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Chapter 12

Chapter 12 The Somatic Sensory System

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Matthias Niemeier

Chapter 12The Somatic Sensory System Somatic sensation enables our body to feel to ache to chill and to know that its parts are doing it is responsible for touch and pain The somatic motor system is different from other sensory motor systems for two reasons 1 receptors are distributed throughout the body rather than being concentrated at particular locations 2 responds to many different kinds of stimuli therefore we can think of it as a group of at least 4 senses rather than a single onesense of touch temperature pain and positionA single sensory receptor can encode stimulus features such as intensity duration position and sometimes direction BUT a SINGLE stimulus usually activates MANY receptors TOUCHLargest sensory organ two major types hairy and glabrous hairless Skin has an outer layer called the epidermis and an inner layer called the dermisFUNCTION serves as protection prevents evaporation of body fluidsand provides direct contact with the world Mechanoreceptors of the Skin Mechanoreceptors are sensitive to physical distortions such as bending and stretching they monitor contact with the skin pressure in the heart and blood vessels and stretching of the digestive organs urinary bladder etcStretchingchanges in tension of the surrounding membrane gates the mechanosensitive ion channels in the unmyelinated axon branches of mechanoreceptors STRUCTURE DESCRIPTION Receptive AdaptionTuning Field SizeNerve terminal Merkelsflattened nondisks neural epithelial SMALL SLOW cell synapselike junction Meissners Located in the corpuscles ridges of SMALL RAPID 50 Hz glabrous skin Pacinian Largest receptor corpuscle located deepwithin the LARGE RAPID 200300Hz dermis Ruffinis Found in both endings hair and LARGE SLOW Very low glabrous skinfreqTo distinguish between different kinds of mechanical energy we have mechanoreceptors that vary in their preferred stimulus frequencies pressures and receptive field sizesSIGNAL TRANSDUCTION Bending of hairDeformation of follicle and surrounding tissueStretchbendflatten nerve endingsincreasedecrease AP firing rate different mechanical sensitivities of mechanoreceptors mediate different sensationsVibration and the Pacinian Corpuscle The selectivity of a mechanoreceptive axon depends primarily on the structure of its special endingExample Pacinian capsuleCapsule compressedenergy transferred to nerve terminalmembrane deformedmechanosensitive channels opencurrent flow generates receptor potentialdepolarizationfiring of action potential if depolarization is large enough
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