NOVEL CLASSES OF RESPONSIVE AND UNRESPONSIVE C-NOCICEPTORS IN HUMAN SKIN

One hundred ninety-four cutaneous C-fibers were recorded from the huma n peroneal nerve; 118 units were found by mechanical stimuli and 76 un its were detected by electrical stimulation through a surface electrod e. Needle electrodes were then inserted for electrical intradermal sti mulation in the innervation territory of the units. Afferent and effer ent sympathetic C-fibers were identified by slowing of conduction velo city after activation either by physical or chemical skin stimuli, or by arousal maneuvers eliciting sympathetic reflexes. In addition to me chano-heat-responsive C units (CMH) also found in previous studies, we here report on novel classes of C nociceptors in human skin, namely, units responding only to mechanical stimuli (CM), units responding onl y to heating (CH), and units that were insensitive to mechanical and h eating stimuli and also to sympathetic provocation tests (CM(i)H(i)). With the electrical search technique we found 45% CMH, 13% CM, 6% CH, 24% CM(i)H(i), and 12% sympathetic units. Excitation by topically appl ied mustard oil occurred in 58% of CMH units, and in one-third of CM a nd CM(i)H(i) units, respectively. Some CM, CH, and CM(i)H(i) units wer e sensitized to heating and/or to mechanical stimuli after topical app lication of mustard oil or capsaicin. These units then acquired respon siveness to a stimulus modality to which they previously were insensit ive. Such recruitment of previously silent nociceptors implies spatial summation to the nociceptive barrage at central levels, and may contr ibute both to primary hyperalgesia to heat and pressure after chemical irritation, and to secondary hyperalgesia as a consequence of central sensitization.