Partial reversal of conduction slowing during repetitive stimulation of single sympathetic efferents in human skin.
OBJECTIVE
To describe and identify the function of a class of human C fibre with an unusual response to repetitive electrical stimulation. Other C fibres slow progressively at 2 Hz (type 1), reach a latency plateau (type 2) or hardly slow at all (type 3).


METHODS
C fibres innervating hairy skin were recorded by microneurography in the superficial peroneal nerves of 19 healthy volunteers. Baseline electrical stimulation of the skin was at 0.25 Hz, and activity-dependent slowing recorded during stimulation at 2 Hz for 3 min and after a 3-min pause in stimulation.


RESULTS
In 41 units, there was a partial recovery of latency during repetitive stimulation. These were classified as 'type-4' units, and identified as sympathetic efferents, since they exhibited spontaneously activity, which was enhanced by manoeuvres that increase sympathetic outflow (15 of 16 cases) and/or suppressed by a proximal anaesthetic block (eight of eight cases). The peak slowing during 2 Hz trains averaged 6.47 +/- 2.06% (mean +/- SD, n=41), but after 3 min the slowing had reduced to 4.90 +/- 2.20%, which was less than in all type 1 (nociceptor) fibres but similar to that in type 2 (cold) fibres. Compared with cold fibres, type-4 sympathetic fibres slowed more after the first 10 impulses at 2 Hz (2.57 +/- 0.45%) and also after a pause in stimulation (1.66 +/- 0.51%).


CONCLUSIONS
The distinctive activity-dependent slowing profiles of these type-4 sympathetic C units may help identification in vitro, and suggest that hyperpolarization-activated channels have a particularly prominent role in the axonal membrane.
Campero
M
M
Good Samaritan Hospital & Medical Center and Oregon Health Sciences University, Portland, OR, USA.
Serra
J
J
Bostock
H
H
Ochoa
J L
JL
D000293:Adolescent; D000328:Adult; D001369:Axons / Q000502:physiology; D004525:Efferent Pathways / Q000502:physiology; D004558:Electric Stimulation / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D036421:Nerve Fibers, Unmyelinated / Q000502:physiology*; D009431:Neural Conduction / Q000502:physiology*; D009476:Neurons, Efferent / Q000502:physiology*; D009619:Nociceptors / Q000502:physiology; D010543:Peroneal Nerve / Q000502:physiology; D012879:Skin Physiological Phenomena; D013564:Sympathetic Nervous System / Q000502:physiology*