Spontaneous discharge of afferents in a neuroma reflects original receptor tuning.

Injured afferent axons trapped in chronic nerve-end neuromas frequently generate spontaneous discharge. We asked whether the patterns of discharge originating at such sites of ectopic electrogenesis bear a consistent relationship to the patterns of discharge characteristic of the corresponding intact afferent types before injury. Nerve-end neuromas were created in electrosensory lateral line nerves in 3 species of weakly electric Gymnotiform fish. Species were chosen in which normal afferent activity occurs at highly characteristic, non-overlapping, species-specific frequencies. Afferent impulse discharge was recorded in vivo from lateral line nerve end neuromas using the nerve-teasing technique. The distribution of firing frequencies of spontaneously active neuroma afferents was relatively uniform within a given fish species, and differed significantly from species to species. Mean values were somewhat lower than for corresponding intact afferents, but the rank order of frequencies across the species was preserved. These data indicate that differential membrane remodelling after axotomy tends to reestablish normal afferent fiber tuning despite failure of regeneration, and in the absence of peripheral electroreceptor reinnervation.