title: Velocity recovery cycles of single C fibres innervating rat skin.
abstract: 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.
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, Queen Square, London WC1N 3BG, UK.
authors.forename: Annette
authors.initials: A
authors.lastname: George
authors.forename: Jordi
authors.initials: J
authors.lastname: Serra
authors.forename: Xavier
authors.initials: X
authors.lastname: Navarro
authors.forename: Hugh
authors.initials: H
authors.lastname: Bostock
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology; D004558:Electric Stimulation; D004594:Electrophysiology; D005260:Female; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D009412:Nerve Fibers / Q000145:classification / Q000502:physiology; D036421:Nerve Fibers, Unmyelinated / Q000502:physiology*; D009619:Nociceptors / Q000502:physiology; D010812:Physical Stimulation; D051381:Rats; D017207:Rats, Sprague-Dawley; D012867:Skin / Q000294:innervation*