Velocity recovery cycles of C fibres innervating human skin

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Abstract
Velocity changes following single and double conditioning impulses were studied by microneurography in single human C fibres to provide information about axonal membrane properties. C units were identified as mechano-responsive (n= 19) or mechano-insensitive (12) nociceptors, cold-sensitive (8) or sympathetic fibres (9), and excited by single, double and triple electrical stimuli to the skin at mean rates of 0.25–2 Hz. The interval between single or paired (20 ms apart) conditioning stimuli and test stimulus was then varied between 500 and 2 ms, and recovery curves of velocity change against inter-spike interval constructed, allowing for changes in these variables with distance. All fibres exhibited an initial (4–24 ms) relative refractory phase, and a long-lasting (>500 ms) ‘H2’ phase of reduced velocity, attributed to activation of Na+/K+-ATPase. Mechano-responsive nociceptors exhibited an intermediate phase of either supernormality or subnormality, depending on stimulation rate. Mechano-insensitive nociceptors behaved similarly, but all were supernormal at 1 Hz. Sympathetic units exhibited only a long-lasting supernormality, while cold fibres exhibited a briefer supernormal and a late subnormal phase (H1), similar to A fibres. A pre-conditioning impulse doubled H2 and increased H1, but did not augment supernormality or the subnormality of similar time course. Like A fibre supernormality, these phenomena were explained by a passive cable model, so that they provide an estimate of membrane time constant. Nociceptor membrane time constants (median 110 ms, n= 17) were rather insensitive to membrane potential, indicating few active voltage-dependent potassium channels, whereas sympathetic time constants were longer and reduced by activity-dependent hyperpolarisation.