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Neuroscience : Chapter 12 The Somatic Sensory System Somatic sensation can be divided into four broad subcategories: the senses of touch, temperature, pain, and body position In reality, it is a `catch-all` name for everything that does not include the sense of smell, hearing, tasting or sense of balance Touch Skin is the largest organ that we have; there are two layers: the dermis (outer layer) and the epidermis (the inner layer) The sense of touch begins at the skin; there are two main types of skin: hairy and glabrous (hairless) Mechanoreceptors of the Skin Mechanoreceptors: 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 teeth The different types of mechanoreceptors found in the skin include: 1. Pacinian Corpuscle: Lies deep in the dermis, it is one of the largest receptors in the skin (large on can be seen with the naked eye) 2. Ruffini`s Endings: Slightly smaller than Pacinian Corpusles; they are found in both hairy and glabrous skin 3. Meissner`s Corpuscles: Are located in the ridges of glabrous skin; about 1/10 the size of Pacinian Corpuscle 4. Merkles Disks: Consists of nerve terminals and flattened epithelial cells 5. Krause End Bulb: Lie in the border regions of dry skin and mucous membrane; looks 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 endings) There are also differences in how different mechanoreceptors respond to different stimuli; Pacinian Corpuscle + Meissners Corpuscle = respond quickly at first but stop firing after a while (rapidly adapting receptors) Merkels Disks and Ruffinis Endings = Generate 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 it When 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 rate Pacinian Corpuscles are most sensitive to vibrations of about 200-300 HZ Meissners Corpuscles respond best to frequencies of 50 Hz Pacinian Corpuscle and Vibrations As 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 decreases The reason for this is that Pacinian Corpuscles have football-shaped capsule with 20-70 layers of connective tissue wrapped around the nerve ending in the middle When 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 potentials The 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 Two-Point Discrimination Two-Point Discrimination: The ability to discern that two nearby objects touching the skin are truly two distinct points Two-point discrimination various greatly from one point of the body to another. Fingertips have the highest resolutionThere are several reasons for this: 1. There is a higher density of mechanoreceptors on the tip of a finger than anywhere else in the body 2. The receptors on the finger tip have much smaller receptive fields = better discrimination 3. There is more brain tissue devoted to the fingertips 4. There may be specialized neurons devoted to high-res. Discrimination Primary Afferent Axons Primary Afferent Axons: Axons bringing info from the somatic sensory receptors to the spinal cord or brain stem There are many different types of axons, varying in size and diameter. From biggest to smallest they go: Proprioceptors in Skeletal Muscle (A; Group I; diameter = 13-20pm; Speed = 80-120 m/sec) Mechanoreceptors of the skin (B; Group II; Diameter = 6-12pm; Speed = 35-75 m/sec) Pain, and temperature (A; Group III; Diameter = 1-5pm; Speed = 5-30 m/sec) Temperature, Pain, and Itch (C; Group IV; Diamete
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