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Chapter 12

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Neuroscience Chapter 12The Somatic Sensory SystemSomatic sensation can be divided into four broad subcategories the senses of touch temperature pain and body position In reality it is a catchall name for everything that does not include the sense of smell hearing tasting or sense of balance TouchSkin is the largest organ that we have there are two layers the dermis outer layer and the epidermis the inner layerThe sense of touch begins at the skin there are two main types of skin hairy and glabrous hairless Mechanoreceptors of the SkinMechanoreceptors The sensory receptors of the somatic sensory system They monitor contact with the skin as well as pressure in the heart and blood vessels stretching of the digestive organs and urinary bladder and force against the teethThe different types of mechanoreceptors found in the skin include1 Pacinian CorpuscleLies deep in the dermis it is one of the largest receptors in theskin large on can be seen with the naked eye2 Ruffinis EndingsSlightly smaller than Pacinian Corpusles they are found in bothhairy and glabrous skinth3 Meissners Corpuscles Are located in the ridges of glabrous skin about 110 the sizeof Pacinian Corpuscle4 Merkles DisksConsists of nerve terminals and flattened epithelial cells5 Krause End BulbLie in the border regions of dry skin and mucous membranelooks like knotted balls of string Some mechanoreceptors have small receptive fields Meissners Corpuscles and Merkels Disks while others have large receptive fields Pacinian Corpuscles and Ruffinis endingsThere are also differences in how different mechanoreceptors respond to different stimuli Pacinian CorpuscleMeissners Corpuscle respond quickly at first but stop firing after awhile rapidly adapting receptorsMerkels Disks and Ruffinis EndingsGenerate a more sustained response during a long stimulus slowly adapting Hairs grow from follicles that are embedded into the skin these follicles are innervated by free nerve endings look like epithelial cells that outline the hair There are also follicle receptors that may wrap around it or run parallel to itWhen the hairs in the follicle bend they cause deformation in the follicle and the follicle receptors This then alters the nearby nerve endings which increase or decrease their AP firing ratePacinian Corpuscles are most sensitive to vibrations of about 200300 HZ Meissners Corpuscles respond best to frequencies of 50 Hz Pacinian Corpuscle and VibrationsAs aforementioned the Pacinian Corpuscle is rapidly adapting it responds to pressure with a high firing rate at first but after a while the firing rate decreasesThe reason for this is that Pacinian Corpuscles have footballshaped capsule with 2070 layers of connective tissue wrapped around the nerve ending in the middleWhen the capsule is compressed energy is transferred through the layers of connective tissue to the nerve endings where mechanosensitive ion channels open and cause APs However the layers of connective tissue have vicious fluid between them and if pressure is maintained the layers slip passed one another in a way that relieves the pressure on the nerve ending causing the cessation of the action potentialsThe connective tissue layers make the receptor more sensitive to vibrations and less sensitive to constant pressure If a Pacinian Corpuscle is stripped of its connective tissue it becomes less sensitive to vibrations and more sensitive to constant pressure TwoPoint DiscriminationTwoPoint Discrimination The ability to discern that two nearby objects touching the skin are truly two distinct pointsTwopoint discrimination various greatly from one point of the body to another Fingertips have the highest resolution
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