Recent studies on human cutaneous C fibres by microneurography have shown that different functional classes of C fibre (e.g. polymodal nociceptors, mechano-insensitive nociceptors, cold fibres, sympathetic fibres) exhibit different profiles of activity-dependent velocity changes when stimulated at different rates (Serra et al. 1999; Weidner et al. 1999). These differences suggest that different specific membrane properties are responsible, probably including differences in the electrogenic Na+ pump. To obtain more information about the different membrane properties, we have explored the short-lived effects (up to 500 ms) of single and double conditioning impulses on conduction velocity. The effects of single conditioning stimuli were recently investigated by Weidner et al. (2000). They stimulated single C units regularly at 0.25 Hz and measured the effects (up to 2000 ms) of additional, interpolated impulses on the velocity of the next impulse. They found that 16 mechano-insensitive units exhibited a supernormal period of increased velocity, which peaked at an interstimulus interval of 69 ± 10 ms, whereas 20 mechano-sensitive units exhibited only subnormality, with a time course that resembled a mirror-image of the supernormality of the mechano-insensitive units. However, all units conducted more slowly when preceded by an extra stimulus at intervals of 1000 ms or more, and this long-lasting slowing was linearly related to the number of interpolated impulses (1, 2 or 4). The long-lasting slowing is clearly analogous to the post-tetanic depression and hyperpolarisation (H2) of myelinated axons, and presumably reflects membrane hyperpolarisation by the electrogenic Na+ pump (Rang & Ritchie, 1968). However, the mechanism of the supernormality was not specifically addressed by Weidner et al. (2000).
Three different mechanisms have been proposed for supernormal conduction, or for the associated negative (depolarising) afterpotential, in unmyelinated axons. First, Frankenhaeuser & Hodgkin (1956) attributed the negative afterpotential in the squid giant axon to a transient increase in the K+ concentration in the immediate vicinity of the axon following activity. This explanation was applied to mammalian C fibres by Greengard & Straub (1958) and many subsequent studies have concentrated on the relationship between afterpotentials or supernormality and extracellular K+ (Zucker, 1974; Kocsis et al. 1983; Shin & Raymond, 1991). However, Bliss & Rosenberg (1979) argued against this mechanism for the supernormality in tortoise olfactory nerve, and suggested that it might be due to a spike-dependent increase in Na+ channel conductance. Finally, Barrett & Barrett (1982). who showed that the depolarising afterpotential in amphibian A fibres is primarily a passive electrical phenomenon, suggested that the depolarising afterpotential in unmyelinated axons might have a similar origin. This explanation, which was recently supported by the microneurography experiments of Weidner et al. (2002). has the interesting implication that the time course of the afterpotential (and therefore of supernormality) should reflect the passive membrane time constant of the axons. To distinguish between these three proposed mechanisms, we have tested the effects of a second conditioning impulse on C fibre recovery cycles. If the velocity changes are due to ion accumulation, they should roughly double after a second impulse, whereas if they are passive, as in myelinated fibres, then a second conditioning impulse should have only a minimal effect on the recovery cycle. Slow changes in channel properties might be expected to result in an intermediate effect of a second conditioning impulse.
A complication that occurs when measuring the velocity recovery cycle of a C fibre, especially over the long distances sometimes achieved in humans, is that the time interval between the conditioning and test impulses is not constant but changes as the second impulse speeds up or slows down. These changes in the conditioning-test interval can be substantial, and cause serious distortion of the shape of the recovery cycle with conduction distance, especially when a second conditioning stimulus is added. The distortion is greater the longer the conduction distance, and Weidner et al. (2002) found that when recording over distances of 300–400 mm, the amount of speeding up recorded during the supernormal period is often limited by the second impulse catching up the first and reaching the ‘entrainment interval’, of about 10 ms, when the velocities become the same. We have minimised this problem in two ways: first by recording over shorter distances (mean 108 mm) to reduce the absolute latency changes, and secondly by recording the recovery cycles with sufficient time resolution that we were able to make corrections for the latency changes and infer the underlying relationship between velocity and interspike interval. In this study we therefore recorded recovery cycles with as many as 48 interstimulus intervals, from 2 to 500 ms, and with both a single conditioning stimulus and with two conditioning stimuli 20 ms apart. Our results provide the first data on recovery cycles of cold-specific and sympathetic C fibres, and provide strong support for Barrett & Barrett's (1982) hypothesis of the passive origin of supernormality in C fibres.