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Chapter 12


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INTRODUCTION Somatic sensation enables our body to feel to ache to chill and to know what its parts are doing It is sensitive to many kinds of stimuli The somatic sensory system is different from other sensory systems in two interesting waysits receptors are distributed throughout the body rather than being concentrated at small specialized locationsit responds to many different kids of stimuli grouped into four sensestouch temperature pain and body position A single stimulus usually activates many receptors The central nervous system interprets the activity of the vast receptor array and uses its to generate coherent perceptionsTOUCH Touch begins at the skin There are two major types of skin hairy and glabrous hairless Skin has an outer layer the epidermis and an inner layer the dermis Skin performs an essential protective function and it prevents the evaporation of body fluids into the dry environment we live inMechanoreceptors of the Skin Most of the sensory receptors in the somatic sensory system are mechanoreceptors which are sensitive to physical distortion such as bending or stretching These receptors monitor contact with the skinpressure in the heart and blood vesselsstretching of the digestive organs and urinary bladderforce against the teeth Each mechanoreceptor contains unmyelinated axon branches which have mechanosensitive ion channels whose gating depends on stretching or changes in tension of the surrounding membraneSee Fig 121 The largest and best studied receptor is the Pacinian corpuscle which lies deep in the dermis Ruffinis endings are found in both hairy and glabrous skin slightly smaller than the Pacinian corpuscles Meissners corpuscles are located in the ridges of glabrous skin Merkels disks are located within the epidermis and consist of a nerve terminal and flattened nonneural epithelial cells In Krause end bulbs borders regions of dry skin and mucous membraneSkin can be vibrated pressed pricked and stroked and its hair can be bent or pulled The skin is able to tell these different stimuli apart We have mechanoreceptors that vary in their preferred stimulus frequencies pressures and receptive field sizes Mechanoreceptors also vary in the persistence of their responses to long lasting stimuli For some animals hair is a major sensory system Hairs grow from follicles embedded in the skin Each follicle is richly innervated by free nerve endings that either wrap around it or run parallel to it There are several types of hair follicles In all types the bending of the hair causes
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