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 acti vity 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 i n axonal excitability caused by electrical stimulation. Stimuli were a pplied both at fixed frequencies and at the time intervals of impulses previously recorded during response to natural stimulation. Latency i ncreased following both these forms of electrical stimulation, as well as after natural stimulation of the receptive fields. The latency inc rease was correlated with the number of impulses and the frequency of the preceding discharge in all of 4 nociceptors and 13 cold fibers stu died for this feature. Increase of latency by electrical or natural st imulation led to reduced responsiveness to natural stimulation. The ma gnitude and time course of latency changes were correlated with fiber modality. In 32 nociceptors the latency increased continuously with ti me during a stimulus train, whereas in 21 cold fibers there was only a n initial increase in latency over the first few seconds, after which the latency remained at a plateau even as the firing response continue d. Paralleling this slowing, impulse failure occurred more frequently during repetitive stimulation in both A delta and C nociceptors than i n 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'' fi bers slowed significantly more than ''Type I'' or cold fibers. The res ults support the hypotheses (1) that the pattern of latency changes du ring 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.