Mechanosensation

Mechanosensation is a response mechanism to mechanical stimuli. The physiological foundation for the senses of touch, hearing and balance, and pain is the conversion of mechanical stimuli into neuronal signals: mechanosensation. Mechanoreceptors of the skin, called cutaneous mechanoreceptors, are responsible for touch. Tiny cells in the inner ear, called hair cells, are responsible for hearing and balance. States of neuropathic pain, such as hyperalgesia and allodynia, are also directly related to mechanosensation. A wide array of elements are involved in the process of mechanosensation, many of which are still not fully understood.

Cutaneous mechanoreceptors are physiologically classified with respect to conduction velocity, which is directly related to the diameter and myelination of the axon.

Mechanoreceptors that possess a large diameter and high myelination are called low-threshold mechanoreceptors. Fibers that respond only to skin movement are termed rapidly adapting mechanoreceptors (RA), while those that respond also static indentation are termed slowly adapting mechanoreceptors (SA).

Aδ fibers are characterized by thin axons and thin myelin sheaths, and are either D-hair receptors or nociceptive neurons. Aδ fibers conduct at a rate of up to 25 m/s. D-hair receptors have large receptive fields and very low mechanical thresholds, and have been shown to be the most sensitive of known cutaneous mechanoreceptors. A-fiber mechanoreceptors (AM) also have thin myelination and are known for their "free" nerve endings. It is believed that A-fiber mechanonociceptors have high mechanical sensitivity and large receptive fields, and are responsible for rapid mechanical and heat pain.

C fibers have slow conduction velocities of less than 1.3 m/s because they do not have a myelin sheath at all. C fibers account for 60-70% of primary afferent neurons that innervate the skin. C fibers are activated by both mechanical and thermal stimuli, and also respond to algesic chemicals, such as capsaicin. Some C fibers respond only to mechanical stimuli. Therefore, classification of C fibers are broken down further. C-fiber nociceptors which respond to both mechanical and thermal stimuli include C-mechanoheat (C-MH), C-mechanocold (C-MC), and C-mechanoheatcold (C-MHC). C-fiber nociceptors that respond only to mechanical stimuli are called C-mechanonociceptors (C-M). Other groups of C fibers include C-fiber low threshold mechanoreceptors (C-LT), which are involved in nondiscriminative touch, and mechanically insensitive afferents (MIA), which lack mechanosensitivity and are also known as “silent” or “sleeping” nociceptors. C fibers called C-mechano insensitive, heat insensitive (C-MiHi) account for about 15-25% of all C fibers.

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