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Lecture 17

ANP1106 Lecture 17: Physiology notes Part 3

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University of Ottawa
Anatomy and Physiology
Jacqueline Carnegie

SOMESTHESIA: SIMPLE RECEPTORS The simple receptors in the somatic system detect the stimuli to the external body and those in the vegetative system detect stimuli to the internal organs. There are 2 kinds of simple receptors free nerve endings - including • non- specialized naked or free nerve endings which detect pain, temperature, or strong pressure throughout the body Merkel discs which are for the detection of light touch at the bottom of the epidermis hair follicle receptors which detect movement of hair encapsulated dendritic endings - including • Meissner's corpuscle which is of greater density in the hands, lips, and nipples where they have a discriminative tactile function Krause corpuscles which are like Meissner's corpuscle but are located in mucous membranes like the mouth Pacinian corpuscle which is in the dermis and the subcutaneous tissue where they respond to changes in pressure since the capsule partly adapts to maintained pressure Ruffini corpuscle which is in the dermis, the subcutaneous tissue, and the articular capsules where they respond to stretching Golgi tendon organs which are in the tendon and respond to applied tension Muscle spindles which are in the muscle and respond to changes in muscle length SOMESTHESIA: P AIN S TIMULATION AND T RANSDUCTION A painful stimulus will cause the release of substances (serotonin, prostaglandins, leukotrines) that will • stimulate free nerve endings to produce action potentials that will then cause the nerve terminals to release substance P which stimulates the blood vessels and the mastocytes which in turn produce inflammation propagate the action potentials towards the CNS to produce reflex responses and perception • The substances released by damage to cells act like ligands by binding to metabotropic receptors to produce postsynaptic depolarization SOMESTHESIA: T EMPERATURE S TIMULATION AND T RANSDUCTION • A heat thermoreceptor is specialized to detect increases in temperature whereas a cold thermoreceptor is specialized to detect decreases in temperature • Outside the normal range of temperatures for a thermoreceptor, its responses do not provide useful information • Changes in temperature cause a change in the conformation of the receptor such that it opens or closes the ionic channel An increase in temperature will cause the channel of heat thermoreceptor to open whereas it will cause the channel of a cold thermoreceptor to close An decrease in temperature will cause the channel of heat thermoreceptor to close whereas it will cause the channel of a cold thermoreceptor to open SOMESTHESIA: M ECHANICAL S TIMULATION AND TRANSDUCTION • Touch is detected by mechanoreceptors Merkel's discs, hair follicle receptors, Meissner's corpuscles, Krause corpuscles, Ruffini's corpuscles and Pacinian corpuscles in the skin • The capsule of the Pacinian corpuscle allows the receptor to detect decreases in pressure the receptor responds only to increases in pressure when capsule is removed • The pacinian receptor responds to increases and decreases in pressure as during vibratory stimuli. • All mechanoreceptors have channels with gates that open when stretch or pressure is transmitted to the cell membrane with its cytoskeletal cables that are attached to the channel gate SOMESTHESIA: P ROPRIOCEPTORS • Proprioception (a sense associated with posture or movement) is detected by mechanoreceptors • Muscle spindle has a capsule containing intrafusal muscle fibers surrounded with sensory endings (primary and secondary) that measure the change in length of the central part of the intrafusal fibers (this corresponds to the change in length of the extrafusal muscle) • Information on changes in muscle length are used to determine joint angle • Sensitivity of the muscle spindles can be modified by activation of gamma motoneurons that cause contraction of the extremities of the intrafusal fibers SOMESTHESIA: P ROPRIOCEPTORS F UNCTION • Panel A —> the responses of the sensory receptors to an imposed stretch of the muscle Panel B —> stretch is done with activation of gamma motoneurons which increase the senstitivity of the • muscle spindle afferents Panel C —> Only alpha motoneuron activation which produces extrafusal muscle contraction • • Panels A-C are hypothetical examples while panel D shows what normally happens • There is activation of both the alpha and gamma motoneurons so the intrafusal muscle fiber contraction matches that of the extrafusal muscle such that minimal change in sensory activity occurs • Perception of muscle length requires knowledge of both muscle spindle afferent activity and gamma motoneuron efferent activity SOMESTHESIA: S ENSORY P ATHWAYS • By a relay of 3 neurons, information of a stimulus on the body is conveyed to the cerebral cortex for perception The first neuron is the sensory neuron which forwards information to the CNS where it synapses on the second order neuron which in turn projects to the third order neuron in the thalamus which projects to the cortex The second and third neurons form an ascending pathway which conveys information to the sensory cortex • Perception is achieved by the networks of neurons in the cerebral cortex. There are two main pathways to transmit sensory information to the cortex anterolateral pathway medial lemniscus pathway • Stimulus transmitted by the sensory neuron (fir•tStimulus transmitted by the sensory neuron (first order) synapses in the spinal cord on the second order projects up the dorsal columns of the spinal cord • sends projections crossing towards the contral•tsynapse on a second order neuron in the the dorsal side column nuclei (gracilis or cuneatus in the medulla) • ascending to the thalamus to make synapse on t•esends projections crossing to the contralateral side and order neuron up the medial lemniscus to the thalamus where it • Thalamic neuron (third order) sends a projection to thes on a third order neuron primary somatosensory cortex • Thalamic neuron (third order) projects to the primary somatosensory cortex SOMESTHESIA: S OMATOSENSORY C ORTEX AND S ENSORY H OMUNCULUS • Primary somatosensory cortex lies in the postcentral gyrus • Various parts of the body are represented by a homunculus in the primary somatosensory cortex SOMESTHESIA: I NTERNAL O RGAN P AIN AND R EFERRED P AIN Sensory fibers also come from internal organs where they can provide pain information where there’s • internal organ damage • The homunculus doesn’t have sensory representations for visceral organs like the kidneys, stomach, and heart Nociceptors of these visceral organs join the same pathways of other somatic sensations which are represented in the homunculus of the primary somatosensory cortex PERCEPTION 3 L EVELS Receptor level • includes the sensory neuron (first order) which transduces the stimulus into electrical activity and forwards information to the CNS to synapse on the second order neuron Ascending pathway level • includes second and third order neurons that convey information to the primary sensory cortex • Perceptual level contains the cortical neurons that will analyze sensory information to provide perception • a sensation occurs at the unconscious level while perception occurs at the conscious level • Somatic sensory information arrives first in the primary somatosensory cortex (raw information) and then it is analyzed by the surrounding association sensory cortex (processed information) • Lesion of the primary somatosensory cortex will eliminate sensation and perception resulting in the symptom of anesthesia • Lesion of the tactile association cortex can produce tactile agnosia where the patient can identify an object by looking at it (visual cortex is not damaged) but not by manipulating it • Lesion of the visual association cortex can produce a visual agnosia where the patient cannot identify an object by looking at it but can easily identify it by manipulating it (somatosensory cortex is not damaged) D ETECTION , M AGNITUDE , SPATIAL D ISCRIMINATION ,AND F EATURE A BSTRACTION • Perceptual detection is the simplest perception - allows us to know if the stimulus exists • Magnitude estimation allows us to know if the stimulus is weak or strong • Perceive the intensity of a stimulus, the cortex uses the discharge frequency of a single neuron (a low frequency corresponds to a stimulus of weak intensity and a high frequency corresponds to a stimulus of strong intensity) and the number of cells active (more cells means stronger stimulus) • Spatial discrimination allows us to know if the stimulus is large The limit of space discrimination, measured as the minimum distance that 2 points can be perceived, depends on the density of receptors • Feature abstraction allows us to know the characteristics of the stimulus The skin doesn’t have receptors specialized to detect the rough, hard, or smooth characters of a stimulus which are required for us to distinguish among silk, velvet, and leather. • The nervous system analyzes information transmitted by all its touch receptors in order to identify the characters that make it possible to distinguish objects P ATTERN RECOGNITION AND R ETINAL S LI
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