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Chapter 14

ANP1106 Chapter 14: Sensation and sensory Receptors

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University of Ottawa
Anatomy and Physiology
Jackie Carnegie

ANP1106 C Dr. Jackie Carnegie & Dr. W Staines 1. PNS and Reflex Activity: Somatosensory 03.14.2017-03.16.2017 P. 486 – 494, 596-514; P. 435-436, 438, 468-469, 472-474, 516-521 Peripheral Nervous System (PNS): all neural structures outside the brain and spinal cord; includes sensory receptors, peripheral nerves, associated ganglia, autonomic nervous system, and motor endings; provides links to and from external environ  CNS function critically depends on PNS input; PNS receives input from peripheral sensory receptors Sensory Receptors  structures specialized to respond to stimuli; activation of sensory receptors results in a graded depolarization that trigger AP impulse along the nerve axon to the CNS  reflex activity takes place in the spinal cord; sensation and perception occur in the cerebral cortex o i.e tendon stretch reflex, withdrawal reflex Receptor Class by Stimulus Type – name indicates stimulus that activates the receptor Mechanoreceptors: respond to mechanical force; i.e touch, pressure, vibration, stretch, and itch Thermoreceptors: sensitive to changes in temperature  transient receptor potential (TRP) family of proteins – free nerve endings with membrane channels that change their permeability (and therefor axon firing rates) across specific temperature ranges Photoreceptors: respond to light energy (i.e retina of eye) Chemoreceptors: respond to chemicals (i.e smell, taste, changes in blood chemistry) Nociceptors: sensitive to potentially damaging stimuli that result in pain or tissue damage; i.e heat, extreme cold, excessive pressure, inflammatory chemicals  stimulate subtpyes of thermoreceptors, mechanoreceptors, and chemoreceptors Receptor Class by Location Exteroceptors: respond to stimuli arising outside the body, found near body surface  include touch, pressure, pain, and temperature in skin; and special sense receptors (olfactory, touch, gustatory, auditory, equilibrium) interoceptors: respond to stimuli arising within the body, found in internal viscera and blood vessels  sensitive to chemical changes, tissue stretch, and temperature changes; related to autonomic functions  their activity causes us to feel pain, discomfort, hunger, or thirst; we are unaware of their activity Proprioceptors – “sense of self”; respond to internal stimuli in restricted location; degree of stretch in skeletal muscles, tendons, joints, ligaments, and CT coverings of bones and muscles  constantly advise brain of the body movements by monitoring the organ’s stretch; give info concerning movements and position of the body  in limbs, the proprioceptors are sensors that provide info about joint angle, muscle length, and muscle tension which is integrated to give info about the position of the limb in space o muscle spindle is a proprioceptor that provides information about changes in muscle length  intrafusal muscle fibers with sensory nerve endings wrapped around it o Golgi tendon organ is proprioceptor that provides info about changes in muscle tension Receptor Classification by Receptor Structure – receptors are structurally classified as either simple or complex most sensory receptors belong to general senses – simple receptors that are modified dendritic endings of sensory neurons  most are widely spread for tactile sensation (touch, pressure, stretch, and vibration), temperature monitoring, and pain as well as muscle sense from proprioceptors  nonencapsulated nerve endings – abundant in epithelia and connective tissue; most are nonmyelinated o Thermoreceptor detect changes in temp - those responding to heat is deeper in skin  temperatures outside of thermoreceptor range activates nociceptors & perceive as pain o vanilloid receptor detects painful stimuli; itch receptor located in dermis, contains histamine o Tactile (Mekel) Disc – deepest in epidermis, function as light touch receptors o Hair follicle receptors – nerve endings wrap around hair follicles, detect bending of hair  Encapsulated Nerve Endings – consist of one or more fiber terminals of sensory neurons enclosed in CT capsule; all are mechanoreceptors but vary in size, shape, and distribution in body o Tactile corpuscles (Meissner’s corpuscles) – sensory terminals covered by Schwann cells and thin egg-shaped CT capsule; found in dermal papillae in hairless skin areas (nipples, fingertips, soles of feet) o Lamellar corpuscles – scattered deep in dermic and subcutaneous tissue; stimulated by deep pressure; one dendrite is surrounded by capsule of ~60 layers of collagen o Bulbous corpuscles – in dermis, subcutaneous tissue, and joint capsules; spray of nerve endings enclosed in flattened capsule; respond to deep continuous pressure o Muscle spindles – fusiform proprioceptors in perimysium of skeletal muscle  each spindle has intrafusal fibers (modified skeletal muscle fibers) in CT capsule  detect muscle stretch and initiate reflex to resist stretch o tendon organs – proprioceptors in tendons, close to muscle insertion  bundles of collagen fibers in layered capsule with sensory terminals coiling btwn and around the fibers  muscle contraction stretches tendon fibers resulting with compression of nerve fibers to activate them – initiate reflex to contract muscle to relax o joint kinesthetic receptors – proprioceptors to monitor stretch in articular capsule that enclose synovial joints; 4 types: lamellar, bulbous, free nerve endings, and receptors like tendon organs  provide info on joint position and motion complex receptors are found in special sense organs, they are in contact with a free nerve ending  photoreceptors & retinal bipolar cell axons, hair cells & auditory axons nerve, taste and taste nerve axons Sensory Integration (from sensation to perception) Sensation: awareness of changes in the internal and external environment; Perception: conscious interpretation of those stimuli, determines how we respond  i.e pebble in your shoe – sensation of localized deep pressure, perception of discomfort Receptor level sensory receptors/circuit level ascending pathways/perception level cerebral cortex Organization of the Somatosensory System Somatosensory system: part of sensory system serving the body wall and limbs, receives input from exteroceptors, proprioceptors, and interoceptors; 3 main levels of neural integration: 1) Receptor level – sensory receptors and transmission to CNS; Sensation occurs when a stimulus excites a receptor and APs reach the CNS  Requirements: o energy matches the specificity of receptor – touch receptor sensitive to mechanical pressure, stretch, and vibration but not light  more complex receptor = more specific o stimulus must be applied in sensory receptor’s receptive field (area the receptor monitors); smaller receptive field = easier for brain to locate stimulus accurately = higher acuity  Smaller receptor field the higher the acuity (more neurons = more sensitive); larger receptor field, the lower the acuity (less neurons in area = less sensitive)  Overlapping stimulation btwn neighboring receptive fields provides general info about the location of a stimulus o stimulus energy must be converted to a graded potential (which can be depolarizing or hyperpolarizing) via transduction process  transduction – the conversion of an environmental physical property (stimulus) into a neuronal electrochemical property  stimulus causes membrane to be deformed, which causes the interior to be depolarized (more +ve) and creates graded potential, graded potential reaches node of Ranvier and creates AP  when receptor region is part of sensory neuron, the graded potential is a generator potential b/c it generates an AP in a sensory neuron  when receptor is a separate cell (case for most special senses), graded potential is a receptor potential b/c it occurs in a separate receptor cell – receptor potential changes amount of NTs released by receptor cell onto sensory neuron, NTs generate graded potentials in sensory neuron o graded potential in first order sensory neuron must reach threshold so voltage-gated ion channels on axon are opened and nerve impulse are generated and propagated to CNS o i.e sensory receptor ending, cell body in peripheral ganglion, synapses in spinal cord or medulla Adaptation – stimulus strength, duration, and pattern is encoded in the frequency of nerve impulses; greater the frequency = stronger stimulus; many sensory receptors adapt – change sensitivity when there’s a constant stimulus  Phasic receptors: fast adapting, gives bursts of impulses in beginning and end of stimulus; o i.e receptors responding to pressure, touch, and smell adapt quickly  Tonic Receptors: provide sustained response with little or no adaptation; info has protective importance o i.e Merkel’s discs, Ruffini’s corpuscles (touch receptors), and interceptors that respond to chemical levels in the blood o Pain receptors and proprioceptors do not exhibit adaptation 2
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