Functional Attributes Discriminating Mechano-Insensitive and Mechano-Responsive C Nociceptors in Human Skin

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Abstract
Microneurography was used in healthy human subjects to record action potentials from unmyelinated nerve fibers (C units) in cutaneous fascicles of the peroneal nerve.Activity-dependent slowing (n= 96) and transcutaneous electrical thresholds (n= 67) were determined. Eight units were sympathetic efferents according to their responses to sympathetic reflex provocations. Mechano-heat-responsive C units (CMH) (n= 56) had thresholds to von Frey hair stimulation ≤90 mN (6.5 bar). Mechano-insensitive C units (n= 32) were unresponsive to 750 mN (18 bar). Twenty-six mechano-insensitive units responded to heat (CH), and the remaining six units did not respond to physical stimuli but were proven to be afferent by their response to intracutaneous capsaicin (CMiHi).Mechano-insensitive units had significantly slower conduction velocity (0.81 ± 0.03 m/sec), and CH units had higher heat thresholds (48.0 ± 0.6°C) compared with CMH units (1.01 ± 0.01 m/sec; 40.7 ± 0.4°C). Transcutaneous electrical thresholds were 35 mA for CH and CMiHiunits. Activity-dependent slowing was much more pronounced in mechano-insensitive than in mechano-responsive units, without overlap. Sympathetic efferent C units showed intermediate slowing, significantly different from CMH, and completely separate from CH and CMiHiunits. The activity-dependent slowing of conduction provides evidence for different membrane attributes of different classes of C fibers in humans.