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SENSORY RECEPTORS, NEURONAL CIRCUITS FOR PROCESSING INFORMATION Physio 1 Types of sensory receptors Five basic types of sensory receptors 1. Mechanoreceptors 2. Thermoreceptors 3. Detects physical or chemical damage Electromagnetic receptors 5. Detects changes in temperature Nociceptors 4. Detects mechanical compression or stretching of receptor tissues Detects light in the retina Chemoreceptors Detects taste, smell, O2 levels, osmolality, CO2 concentration, others How do two types of sensory detect different stimuli? By differential sensitivities Each type of receptor is sensitive to one type of stimulus Example: Rods & cones of the eyes sensitive to light, nonresponsive to heat, cold Pain receptors in skin not stimulated by touch but active when the tactile stimuli is severe enough to damage the tissues Modality of sensation—the “labeled line "principle Each nerve tract terminates at a specific point in the nervous system Type of sensation felt when nerve fiber is stimulated is determined by the point in the nervous system to which the fiber leads Pain fibers activated regardless of cause Fibers pain sensation in the ear terminate in the auditory areas of brain This specificity of nerve fibers for transmitting only one modality of sensation is called the labeled line principle Transduction of sensory stimuli to nerve impulses Transduction: conversion of an impulse to an action potential or change in the transmembrane potential Mechanisms on how this is achieved 1. Mechanical deformation of receptor- opens ion channels 2. Chemical application to membrane- opens ion channels 3. Change of temperature of the membrane- alters permeability 4. Effect of electromagnetic radiation- allows ions to flow Sensory Pathway Stimulus Sensory receptor (= transducer) Afferent sensory neurons CNS Integration, perception Sensory Pathways Stimulus as physical energy sensory receptor acts as a transducer Stimulus > threshold action potential to CNS Integration in CNS cerebral cortex or acted on subconsciously Film somatosensory system https://www.youtube.com/watch?v=6OpeT6hcwBQ&list=PLNCmf4qh1CfFRfLc kNpsKoXKEvuw595Qt Mechanoreceptors Specialized to Receive Tactile Information Four major types of encapsulated mechanoreceptors are specialized to provide information to the central nervous system about touch, pressure, vibration, and cutaneous tension: 1. Meissner's corpuscles, 2. Pacinian corpuscles, 3. Merkel's disks, 4. Ruffini's corpuscles These receptors are referred to collectively as low-threshold (or high-sensitivity) mechanoreceptors because even weak mechanical stimulation of the skin induces them to produce action potentials. All low-threshold mechanoreceptors are innervated by relatively large myelinated axons ensuring the rapid central transmission of tactile information. The Major Classes of Somatic Sensory Receptors Receptor type Location Function Rate of adaptation Threshold of activation Meissner's corpuscles Principally glabrous skin Touch, pressure (dynamic) Rapid Low Pacinian corpuscles Subcutaneous tissue, interosseous membranes, viscera Deep pressure, vibration (dynamic) Rapid Low Merkel's disks Touch, pressure (static) Slow Low Stretching of skin Slow Low All skin, hair follicles Ruffini's corpuscles All skin Example of receptor function—the Pacinian Corpuscle Relation between stimulus intensity & receptor potential The frequency of repetitive action potentials transmitted from sensory receptors increases approximately in proportion to the increase in receptor potential Very intense stimulation of receptor causes progressively less and less additional increase in numbers of action potentials. Adaptation of receptors Receptors adapt either partially or completely to any constant stimulus after a period of time When a continuous sensory stimulus is applied, the receptor responds at a high impulse rate at first and then at a progressively slower rate until finally the rate of action potentials decreases to very few or often none at all. Adaptation is reduction in sensitivity due to repeated stimulation Mechanisms by which receptors adapt Tonic receptors Slowly adapting receptors detect continuous stimulus strength Slowly adapting receptors continue to transmit impulses to the brain as long as the stimulus is present Examples: Impulses from the muscle spindles and Golgi tendon apparatus allow the NS to know the status of muscle contraction and load on the muscle tendon at each instant Receptors of the macula in the vestibular apparatus Pain receptors Baroreceptors of the arterial tree Chemoreceptors of the carotid and aortic bodies Phasic receptors (or Rate receptors, Movement Receptors) Rapidly adapting receptors detect change in stimulus strength Stimulated React only when the stimulus strength changes strongly while a change is actually taking place Example: Pacinian corpuscle- sudden pressure applied to the tissue excites this receptor for a few milliseconds, and then the excitation is over even though the pressure continues. Later, it transmits a signal again when the pressure is released Transmission of signals of different intensity in nerve tracts Spatial and Temporal Summation Spatial summation Increasing signal strength is transmitted by using progressively greater number of fibers Temporal summation Increasing signal strength by increasing the frequency of nerve impulses in each fiber Neuronal Pools The CNS composed of thousands of neuronal pools Each input fiber divides hundreds of times to synapse with dendrites or cell bodies of the neurons in the pool Amplifying divergence Input signals spreads to an increasing number of neurons as it passes through successive orders of neurons in its path Somatic Sensations. General Organization Tactile & Position Senses Somatic senses are the nervous mechanisms that collect sensory information from all over the body. In contradistinction to the special senses (hearing, vision, smell, taste, & equilibrium) Classification of somatic senses (Physiologic types) 1. Mechanoreceptive somatic sensesstimulated by mechanical displacement of some tissue of the body a. Tactile sensations b. Position sensations 2. Thermoreceptive senses- detect heat and cold 3. Pain sense- activated by factors that damage the tissues Let us focus on the mechanoreceptive tactile & position senses Tactile Senses Touch Pressure Vibration Tickle Position Static Rate Senses position sense of movement sense Detection & Transmission of Tactile Sensations Although touch, vibration & pressure are frequently classified as separate sensations, they are detected by the same types of receptors. Three principal differences 1. Touch sensation results from tactile receptors in the skin or in tissues beneath the skin 2. Pressure sensation generally results from deformation of deeper tissues 3. Vibration sense results from rapidly repetitive sensory signals Tactile receptors Sensory pathways for transmitting somatic signals into the central nervous system Almost all sensory information from the somatic segments of the body enters the spinal cord through the dorsal roots of the spinal nerves Signals are carried through 1. The dorsal column-medial Lemniscal system 2. Anterolateral system Somatosensory tracts https://www.youtube.com/watch?v=vrmKqH8d1RM&list=PLYLMBYHSuMTmOku UvY86IUdj_Sjb7pKVN&index=2 Carries signals upward to the medulla of the brain mainly in the dorsal columns of the cord. Then after the signals synapse and cross to the opposite side in the medulla, they continue upward through the brain stem to the thalamus by way of the medial lemniscus