Functional attributes discriminating mechano-insensitive and mechano-responsive C nociceptors in human skin

Microneurography was used in healthy human subjects to record action potent ials from unmyelinated nerve fibers (C units) in cutaneous fascicles of the peroneal nerve. Activity-dependent slowing (n = 96) and transcutaneous electrical threshold s (n = 67) were determined. Eight units were sympathetic efferents accordin g to their responses to sympathetic reflex provocations. Mechano-heat-respo nsive C units (CMH) (n = 56) had thresholds to von Frey hair stimulation le ss than or equal to 90 mN (6.5 bar). Mechano-insensitive C units (n = 32) w ere unresponsive to 750 mN (18 bar). Twenty-six mechano-insensitive units r esponded to heat (CH), and the remaining six units did not respond to physi cal stimuli but were proven to be afferent by their response to intracutane ous capsaicin (CMiHi). Mechano-insensitive units had significantly slower conduction velocity (0.8 1 +/- 0.03 m/sec), and CH units had higher heat thresholds (48.0 +/- 0.6 de grees C) compared with CMH units (1.01 +/- 0.01 m/sec; 40.7 +/- 0.4 degrees C). Transcutaneous electrical thresholds were <9 mA for CMH units and >35 mA for CH and CMiHi units. Activity-dependent slowing was much more pronoun ced in mechano-insensitive than in mechano-responsive units, without overla p. Sympathetic efferent C units showed intermediate slowing, significantly different from CMH, and completely separate from CH and CMiHi units. The ac tivity-dependent slowing of conduction provides evidence for different memb rane attributes of different classes of C fibers in humans.