Ns10 Sensory Receptors Corrected

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NS10
Lecture 10 of 63 in the Neuroscience Module

"Sensory Receptors" [Physiology]

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Ns10 Sensory Receptors Corrected

  1. 1. NS10 – SOMATOSENSORY PATHWAY NS10 – SOMATOSENSORY PATHWAY (corrections/clarifications are in blue) I. OVERVIEW A. Function of the somatosensory pathway: 1. To collect sensory information from sensory receptors in the skin, viscera, joints and muscles 2. To transmit sensory information to the brain for evaluation and integration. B. Classification based on the source of sensory information 1. Proprioreception – information about the relative position and movement of body segments to each other and in space – muscle tension, muscle length, muscle velocity 2. Exteroreception – information about the external environment a. Mechanoreception: discriminative touch - form, texture, vibration, stretch b. Nociception: pain - sharp pain, burning pain c. Thermoreception: temperature - warmth, cold 3. Interoreception – information from the internal organs C. Components of the somatosensory pathway 1. Sensory receptors: mechanoreceptors, nociceptors, thermoreceptors, proprioreceptors 2. Sensory nerve fibers a. first order neurons – primary sensory afferents – unipolar neurons (dorsal root ganglion cells) b. second order sensory neurons c. third order sensory neurons (thalamus) d. fourth order sensory neurons (somatosensory cortex) 3. Higher centers – cerebral cortex, cerebellum, periaqueductal gray D. Organization of the somatosensory pathways 1. Dorsal spinal cord pathways a. dorsal column/ medial leminiscal pathway (body; discriminative touch) b. trigeminal pathway (face; depends on final target can be for touch, pain or proprioception) c. spinocerebellar pathway (body; proprioception) 2. Ventral spinal cord pathways a. spinothalamic pathway (body; pain & temperature) b. spinoreticular pathway body; attentional, arousal) c. spinomesencephalic pathway (body; emotional response) E. Transmission and interpretation of somatosensory information 1. Receptor perception of stimuli 2. Receptive field 3. Sensory modality: an aspect of a stimulus; coding of information (e.g. mechanoreception, nociception0 4. Types of sensory afferents 5. Adaptation: ability of receptor to change its response to constant stimuli. 6. Dermatome: an area of skin innervated by a pair of dorsal root nerve. The body can be divided into 1
  2. 2. NS10 – SOMATOSENSORY PATHWAY regions (dermatomal map) that are supplied by a single nerve and this map can be used to identify sites of nerve damage E. Transmission and interpretation of somatosensory information 7. Modality segregation – sensory neurons are organized according to the type of information that they transmit and become part of different ascending spinal tracts. 7. Somatosensory map - a complete map of the body exists in the somatosensory cortex. II. SOMATOSENSORY RECEPTORS A. MECHANORECEPTORS TABLE 1: Comparison of cutaneous mechanoreceptors RECEPTIVE GROSS VS. DETECTION AXON STIMULI ADAPTATION FIELD FINE OF STIMULI TYPE large, >20 gross Pacinian unlocaliized beginning & rapid mm (70- vibratiion end corpuscle deep 1000Hz) small, 3-4 Meissner’s fine beginning & skin stretch rapid mm (10-200 Hz) end corpuscle superficial myelinated type Aβ small, 3-4 Merkel’s fine skin curvature slow mm sustained (30 Hz) disc superficial large, >10 Ruffini’s directional gross slow mm sustained skin stretch (15 Hz) ending deep 2
  3. 3. NS10 – SOMATOSENSORY PATHWAY B. THERMORECEPTORS & NOCICEPTORS TABLE 2: COMPARISON OF THERMO AND NOCICEPTORS NERVE ENDING AXON TYPE DETECTION OF RECEPTIVE FIELD STIMULI THERMORECEPTORS free unmyelinated 30-45oC 1 -5 mm Warm receptors type C free lightly myelinated 10 - 35 oC 1-5 mm Cold receptors type Aδ NOCICEPTORS lightly myelinated strong pressure free 2-3 mm Mechanical type Aδ from sharp objects nociceptor lightly myelinated burning heat or free 2-3 mm Thermal type Aδ extreme cold nociceptor lightly myelinated extreme in pH, free 2-3 mm Chemical type Aδ irritants nociceptor combination of Polymodal unmyelinated mechanical, thermal free type C (heat & cold) and nociceptor chemical stimuli C. PROPRIORECEPTORS TABLE 3: COMPARISON OF PROPRIOCEPTORS RECEPTOR FUNCTION STIUMLI AXON TYPE Ruffini’s ending prevent hyperflexion, stretch JOINT RECEPTOR Panciniform hyperextension corpuscles encapsulated myelinated ending with a stretch change in muscle type Aα MUSCLE SPINDLE primary and tendon tap length secondary ending terminals within GOLGI TENDON change in muscle collagen fibers of tension ORGAN stretch tendon 3
  4. 4. NS10 – SOMATOSENSORY PATHWAY Table 4. Comparison of dorsal spinal cord pathways (pathways that are part of the learning objectives are in red) DORSAL SPINAL Dorsal column/ medial trigeminal Spinocerebellar CORD lemniscal PATHWAYS body face body REGION conscious perception of touch, pain/temperature, subconsious proprioceptive SENSORY touch proprioception posture, balance, INFORMATION coordination of skilled movement mechanorecpetors of skin, mechanoreceptors, muscle and joint receptors RECEPTORS joints, muscle nociceptors, from lower body thermoreceptors, proprioceptors dorsal root ganglia: cranial nerve ganglia dorsal root ganglia FIRST ORDER - fasciculus gracilis (T7 and NEURONS below; legs and trunk) - fasciculus cuneatus (above T7; arms and trunk) brain stem or trigeminal nuclei synapse on neuron in SECOND ORDER caudal medulla: Clarke’s Nucleus NEURONS - gracilis nucleus project through projects ipsilaterally via - cuneate nucleus tigeminothalamic tract posterior spinocerebellar tract axons run ventrally = decussate internal arcuate fibers; then decussate to become medial lemniscus VPL (ventral posterior VPMN (ventral THIRD ORDER lateral) nucleus of the posteromedial) nucleus of NEURONS thalamus thalamus pass through posterior limp of internal capsule somatosensory cortex somatosensory cortex cerebellum HIGHER CENTER 4
  5. 5. NS10 – SOMATOSENSORY PATHWAY Table 5. Comparison of ventral spinal cord pathways VENTRAL SPINAL anterolateral spinoreticular tract spinomesencephalic CORD PATHWAYS (spinothalamic) tract body body body REGION pain & temperature attentional mechanism & emotional response; SENSORY arousal; noxious stimuli noxious stimuli INFORMATION mechanoreceptors nocieptors nocieptors RECEPTORS (touch = anterior stt) nociceptors thermoreceptors (pain & temp = lateral stt) dorsal root ganglia dorsal root ganglia dorsal root ganglia FIRST ORDER NEURONS dorsal horn dorsal horn dorsal horn SECOND ORDER NEURONS decussate in spinal cord ascend as ipsilateral ascend as ipsilateral spinoreticular tract spinoreticular tract ascend in either anteror or lateral spinothalamic tract anterior and lateral tracts run together as spinal lemniscus in medulla pass through pons and midbrain VPL (ventral posterior THIRD ORDER lateral) nucleus of the NEURONS thalamus somatosensory cortex medullary, periaqueductal gray (PAG) HIGHER CENTER pontine, and in midbrain mesencephalic reticular formation mediates endogenous analgesia & control of aversive behaviour 5

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