: otitis media 5- Teeth: toothache 6- Systemic disorders: anaemia  (4) Neuropathic pain Chronic type of pain caused by damage or pathological changes in the nerve fibers of the peripheral or central nervous system Examples: 1- Trigeminal neuralgia 2- Herpes zoster 3- Phantom limb pain 4- Peripheral neuropathy e.g. diabetic neuropathy 5- Sciatica Characters: 1- Excruciating, electric, burning, shooting pain 2- Occurring in bouts or paroxysms 3- Accompanied by hyperalgesia & parasthesia 4- Partially responsive to opioid therapy Which of the following statements did not fit with thermal receptors? a. Located immediately under the skin b. Has small receptive field c. Cold receptors more numerous than warm receptors d. They are rapidly adapting receptors Which of the following structures produce intracranial when stimulaed by painful stimuli? a. Arachnoid matter b. Brain tissue c. Intracranial veins d. Pia matter Explain mechanism of referred pain: ………………………………………………… …………………………………………………. …………………………………………………. ………………………………………………….. ……………………………………………………