Velocity recovery cycles of C fibres innervating human skin.

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.