Modality-dependent Modulation of Conduction by Impulse Activity in Functionally Characterized Single Cutaneous Afferents in the Rat

Cutaneous afferents exhibit changes in excitability after impulse activity that are correlated with functional modality but are independent of axonal diameter, as studied in 39 cold fibers and 51 nociceptors of the rat. Latency of conducted impulses was used to indicate changes in axonal excitability caused by electrical stimulation. Stimuli were applied both at fixed frequencies and at the time intervals of impulses previously recorded during response to natural stimulation. Latency increased following both these forms of electrical stimulation, as well as after natural stimulation of the receptive fields. The latency increase was correlated with the number of impulses and the frequency of the preceding discharge in all of 4 nociceptors and 13 cold fibers studied for this feature. Increase of latency by electrical or natural stimulation led to reduced responsiveness to natural stimulation. The magnitude and time course of latency changes were correlated with fiber modality. In 32 nociceptors the latency increased continuously with time during a stimulus train, whereas in 21 cold fibers there was only an initial increase in latency over the first few seconds, after which the latency remained at a plateau even as the firing response continued. Paralleling this slowing, impulse failure occurred more frequently during repetitive stimulation in both A delta and C nociceptors than in velocity-matched cold fibers of either class. Based on the magnitude of latency increases during stimulus trains at different frequencies, two distinct patterns were discerned in A nociceptors: "Type II" fibers slowed significantly more than "Type I" or cold fibers. The results support the hypotheses (1) that the pattern of latency changes during activity are signatures for the modality in a given fiber; and (2) that endogenous, activity-dependent processes of the axon contribute to adaptation and encoding in cutaneous sensory afferents.