One hundred ninety-four cutaneous C-fibers were recorded from the human peroneal nerve; 118 units were found by mechanical stimuli and 76 units were detected by electrical stimulation through a surface electrode. Needle electrodes were then inserted for electrical intradermal stimulation in the innervation territory of the units. Afferent and efferent sympathetic C-fibers were identified by slowing of conduction velocity after activation either by physical or chemical skin stimuli, or by arousal maneuvers eliciting sympathetic reflexes. In addition to mechano-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 only to heating (CH), and units that were insensitive to mechanical and heating stimuli and also to sympathetic provocation tests (CMiHi). With the electrical search technique we found 45% CMH, 13% CM, 6% CH, 24% CMiHi, and 12% sympathetic units. Excitation by topically applied mustard oil occurred in 58% of CMH units, and in one-third of CM and CMiHi units, respectively. Some CM, CH, and CMiHi units were sensitized to heating and/or to mechanical stimuli after topical application of mustard oil or capsaicin. These units then acquired responsiveness to a stimulus modality to which they previously were insensitive. Such recruitment of previously silent nociceptors implies spatial summation to the nociceptive barrage at central levels, and may contribute both to primary hyperalgesia to heat and pressure after chemical irritation, and to secondary hyperalgesia as a consequence of central sensitization.
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