Lecture 9 - Somatosensory Receptors PDF

Somatosensory Receptors Dr. Kelly C. S. Roballo [email protected] VCOM-Main Building Room 341 Learning Objectives 1) 2) 3) 4) 5) 6) Describe the cutaneous and proprioceptive mechanoreceptors Describe the neuronal order in all sensory pathways from the cerebrum through the brain stem and spina...

Somatosensory Receptors Dr. Kelly C. S. Roballo [email protected] VCOM-Main Building Room 341 Learning Objectives 1) 2) 3) 4) 5) 6) Describe the cutaneous and proprioceptive mechanoreceptors Describe the neuronal order in all sensory pathways from the cerebrum through the brain stem and spinal cord. Identify the sensory pathways through all sections of the central nervous system including modulatory tracts and neurons. Identify the somatotopic organization of the somatosensory system. Describe neurological deficits related to injury to the sensory pathways. Identify the primary somatosensory cortex and the sensory associational areas. Chapter 11- basic clinical neuroscience Somatosensory pathway GENERAL SOMATIC SENSES A variety of sensory endings in the skin transduce mechanical, thermal, and chemical stimuli (1) mechanoreceptive somatic senses, which include both tactile and position sensations that are stimulated by mechanical displacement of some tissue of the body (2) thermoreceptive senses, which detect heat and cold, chemoreceptors are sensitive to chemical alterations (3) pain sense, which is activated by any factor that damages the tissues. Sensors in the Skin Boron & Boulpaep, Medical Physiology Mechanoreceptors detecting innocuous stimuli in the skin (6 different types) 1. Hair follicles can detect light touch; 2. Meissner corpuscles in the dermal papillae detect indentation and slipping of objects; 3. Pacinian corpuscles in the deeper dermis detect vibration; 4. Merkel complexes in the basal epidermis create an understanding of structure and texture; 5. Ruffini corpuscles detect stretch; 6. C-fiber LTMs (tactile) detect pleasant, light tactile sensations; Costanzo, Physiology Blumenfeld, Neuroanatomy Thermoreceptors Warmth receptors begin firing above about 30°C and increase their firing rate until 44°C to 46°C Beyond 46°C, the rate falls off steeply and a sensation of pain begins Cold receptors steady discharge rate increases as the temperature falls to 24°C to 28°C Below 10°C, firing ceases and cold becomes a very effective local anesthetic Boron & Boulpaep, Medical Physiology Nociceptors • Nociceptors help signal pain that is related to temperature, pressure, and chemicals • When it comes to pain, nociceptors only signal when the body has reached a point of tissue damage • Different fibers relay pain information; these are A-delta and C fibers • Painful temperatures, uncomfortable pressures, and chemicals mostly use C-fibers. C-fibers vary to be able to sense all three types of stimuli. • A-delta fibers are small and unmyelinated and are primarily involved in thermal and mechanosensitive pain. • Nociceptors utilize mostly glutamate but also substance P, calcitonin gene-related peptide, and somatostatin to signal pain Nociceptors Relationship between Receptor Potential and Action Potentials The more the receptor potential rises above the threshold level, the greater becomes the action potential frequency. Guyton & Hall, Medical Physiology Adaptation of Receptors All sensory receptors adapt either partially or completely to any constant stimulus after a period of time Guyton & Hall, Medical Physiology Two-point discrimination Two-point sense: ability to distinguish stimulation by one or two points applied to the skin *Stereognosis: recognize objects by touch alone Light touch *Graphesthesia: ability to recognize number and letters drown on the skin * Require intact light touch pathways ad memory; in other words, the objects, numbers, or letters must be known to the individual being tested Black dots indicate an area of maximal sensitivity of a single Pacini’s corpuscle Each dot represents the entire receptive field of a single Meissner’s corpuscle The green area is the receptive field of a corpuscle 1. Pacinian corpuscles in the deeper dermis detect vibration; 2. Meissner corpuscles in the dermal papillae detect indentation and slipping of objects; Boron & Boulpaep, Medical Physiology Inhibitory circuitry for the two-point discrimination Use of Inhibition in sensory neurons Two-point discrimination test • Nerve examination • Mapping relative density receptors Peripheral Sensitization • Following a painful stimulus (cuts, scrapes, bruises), stimuli in the area of the injury that would ordinarily be perceived as slightly painful are perceived significantly more • Results from the interaction of nociceptors with the “inflammatory soup” of substances released when tissue is damaged • Arachidonic acid, lipid metabolites, bradykinin, histamine, serotonin, prostaglandins, nerve growth factor Boron & Boulpaep, Medical Physiology Glossary • Kinesthesia: sense of body movement • Mechanoreceptor: sensory receptor modified to respond to mechanical disturbance such as being bent, touch, pressure, motion, and sound • Perception: individual interpretation of a sensation; a brain function • Proprioception: sense of limb position; • Reception: receipt of a signal (such as light or sound) by sensory receptors • Receptive field: region in space in which a stimulus can activate a given sensory receptor • Receptor potential: membrane potential in a sensory receptor in response to detection of a stimulus • Sensory receptor: specialized neuron or other cells associated with a neuron that is modified to receive specific sensory input • Sensory transduction: conversion of a sensory stimulus into electrical energy in the nervous system by a change in the membrane potential • Vestibular sense: sense of spatial orientation and balance