Pathophysiology of Cancer Pain

A-fiber nociceptors also respond to mechanical, heat, and chemical stimuli and therefore are polymodal in nature like C-fibers. Two types of A-fiber nociceptors have been identified based on their profile of response to heat stimuli. Type I A-fiber nociceptors have very high thresholds under normal circumstances, and, because of this, are referred to as high-threshold mechanoreceptors bymany investigators (13-15). However, many of these nociceptors also respond well to intense heat stimuli (16), and therefore are likely involved in signaling the pain associated with intense heat. Type I A-fiber nociceptors are particularly prevalent on the glabrous skin of the hand in monkey (17) and have also been described in cat, rabbit, and man (18, 19). Themean conduction velocity forType IA-fiber nociceptors inmonkey is 30m=sec and extends as high as 55m=sec. Thus, by conduction velocity criteria, these nociceptors fall into a category between that of Ad and Ab fibers.