Activity-dependent slowing of conduction velocity provides a method for identifying different functional classes of C-fibre in the rat saphenous nerve.

Repetitive firing of nerve fibres results in the slowing of their conduction velocity. The extent of conduction velocity slowing throughout a standard electrical stimulus (20 s, 20 Hz, 2 x electrical threshold) was examined in identified C-fibres dissected from the saphenous nerve of anaesthetized rats. The aim of this study was to establish whether the different functional classes of C-fibre could be identified on the basis of their activity-dependent slowing of conduction velocity. Following 20 s of stimulation at 20 Hz, nociceptive C-fibres showed a significantly greater slowing of conduction velocity (mean +/- S.E.; polymodal and heat nociceptors = 29.2% +/- 0.7, n = 53; mechanical nociceptors = 27.7% +/- 1.7, n =13) than cold thermoreceptive fibres (10.8% +/- 0.6, n = 10), mechanoreceptors (14.4% +/- 0.8, n = 17) and spontaneously active sympathetic efferent units (14.9% +/- 0.8, n = 24). The degree of conduction velocity slowing shown by a unit was not correlated with its resting conduction velocity. There was little overlap of the degree of conduction velocity slowing between the nociceptive and non-nociceptive fibres. Also, there was little overlap of conduction velocity slowing between the mechanoreceptors and the cold units, particularly after just 6 s of stimulation at 20 Hz. Units for which no receptive field to mechanical or thermal stimuli could be found showed a bimodal distribution of conduction velocity slowing. In the saphenous nerve, such inexcitable units will be of three main types--sympathetic efferent units, "sleeping" or "silent" nociceptors and non-cutaneous afferent fibres. Those inexcitable units slowing in conduction velocity by greater than 20% showed a similar distribution to the polymodal nociceptors and those inexcitable units slowing by less than 20% showed a similar distribution to the spontaneously active sympathetic units. Twenty-three of the 61 units without mechanical or thermal receptive fields were investigated using electrical skin stimulation and topical application of 5 or 10% mustard oil. Afferent fields could not be found for any of the nine units that slowed in conduction velocity by less than 20%. Afferent fields were detected for 11 of the remaining 14 insensitive units, which all showed a greater than 20% slowing from resting conduction velocity. Therefore, one can distinguish nociceptive and non-nociceptive afferent fibres simply by looking at the axonal property of activity-dependent slowing of conduction velocity. Moreover, it is possible to use this axonal property to separate the two classes of non-nociceptive afferent C-fibre (i.e. mechanoreceptors and cold thermoreceptors). In addition, one can also use this parameter to differentiate between the afferent and non-afferent populations of inexcitable C-fibres. The ability to identify a particular fibre type on the basis of an axonal property provides a useful tool for the functional classification of fibres in experiments where axons are separated from their terminals.