Identification of afferents in a fine nerve strand by selective elimination of signals from C-fibers by electric current pulses.

Fine nerve strands teased mechanically from saphenous nerve in anesthetized cats contained 2-5 intact myelinated (A) and up to 15 unmyelinated (C) fibers. An electric current pulse of 3 microA applied through a platinum wire electrode to such a strand during 10 s irreversibly eliminated spikes from C-fibers, yet did not disturb spikes of 80% of A-fibers and decreased the amplitude of the spikes of other A-fibers by not more than 3-fold. When this decrease took place the spike wave form was not distorted and could still be recognized. To diminish the influence of electric current upon signals of some A-fibers current pulses of gradual increasing amplitude in the range of 1-3 microA were applied repetitively until the C-wave in the compound action potential was suppressed. The selective elimination of the signals of C-fibers in fine nerve strands may be used as a method to assist the identification of individual fiber type (A or C). This method is particularly useful in experiments with chemo- and thermo-receptive afferents where the standard collision technique is difficult to apply.