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Dr. R Ghasemi 1 2 Dr. R Ghasemi Neurophysiology SENSORY SYSTEM By Dr. Rasoul Ghasemi Outlines 4 Introduction Sensory System Principles Categorization Tactile Sensation Receptor types Dorsal Root Ganglion Nerve fibers Dr. R Ghasemi Outlines 5 Proprioceptors Thermoreceptors Pathways to the CNS Somatosensensory cortex Pain Pain pathways Control of pain Hyperalgesia Dr. R Ghasemi Somatosensory system Four functions of Sensory systems 7 What do the senses do for us? Perception control of movement regulation of internal organs maintenance of arousal Dr. R Ghasemi Sensory system 8 1- General somatic senses receptors Touch are widely spread (Discriminative touch) Movement, Pain (nociception) Temperature Dr. R Ghasemi pressure, vibration Sensory system 9 2- Special somatic senses Hearing Balance Vision Smell Dr. R Ghasemi Sensory system 10 3- Proprioceptive senses : Detect stretch in tendons and muscle Body sense – position and movement of body in space 4- Visceral sensory: General visceral senses – stretch, pain, temperature, nausea, and hunger Widely felt in digestive and urinary tracts, and reproductive organs 5- Special visceral senses - taste Dr. R Ghasemi The sensory systems encode four elementary attributes of stimuli 11 Modality location intensity timing Dr. R Ghasemi Sensory Receptors Sensory Receptor Neurons 13 Structures vary – – – – a. naked nerve endings surrounded by accessory tissues Pacinian corpuscle b. cilia like structures - mechanosensory, olfactory, retina c. those with true axons - mechanosensory, olfactory use electrical signaling d. those without true axons - gustatory, primarily use chemical signaling (may generate an AP) Sensory cells respond to a specific stimulation – can respond to other stimulation if great enough (eyeball trick) Excite downstream neurons – if these are excited then will perceive the appropriate stimulation (amputated limbs) Dr. R Ghasemi Classes of receptors 14 Exteroceptors Detect stimuli near the outer surface of the body and include those from the skin (thermal, touch, pressure and vibration Interoceptors Detect stimuli from inside the body pH, oxygen level/carbon dioxide in the blood, concentration, osmolality of body fluids, distention and spasm (e.g., gut), and flow (e.g., urethra) Dr. R Ghasemi Classes of receptors (continued) 15 Proprioceptors Monitor joint position & muscle contraction DO NOT ADAPT Structurally complex – are 2 types Tendon organs – monitor tendon strain Muscle spindles – monitor muscle length Most information from these receptors is monitored subconsciously Are vital for normal skeletal motor function Dr. R Ghasemi Types of Sensory Receptors 16 A ) Mechanoreceptors (Skin tactile sensibilities) Muscle endings Muscle spindles Golgi tendon receptors Hearing Equilibrium Sound receptors of cochlea Vestibular receptors Arterial pressure Baroreceptors of carotid sinuses and aorta Dr. R Ghasemi Types of Sensory Receptors (continued) 17 B) Thermoreceptors Cold receptors Warm receptors C) Nociceptors Pain (Free nerve endings) D) Electromagnetic receptors Rods Cones Dr. R Ghasemi Types of Sensory Receptors (continued) 18 E) Chemoreceptors Taste Receptors Smell Receptors in or on surface of medulla and in aortic and carotid bodies Blood glucose, amino acids, fatty acids Receptors of aortic and carotid bodies Blood CO2 Receptors of olfactory epithelium Arterial oxygen of taste buds Receptors in hypothalamus Osmolality Neurons in or near supraoptic nuclei Dr. R Ghasemi Sensory Physiology: General Principles 19 Sensory systems convert one form of energy into an electrical signal This conversion of one energy form (eg. light) into an electrical signal (receptor potential) is known as stimulus transduction Dr. R Ghasemi General Principles 20 The process of stimulus transduction involves the opening or closing of ion channels on a specialized plasma membrane. The size of the receptor potential, just like the synaptic potential is graded and is determined by: • • • • The magnitude of the stimulus The rate of change of the stimulus strength The temporal summation of other receptor potentials The degree of ‘adaptation’ Dr. R Ghasemi Spatial and Temporal Summation 21 Dr. R Ghasemi General Principles 22 Receptor potentials have the same properties as synaptic potentials A receptor may be either a specialized nerve ending of an afferent neuron or a separate cell that is intimately associated with the peripheral endings of the neuron. Dr. R Ghasemi Receptor Potentials 23 Dr. R Ghasemi Receptor Adaptation 24 Different rates of adaptation Slowly adapting (SA) steady pattern of firing Rapidly adapting (RA) fire only at onset of stimulus Dr. R Ghasemi Sensory receptors that produce a steady rate of firing in response to an unchanging stimulus are called “tonic receptors” Dr. R Ghasemi 25 Sensory receptors that reduce their rate of firing in response to an unchanging stimulus are called phasic receptors Dr. R Ghasemi 26 Adaptation of sensory receptors can be produced by changes the mechanical properties of the receptor or by the electrical properties of the receptor or the synapse The Pacinian corpuscle is a very rapidly adapting receptor Dr. R Ghasemi 27 Dr. R Ghasemi 28 Dorsal Root Ganglion 29 Dorsal Root Ganglion Neuron Is the Sensory Receptor in the Somatic Sensory System Dr. R Ghasemi 30 Dr. R Ghasemi Receptors 31 Bare nerve ending (Protopathic sensations ) Encapsulated nerve ending (Epicritic sensations ) In superficial layers Meissner's corpuscle/ Merkel disk receptor In deep subcutaneous tissue Pacinian Dr. R Ghasemi corpuscle/ Ruffini ending The somatic sensory receptors 32 Dr. R Ghasemi Dr. R Ghasemi 33 Dr. R Ghasemi 34 Tactile receptors 35 Merkel receptor fires continuously while stimulus is present. Responsible for sensing fine details Meissner corpuscle fires only when a stimulus is first applied and when it is removed. Responsible Dr. R Ghasemi for controlling hand-grip Figure 14.1 A cross section of glabrous (without hairs or projections) skin, showing the layers of the skin and the structure, firing properties and perceptions associated with the Merkel receptor and Meissner corpuscle - two mechanoreceptors that are near the surface of the skin. Dr. R Ghasemi Tactile receptors- continued 37 Two types located deeper in the skin Ruffini cylinder fires continuously to stimulation Associated with perceiving stretching of the skin Pacinian corpuscle fires only when a stimulus is first applied and when it is removed. Associated with sensing rapid vibrations and fine texture Dr. R Ghasemi 38 Figure 14.2 A cross section of glabrous skin, showing the structure, firing and perceptions associated with the Ruffini cylinder and the Dr.properties R Ghasemi Pacinian corpuscle - two mechanoreceptors that are deeper in the skin. Distribution Dr. R Ghasemi 39 Distribution Dr. R Ghasemi 40 Receptive Fields 41 Skin region influencing single neuron small, sharply defined fine spatial detail, tactual acuity large, less distinct regions fine acuity not as important ~ Dr. R Ghasemi Receptive Field Size Adaptation Small Rapid Slow Dr. R Ghasemi Large Meissner’s Corpuscle Merkel’s Disks Pacinian Corpuscle Ruffini Ending 42 Two-point discrimination Dr. R Ghasemi 43 Two-Point Discrimination Dr. R Ghasemi 44 Receptor Threshold 45 Dr. R Ghasemi Dr. R Ghasemi 46 The Spatial Characteristics of Objects Are Signaled by Populations of Mechanoreceptors Dr. R Ghasemi 47 Dr. R Ghasemi 48 Dr. R Ghasemi 49 Dr. R Ghasemi 50 Lateral Inhibition Can Aid in Two-Point Discrimination 51 Dr. R Ghasemi Thermal Receptors 52 Humans recognize four distinct types of thermal sensation: cold, and cool, warm, and hot Unlike mechanoreceptors, cold receptors and warmth receptors fire action potentials continuously at low rates (2-5 spikes per second) Dr. R Ghasemi Dr. R Ghasemi 53 Interoceptors Detect stimuli from inside the body and include receptors that respond to pH, oxygen level in arterial blood, carbon dioxide concentration, osmolality of body fluids, distention and spasm (e.g., gut), and flow (e.g., urethra) Dr. R Ghasemi 54 54 Internal mechanoreceptors Stretch receptors that monitor changes in organ pressure in distensible organs Rapidly adapting They monitor BP, respiration, digestion, and urinary control Dr. R Ghasemi 55 Chemoreceptors… Only respond to dissolved chemicals 56 Rapidly adapting: found in olfaction, taste & the CNS at: Medulla – receptors are sensitive to pH/CO2 changes in CSF– trigger respiratory adjustments Aortic/Carotid bodies – sensitive to changes in pH/CO2/O2 blood levels – trigger adjustments in respiration and cardiovascular activity Dr. R Ghasemi Proprioceptors…. 57 Monitor joint position & muscle contraction DO NOT ADAPT Structurally complex – are 2 types Tendon organs – monitor tendon strain Muscle spindles – monitor muscle length Most information from these receptors is monitored subconsciously Are vital for normal skeletal motor function Dr. R Ghasemi Muscle spindles respond to muscle length or velocity Golgi tendon organs respond to muscle force or contraction Dr. R Ghasemi 58 Nerve Fibers That Transmit Different Types of Signals, and Their Physiologic Classification 59 Dr. R Ghasemi Afferent Fibers 60 Dr. R Ghasemi Dr. R Ghasemi 61 Somatosensory Pathways 62 The Anterolateral Pathway (also called Spinothalamic) The Dorsal Column Pathway The Spinocerebellar Pathway (important for proprioception) Dr. R Ghasemi 63 Pathways are composed of a first, second and third order neuron First Order neuron Second Order neuron Third Order neuron Dr. R Ghasemi Sensory Pathways for Transmitting Somatic Signals into the CNS 64 A) the dorsal column–medial lemniscal system Touch sensations requiring a high degree of localization of the stimulus Touch sensations requiring transmission of fine gradations of intensity Phasic sensations, such as vibratory sensations Sensations that signal movement against the skin Position sensations from the joints Pressure sensations having to do with fine degrees of judgment of pressure intensity Dr. R Ghasemi Dorsal column pathway Carries fine touch, vibration and conscious proprioception signals 1st neuron enters spinal cord through dorsal root; ascends to medulla (brain stem) 2nd neuron crosses over in medulla; ascends to thalamus 3rd neuron projects to somatosensory cortex Dr. R Ghasemi 65 Dorsal Columns/Medial Lemniscal System Fasiculus cuneatus (upper body) Neuron #1 Dr. R Ghasemi Fasiculus gracilis (lower body) Origin: Course: Termination: Dorsal root ganglion (cervical or lumbar) Fasiculus gracilis/cuneatus Nucleus cuneatus (upper body) Nucleus gracilis (lower body) 66 Dorsal Columns/ Medial Lemniscal System Nucleus gracilis Nucleus cuneatus Internal Arcuate Fibers Medial Lemniscus Neuron #2 Dr. R Ghasemi Origin: Course: Termination: Nucleus gracilis/cuneatus Medial Lemniscus VPL (ventral posterolateral nuclues) of Thalamus 67 Dorsal Columns/ Medial Lemniscal System To primary somatosensory cortex VPL Nucleus of Thalamus Neuron #2 Dr. R Ghasemi Origin: Course: Termination: Laterality: Nucleus gracilis/cuneatus Medial Lemniscus VPL of Thalamus CONTRA 68 dorsal cloumn pathway Dr. R Ghasemi 69 Dorsal column pathway Primary somatosensory cortex (S1) in parietal lobe Dorsal column nuclei Thalamus Medulla Dorsal column Dr. R Ghasemi Medial lemniscus Spinal cord 70 Spinocerebellar pathway Carries unconscious proprioception signals Receptors in muscles & joints 1st neuron: enters spinal cord through dorsal root 2nd neuron: ascends to cerebellum No 3rd neuron to cortex, hence unconscious Dr. R Ghasemi 71 Spinocerebellar tract damage 72 Cerebellar ataxia Clumsy movements Incoordination of the limbs (intention tremor) Wide-based, reeling gait (ataxia) Alcoholic intoxication produces similar effects! Dr. R Ghasemi Sensory Pathways for Transmitting Somatic Signals into the CNS 73 B) The anterolateral system Pain Thermal sensations Crude touch and pressure sensations Tickle and itch sensations Sexual sensations Dr. R Ghasemi Somatosensory Pathways 74 Thalamus ventral posterior nuclei (VP) pain & touch still segregated pain also to intralaminar nuclei also input from cortex Input to S1 Dr. R Ghasemi Dr. R Ghasemi 75 Dr. R Ghasemi 76 Dr. R Ghasemi 77 Dr. R Ghasemi 78 Somatosensory cortex 79 S1 - Postcentral Gyrus Somatotopic Organization topographic representation of body Distorted Homunculus disproportionate amount of cortex for body parts high sensitivity: large cortical area Dr. R Ghasemi Dr. R Ghasemi 80 Dr. R Ghasemi 81 Dr. R Ghasemi 82 Dr. R Ghasemi 83 Dr. R Ghasemi 84 Dr. R Ghasemi 85 Dr. R Ghasemi 86 Primary Somatosensory Cortex 87 Dr. R Ghasemi Dr. R Ghasemi 88 Dr. R Ghasemi 89 Lesions in Somatosensory Areas Produce Specific Sensory Deficits 90 Tabes dorsalis late consequences of syphilitic infection Destroy large-diameter neurons in the DRG Disruption of SSI Dr. R Ghasemi Spinothalamic damage spinothalamic pathway Left spinal cord injury Brown-Séquard Syndrome Dr. R Ghasemi 91 Pain 92 Dr. R Ghasemi Pain 93 Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage” International Association for the Study of Pain Dr. R Ghasemi Why feel pain? 94 Gives conscious awareness of tissue damage Protection: Remove body from danger Promote healing by preventing further damage Avoid noxious stimuli Elicits behavioural and emotional responses Dr. R Ghasemi Pain Is Mediated by Nociceptors free nerve endings in skin respond to noxious stimuli 95 Selectively respond to stimuli that can damage tissue Histamine, K+ released from injured cells, Bradykinin Substance P and other related peptides Acidity (decreases in the local pH around the nerve terminals), ATP, serotonin, and acetylcholine. Dr. R Ghasemi Pain Dr. R Ghasemi 96 Pain Receptors and Their Stimulation 97 Pain Receptors Are Free Nerve Endings Three Types of Stimuli Excite Pain Receptors Mechanical, Thermal Chemical Polymodal Tissue Ischemia as a Cause of Pain Muscle Spasm as a Cause of Pain Dr. R Ghasemi Pain…. 98 • Superficial Somatic Pain arises from skin areas • Deep Somatic Pain arises from muscle, joints, tendons & fascia • Visceral Pain arises from receptors in visceral organs – localized damage (cutting) intestines causes no pain – diffuse visceral stimulation can be severe • distension of a bile duct from a gallstone • distension of the ureter from a kidney stone Dr. R Ghasemi Acute and chronic pain 99 Fast Pain (acute) occurs rapidly after stimuli (.1 second) sharp pain like needle puncture or cut not felt in deeper tissues larger A nerve fibers Slow Pain(chronic) begins more slowly & increases in intensity in both superficial and deeper tissues smaller C nerve fibers Dr. R Ghasemi Two Types of Peripheral Pain Neurons 100 A-delta fibers Thick, myelinated, fast conducting neurons Mediate the feeling of initial fast, sharp, highly localized pain. C fibers Very thin, unmyelinated, slow-conducting Mediate slow, dull, more diffuse, often burning pain. Dr. R Ghasemi 101 spinothalamic pathway to reticular formation Aδ nerve C nerve nociceptor nociceptor Impulses transmitted to spinal cord by Myelinated Aδ nerves: fast pain (80 m/s) Unmyelinated C nerves: slow pain (0.4 m/s) Dr. R Ghasemi Dr. R Ghasemi 102 Ascending Pathways: 103 Impulses ascend to brain via: Spinothalamic tract(fast pain) Spinoreticular tract to reticular formation (slow pain) Spinomesencephalic tract Cervicothalamictract Spinohypothalamic tract Dr. R Ghasemi Central Pain Pathways: Fast Pain 104 Fast pain and A-delta fibres A-delta fibers synapse on cells in the spinal cord that lead to an area of the thalamus called the ventrobasal complex ventrobasal complex also receives neurons that mediate touch sends its output to the somatosensory cortex allows us to localize where pain originates Dr. R Ghasemi Central Pain Pathways: Slow Pain 105 Slow pain and C fibres C fibres synapse on cells in the spinal cord Relays to a midline nucleus in the thalamus and to the limbic system responsible for motivational and emotional aspects of pain Those connections are important for the interpretation of pain. Dr. R Ghasemi Ascending Pathways: Dr. R Ghasemi 106 Visceral pain 107 Notable features of visceral pain: Often accompanied by strong autonomic and/or somatic reflexes Poorly localized; may be “referred” Mostly caused by distension of hollow organs or ischemia (localized mechanical trauma may be painless) Dr. R Ghasemi Visceral Pain 108 Causes of True Visceral Pain Ischemia Chemical Stimuli Spasm of a Hollow Viscus Overdistention of a Hollow Viscus Insensitive Viscera Dr. R Ghasemi Afferent innervation of the viscera. 109 Many visceral afferents are specialized nociceptors, as in other tissues small (Ad and C) fibers involved. Large numbers of silent/sleeping nociceptors, awakened by inflammation. Nociceptor sensitization well developed in all visceral nociceptors. Dr. R Ghasemi 110 Dr. R Ghasemi 111 Dr. R Ghasemi Referred pain 112 Pain originating from organs perceived as coming from skin Site of pain may be distant from organ Dr. R Ghasemi Referred pain Convergence theory: This type of referred pain occurs because both visceral and somatic afferents often converge on the same interneurons in the pain pathways. Excitation of the somatic afferent fibers is the more usual source of afferent discharge, so we “refer” the location of visceral receptor activation to the somatic source even though in the case of visceral pain. The perception is incorrect. Dr. R Ghasemi 113 “Pain Gate” Theory Melzack & Wall (1965) 114 A gate, where pain impulses can be “gated” The synaptic junctions between the peripheral nociceptor fiber and the dorsal horn cells in the spinal cord are the sites of considerable plasticity. A “gate” can stop pain signals arriving at the spinal cord from being passed to the brain Reduced pain sensation Natural pain relief (analgesia) Dr. R Ghasemi How does “pain gate” work? 115 The gate = spinal cord interneurons that release opioids. The gate can be activated by: Simultaneous activity in other sensory (touch) neurons Descending nerve fibers from brain Dr. R Ghasemi Applications of pain gate 116 Stimulation of touch fibres for pain relief: TENS (transcutaneous electrical nerve stimulation) Acupuncture Massage Release of natural opioids Natural childbirth techniques Dr. R Ghasemi Pain Relief 117 Aspirin and ibuprofen block formation of prostaglandins that stimulate ociceptors Novocain blocks conduction of nerve impulses along pain fibers Morphine lessen the perception of pain in the brain. Dr. R Ghasemi Dr. R Ghasemi 118 Dr. R Ghasemi 119 Peripheral sensitization to pain: Some definitions: Hyperalgesia increased sensitivity to an already painful stimulus Allodynia normally non painful stimuli are felt as painful (i.e .light touch of a sun-burned skin) Analgesia Dr. R Ghasemi 120 Hyperalgesia 121 The skin, joints, or muscles that have already been damaged are unusually sensitive. A light touch to a damaged area may elicit excruciating pain; Primary hyperalgesia occurs within the area of damaged tissue; Secondary hyperalgesia occurs within the tissues surrounding a damaged area. Dr. R Ghasemi Agents that Activate or Sensitize Nociceptors: Cell injury arachidonic acid prostaglandins vasc. permeability (cyclo-oxygenase) sensitizes nociceptor Cell injury arachidonic acid leukotrienes vasc. permeability (lipoxygenase) sensitizes nociceptor Cell injury tissue acidity kallikrein bradykinin vasc. permeability activates nociceptors synthesis & release of PGs Substance P (released by free nerve endings) sensitize nociceptors vasc. perm., plasma extravasation (neurogenic inflammation) releases histamine (from mast cells) Calcitonin gene related peptide (free nerve endings) dilation of peripheral capillaries Serotonin (released from platelets & damaged endothelial cells) activates nociceptors Cell injury potassium activates nociceptors Dr. R Ghasemi 122 Dr. R Ghasemi 123