Skin

Nociceptors innervating the skin can be differentiated into two major groups on the basis of the conduction velocity of the axon transmitting impulse discharge from the skin to the spinal cord. These are the small myelinated Ad-fibers and unmyelinated C-fibers. Two groups of Ad-nociceptive afferents have been described. Both discharge in response to high-intensity, potentially noxious mechanical stimuli but can be distinguished on the basis of their response to noxious heat. One group with a high thermal threshold (>53°C) exhibits a long response latency to heat stimuli and tends to sensitize in response to successive thermal stimuli. These are known as type I AMH (A mechanoheat) receptors, but because of their high thermal threshold this response is often ignored; thus, they are denoted as high-threshold mechanoreceptors (HTMs). The other group, known as type II AMH fibers, has a lower heat threshold (46°C), tends to respond quickly, and desensitizes in response to repeated stimuli. Some of these fibers exhibit sensitivity to chemical stimuli. Nociceptors innervated by C-fibers also exhibit responses to noxious mechanical and thermal stimuli (threshold averaging about 46°C) and often to chemical stimuli such as lowered pH and bradykinin. These sensory neurons are often referred to as polymodal nociceptors.

Many investigators have identified a population of small-diameter afferent fibers whose terminations in the skin can be identified by electrical stimulation but that exhibit no response to peripheral stimuli of any known modality. These "silent" nociceptors (presumably nociceptors whose sensitivity can be brought to a detectable level by sensitizing stimuli) represent an extreme of the spectrum of nociceptive thresholds in normal skin.

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