Innervation territories of single sympathetic C fibers in human skin.

Microneurography techniques were used to record action potentials from unmyelinated nerve fibers (C fibers) in the cutaneous fascicles of the peroneal nerve in healthy volunteers. C units were identified by their long latency responses to electrical stimulation of their terminals in the skin. Their responsiveness to mechanical or heat stimuli applied to the skin or to sympathetic reflex provocation tests was determined by transient slowing of conduction velocity following activation (marking technique). In a sample of 381 C units, 59 were unresponsive to mechanical and thermal stimulation of their endings, but responded to sympathetic reflex provocation tests, e.g., arousal or deep inspiration. They were classified as sympathetic efferent units. On average, conduction velocities of sympathetic units were lower (0.78 +/- 0.12 m/s, mean +/- SD) than those of mechano-heat (CMH) or mechanoresponsive (CM) afferent C units (0.91 +/- 0.14 m/s). Endings of most of the sympathetic units were located in the skin of toes or in the foot dorsum. Innervation territories of 16 sympathetic units were mapped by means of conditioning transcutaneous electrical stimuli. Twelve units had one continuous skin territory, whereas two units had two and two other units had three and five separate territories, respectively. The mean innervated area was 128 mm2 (range: 24-350 mm2). Innervation territories of sympathetic units were of approximately the same size in different skin regions on the lower leg, foot, or toes. Based on responses to whole body cooling and warming, two units were tentatively classified as vasoconstrictor and sudomotor units, respectively. Eleven units were tested for responsiveness to iontophoresis of acetylcholine in their innervation territories. In five of them, activity was induced that was not due to central reflex activity but instead due to antidromic activation from the peripheral terminals. Iontophoresis of saline or histamine was ineffective. These findings confirm the existence of excitatory cholinergic receptors in the terminal membrane of some sympathetic units, possibly sudomotors.