Pain stimuli are conducted from peripheral nociceptors to the dorsal horn via both unmyelinated and myelinated fibers. Nociceptive nerve fibers are classified according to their degree of myelination, diameter, and conduction velocity. Nociceptors have two different types of axons that transmit pain impulses to the dorsal root ganglion. The first are the Ad-fiber axons. These axons are myelinated and allow action potentials to travel at a very fast rate of approximately 20 m/s toward the central nervous system (CNS). The other type is the more slowly conducting non-myelinated C-fiber axons. These only conduct at speeds of about 2 m/s. Thus, in the classic example of touching a hot stove, the AS fibers transmit the "first pain," a rapid onset well-localized, sharp pain of short duration while the C fibers are responsible for the "second pain" or delayed pain. Second pain is associated with a delayed latency and is described as a diffuse burning, stabbing sensation that is often prolonged and may become progressively worse.
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