The relationship between axonal spike shape and functional modality in cutaneous C-fibres in the pig and rat

Axonal spike shape was examined in identified cutaneous C-fibres dissected from the saphenous nerves of anaesthetized pigs and rats, and was found to vary with functional class. In the pig, the action potential duration for heat nociceptor units (duration at half peak amplitude, 1.25 +/- 0.16 ms, mean +/- S.E.M., n=32) was significantly longer than the duration for polymodal nociceptive units (0.88 +/- 0.11 ms, n=32). Both classes of nociceptive C-fibre had action potentials of longer duration than the low-threshold mechanoreceptor units (0.49 +/- 0.04 ms, n=24) and the inexcitable C-fibres (0.56 +/- 0.06 ms, n=19). Undershoot durations were also longer in nociceptive than non-nociceptive C-fibres. In contrast, spike amplitudes were similar in all classes of C-afferent. In the rat, as in the pig, the polymodal nociceptor units had action potentials of longer duration (0.75 +/- 0.05 ms, n=73) than the mechanoreceptor units (0.60 +/- 0.01 ms, n=23). C-fibres identified as spontaneously active sympathetic efferent units had wider action potentials (main initial peak: 1.01 +/- 0.12 ms, n=22; undershoot: 4.1 +/- 1.23 ms, n=20) than the afferent C-fibres (main peak: 0.69 +/- 0.03 ms, n=130; undershoot: 1.4 +/- 0.09 ms, n=111). All rat C-fibre types had action potentials with main initial peaks of a similar height. However, cold thermoreceptor units had spikes with significantly smaller undershoots compared to nociceptive or inexcitable C-fibres. It is concluded that there are clear differences in axonal spike shape between the different functional classes of C-fibre and, in particular, that nociceptive C-afferents tend to have axonal action potentials of longer duration than non-nociceptive afferents. The ion channels responsible for the longer duration action potentials may provide a target for the development of highly selective analgesic drugs.