Time course of post-excitatory effects separates afferent human C fibre classes

1. To study post-excitatory changes of conduction velocity, action potentia ls were recorded from 132 unmyelinated nerve fibres (C fibres) in cutaneous fascicles of the peroneal nerve using microneurography in healthy human su bjects. The 'marking' technique was used to assess responsiveness to mechan ical and heat stimuli or sympathetic reflex provocation. 2. C fibres were classified into three major classes: mechano-responsive af ferent (n = 76), mechano-insensitive afferent (n = 48) and sympathetic effe rent C fibres (n = 8). 3. During regular stimulation at 0.25 Hz, conditioning pulses were intermit tently interposed. Changes of conduction velocity were assessed for differe nt numbers of conditioning impulses and varying interstimulus intervals (IS Is). For all three fibre classes the latency shift following conditioning p ulses at an ISI of 1000 ms increased linearly with their number (n = 1, 2 a nd 4). However, the absolute degree of conduction velocity slowing was much higher in the 32 mechano-insensitive fibres as compared with 56 mechano-re sponsive or 8 sympathetic fibres. 4. Single additional pulses were interposed at different ISIs from 20 to 20 00 ms. For 20 mechano-responsive fibres conduction velocity slowing increas ed with decreasing ISI (subnormal phase). In contrast, for 16 mechano-insen sitive C fibres the conduction velocity slowing decreased with shelter ISIs , and at values lower than 417 +/- 49 ms (mean +/- S.E.M) the conduction ve locity of the conditioned action potential was faster than before (conducti on velocity speeding). This supernormal phase had its maximum at 69 +/- 10 ms. 5. In this study we provide, for the first time, direct evidence of relativ e supernormal conduction in human mechano-insensitive C fibres. The implica tions for temporal coding in different afferent C fibre classes are discuss ed.