Physiology
Initiation of somatosensation begins with activation of a physical "receptor". These somatosensory receptors tend to lie in organs, most notably in the skin, or muscle. The structure of these receptors is broadly similar in all cases, consisting of either a "free nerve ending" or a nerve ending embedded in a specialised capsule. They can be activated by movement (mechanoreceptor), pressure (mechanoreceptor), chemical (chemoreceptor) and/or temperature. Another activation is by vibrations generated as a finger scans across a surface. This is the means by which we can sense fine textures in which the spatial scale is less than 200 µm. Such vibrations are around 250 Hz, which is the optimal frequency sensitivity of Pacinian corpuscles. In each case, the general principle of activation is similar; the stimulus causes depolarisation of the nerve ending and then an action potential is initiated. This action potential then (usually) travels inward towards the spinal cord.
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