Chapter 12.docx

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University of Toronto Scarborough
Matthias Niemeier

Chapter 12 The somatic sensory system Introduction Somatic sensation Detection of 1 Touch 2 Pain 3 Temperature 4 Body position Unique features of the somatic system 1 Receptors distributed throughout the body2 Group of 4 senses rather than one collective sense 3 A single sensory receptor can encode stimulus features such asa Intensityb Duration c Position d Direction4 A single stimulus activates many receptors5 The central nervous system interprets the activity of the vast receptor array and uses it to generate coherent perceptionsTouch Two major types of skin 1 Hairy 2 Glabrousa HairlessLayers of skin 1 Epidermis a Outer layer2 Dermis a Inner layerFunction of skin1 Protective function2 Prevents evaporation of body fluids into the dry environment3 Largest sensory organMechanoreceptors of the skin Makes up the majority of the sensory receptors in the somatic sensory system 1 Sensitive to physical distortion such as bending or stretching2 Monitor contact with the skin 3 Monitor pressure in the heart and blood vessels 4 Monitor stretching of the digestive organs and urinary bladder 5 Monitor Force against the teeth6 Contains unmyelinated axon branches a These branches have mechanosensitive ion channels their gating depends on changes of tension stretching of the surrounding membrane7 Vary in their preferreda Stimulus frequencies b Pressures c Receptive field sizes8 Vary in the persistence of their responses to long lasting stimulia Rapidly adapting i Meissners and pacinian corpuscles tend to respond quickly at first then stop firing despite continuing stimulus b Slowly adapting i Merkels disks and ruffinis endings tend to generate more sustained response during a long stimulus Pacinian corpuscle Largest receptor in the somatic system that lies deep in the dermis inner layer of skin Ruffinis endings Found in both hairy and glabrous skin Slightly smaller than Pacinian corpusclesMeissners corpuscles 110 the size of pacinian corpuscles and are located in the ridges of glabrous skin Merkels disks Located within the epidermis they consist of a nerve terminal and a flattened nonneural epithelial cell The epithelial cell seems to be the mechanically sensitive part because it makes a synapse like junction with the nerve terminal Krause end bulbs Lie in the border regions of dry skin and mucous membrane around lips and genitals The nerve terminals look like knotted balls of string Skin sensitivity Skin can be 1 Vibrated 2 Pressed 3 Pricked 4 Stroked Skin hairs can be
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