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Chapter 12

Chapter 12: The Somatic Sensory System

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University of Toronto Scarborough
Matthias Niemeier

Sanchika K thFeb 8 2012 Sensory and Motor Chapter 12 The Somatic Sensory System Introduction y Somatic sensation enables our body to feel to ache to chill and to know what its parts are doing y It is sensitive to many stimuli o The pressure of objects against the skin o The position of joints and muscles o Distension of the bladder o Temperature of the limbs and the brain o Most important somatic sensation is responsible for the feeling of pain y The somatic sensory is differentfrom other sensory systems in two interesting ways o Receptors are distributed throughout the bodyNot in just one specific spot o It responds to many different stimuli Touch temperature and painy The somatic sensory system is basically every other system other than seeing hearing tasting smelling and the vestibular sense of balance y A single sensory receptor can encode stimulus features such as intensity duration position and sometimes directionTouch y Sensation of touch begins at skin y There are two major types of skin o HairyBack of hand o GlabroushairlessPalm of hand y Skin has an outer layer called the epidermis y Skin has an inner layer called the dermis y Skin is essential for protection and also it prevents the evaporation of body fluids in the dry environment o Also provides our most direct contact with the world o Largest sensory organ we have y Touch of the skin is transduced into neural signalso The signals make their way to the brain o The brain makes sense of themMechanoreceptors of the Skin y Mechanoreceptors o Sensitive to physical distortion Such and bending or stretching y At the heart of each mechanoreceptors are unmyelinated axon branches Sanchika K thFeb 8 2012 Sensory and Motor o They have mechanosensitive ion channelsThe gating depends on stretching or changes in tension of the surrounding membrane y the largest best studied receptor is the pacinian corpuscle y Ruffinis ending found in both hairy and glabrous skin y Meissners corpuscles tenth the size of Pacinian corpuscle Located in the ridges of glabrous skin y Merkels disk each consists of a nerve terminal and a flattened nonneural epithelial cell y We have mechanoreceptors that vary in their preferred stimulus frequencies pressures and receptive field sizes y Meissners corpuscle and Merkels disk have small receptive fields y Pacinian corpuscles and Ruffinis endings have larger receptive fields y Meissners and Pacinian receptors respond quickly at first but then stop firing even though the stimulus continues o Their receptors are said to be rapidly adapting y Merkels disks and Ruffinis endings are slowly adapting and generate a more sustained response during a long stimulus y Hair grows from follicles embedded in the skin ach innervated by free nerve endings o y The bending of hair causes a deformation of the follicle and surrounding skin tissue o In turn this stretches bends or flattens the nearby nerve endings o Which then increases or decreases the action potential y Pacinian receptors are most sensitive to vibrations of about 200300 Hz y Meissners receptor respond best around 50 Hz y Lower frequencies can activate the Ruffinis ending and Meissners corpusclesVibration and the Pacinian corpuscle y The selectivity of a mechanoreceptive axon depends primarily on the structure of its special ending g the pacinian has a footballshaped capsule y yIn an experiment while stimulated with a probe y When it is in contact with a probe a large receptor potential was generated at the onset and offset of the stimulus during maintained indentation the receptor potential disappeared
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