ACTIVITY-DEPENDENT SLOWING OF CONDUCTION-VELOCITY PROVIDES A METHOD FOR IDENTIFYING DIFFERENT FUNCTIONAL CLASSES OF C-FIBER IN THE RAT SAPHENOUS NERVE

Repetitive firing of nerve fibres results in the slowing of their cond uction 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 nerv e of anaesthetized rats. The aim of this study was to establish whethe r the different functional classes of C-fibre could be identified on t he basis of their activity-dependent slowing of conduction velocity. F ollowing 20 s of stimulation at 20 Hz, nociceptive C-fibres showed a s ignificantly greater slowing of conduction velocity (mean +/- S.E.; po lymodal and heat nociceptors = 29.2% +/- 0.7, n = 53; mechanical nocic eptors = 27.7% +/- 1.7, n = 13) than cold thermoreceptive fibres (10.8 % +/- 0.6, n = 10), mechanoreceptors (14.4% +/- 0.8, n = 17) and spont aneously active sympathetic efferent units (14.9% +/- 0.8, n = 24). Th e degree of conduction velocity slowing shown by a unit was not correl ated 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, part icularly after just 6 s of stimulation at 20 Hz. Units for which no re ceptive 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 - sympathet ic efferent units, ''sleeping'' or ''silent'' nociceptors and non-cuta neous afferent fibres. Those inexcitable units slowing in conduction v elocity by greater than 20% showed a similar distribution to the polym odal 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 rece ptive fields were investigated using electrical skin stimulation and t opical 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 remainin g 14 insensitive units, which all showed a greater than 20% slowing fr om resting conduction velocity. Therefore, one can distinguish nocicep tive and non-nociceptive afferent fibres simply by looking at the axon al property of activity-dependent slowing of conduction velocity. More over, it is possible to use this axonal property to separate the two c lasses 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 in excitable C-fibres. The ability to identify a particular fibre type on the basis of an axonal property provides a useful tool for the functi onal classification of fibres in experiments where axons are separated from their terminals. Copyright (C) 1996 IBRO.