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Sensory and Motor Systems Mechanoreceptors also vary in the persistence of their resPonses to long lasting stimuli Meissners and Pacinian corpuscles respond quickly at first but then stop firing even though the stimulus continues ---these receptors are called rapidly adapting ---Local government MPS are rapidly adapting! Miss P Merkels and Ruffinis are slow adapting and generate a sustained response hairs are part of a sensory receptor system for most animals, hair is a major sensory system i.e a rat , navigates by using its facial vibrissae (whiskers) 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 diff types of hair follicles including some with erectile muscles (i.e facilitate goosbumps) and the details of their innervation differ in all cases, the bending of hair cause a deformation of the follicle and surrounding skin turn stretches, bends or flattens the nearby nerve endings which then respond appropriately by increasing or decreasing their firing frequency. mechanorecpetors of hair follicles may be either slowly adaptign or rapidly adapting diff mechanical sensitvities of mechanoreceptors mediate diff sensations Pacinian corpuscles are most sensitive to vibrations of baout 200-300 Hz Meissners corpuscles around 50Hz VIBRATION AND PACINIAN CORPUSCLE selectivity of a mechanoreceptor axon depends on primarily the structure of its special ending Pac.corpuscle: oval shaped capsule with 20-70 concentric layers of an onion, with a nerve terminal in the middle when the capsule is compressed, energy is transferred to the nerve terminal, its membrane is deformed and mechanosensitive channels open. current flowing through the channel generates a receptor potential, that is depolarizing. if depolarizing is large enough, the axon will fire an AP. But the capsule layers are slick with vsicous fluid in between them.---> therefore,if stimulus pressure is maintained, the layers transfer the stimulus energy in such a way that the axon terminal is no longer deformed and receptor potential dissipates. when pressure is released, the events reverse themselves, the terminal depolarizes again and may fire another action potential.----> rapidly adapting When pac.corpuscle is stripped away of its capsule, the naked nerve terminal becomes much less sensitive to vibrating stimuli and more sensitive to steady pressure. --> therefore, it is the layered capsule that makes the Pac. corpsucle sensitive to vibrating, high frequency stimuli and unresponsive to steady pressure Two-point discrimiantion Primary Afferent axons -skin is richly innervated by axons -Primary Afferent axons: axons bringing information from somatic sensory receptors to the spinal cord or brain stem they enter the spinal cord through the dorsal roots, their cell bodies lie in the dorsal root ganglia
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