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Lecture_9-10 - Somatosensory System I All sensory input...

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10/4/05 Somatosensory System I All sensory input from the body to the brain, not including traditional senses (vision, taste, smell) Somatic sensation is a distributed sense- receptors are spread throughout the body (instead of being concentrated in specific areas) 4 large categories 1. Touch- Pressure, texture, vibration (brail) 2. Pain- painful touch or temperature (signals tissue damage) 3. Temperature (non-painful) 4. Proprioception (muscle feedback) Primary afferent (going to the brain) pathway of somatosensory system : sensory receptors project to the dorsal root ganglion, into the spinal cord then to the brainstem Learn 4 classes of afferent axons Figure 12.9 page 404. A(alpha), A(beta), A(delta), C Larger diameter-->smaller diameter More myelin-->less myelin Touch - receptors located in skin (primarily mechanoreceptors) -mechanoreceptors are generally found in the dermis, some project to the epidermis -Changes can be caused (AP) by bending or deforming the receptor -non-myelinated -also found in epithelium: heart, bladder, intestine -Action potentials are caused by ion channels that are triggered by deforming the receptor Types of touch receptors * Pacinian corpuscles - largest, located in deep dermis in both hairy and glabrous skin * Ruffinis endings - smaller in deep dermis of both hairy and glabrous skin * Meissner's corpuscles - half the size of Ruffini, found in glabrous skin with ridges (finger tips), in superficial dermis (on the edge of the epidermis) * Merkel's Disks - small, flattened nerve ending in epidermis, nerve isn't mechanically sensitive- epithelial cells are mechanoreceptive, they send signals to the disks * Hair follicle receptor - nerve associated with the hair follicle (in hairy skin, obviously) with movement of hair, the membrane is deformed and creates signal * Free nerve ending - not strictly somatosensory- pressure, pain, temp detectors, in hairy and glabrous skin * Krause end bulb - edge of lips from dry outer skin to wet inner skin (mucous membrane) (understand function more that area) properties depend on outer capsule (all same inside: un-myleinated nerve)
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Receptive fields - size determines spatial resolution of the nerve; smaller resolution = finer reception, smaller receptor field = finer sense *Meissner's have small receptive fields *Pacianian corpuscles have large receptive fields Adaptation or Attenuation (stop reacting to/ get used to) - reaction speed is determined by the capsule surrounding the axon Receptive field adaptation 2point discrimination (if you put two points on one part of the body and gradually move them apart, the distance between points shows the sensitivity of that area) Affected by - Receptor density , more dense = shorter discrimination distance - Receptive field, smaller = more precise discrimination - Brain area receiving input , more area =more precise discrimination Spinal nerves (signals are sent back through the spinal nerve) -the nerves project through the dorsal root of the spinal cord -limited number of nerves: one spinal nerve per vertebra -enters at dorsal root, exits at ventral root
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