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J Physiol. 2007 Jan 1;578(Pt 1):213-32. Epub 2006 Oct 5.

Velocity recovery cycles of single C fibres innervating rat skin.

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Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, Queen Square, London WC1N 3BG, UK.


To improve knowledge about axonal membrane properties in nociceptive and non-nociceptive C fibres, we studied impulse-dependent velocity changes by in vivo microneurography in the rat sciatic nerve. Cutaneous C fibres were classified, based primarily on their activity-dependent slowing profile, as Type 1A (mechano-responsive nociceptors; CMR; n = 23), Type 1B (mechano-insensitive nociceptors; CMI; n = 24), Type 2 (cold units; n = 2), Type 3 units (unknown function; n = 4) or Type 4 (presumed sympathetics; n = 23) units. They were excited by single, double and triple electrical stimuli to the skin at mean rates of 0.25, 0.5, 1 and 2 Hz and with interstimulus intervals ranging from 2 to 1000 ms. All CMRs exhibited only postspike subnormality at 0.25 and 0.5 Hz. They gradually developed supernormality with higher stimulation rates, and 12/19 CMRs were supernormal at 1 Hz. The CMIs showed a greater tendency towards supernormality, with 10/21 already supernormal at 0.25 Hz, 17/24 at 0.5 Hz and all were supernormal at 1 Hz. In some CMIs but in none of the CMRs, the supernormal period was directly followed by a peak in late subnormality. Among non-nociceptive fibres, all Type 4 units exhibited long-lasting supernormality independent of the stimulation rate, whereas the cold units showed short-lived supernormality. In both, supernormality increased with higher stimulation rates. Regardless of fibre function or stimulation rate, a second conditioning stimulus always induced additional slowing, providing evidence for a passive origin of supernormality in all rat C fibre subtypes. However, the degree and time-course of extra slowing due to a preconditioning stimulus was highly dependent on fibre function and stimulation rate. These data indicate axonal membrane differences between different functional classes of C fibres, which resemble those previously described in human C fibres.

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