Characteristics of A- and C-fibers ending in a sensory nerve neuroma in the rat.

1. We have studied, in vivo, the degree of spontaneous activity, responsiveness to mechanical and chemical stimuli, and the conduction velocities in C- and A-fibers ending in the neuromas formed 8-66 days after ligation and transection of a cutaneous sensory nerve in the rat. 2. Some of these C- and A-fibers developed ongoing activity. The percentage varied considerably between neuromas in different animals, from 0 to 23% (mean, 4.2%), with no major variation in the incidence as a function of neuroma age. 3. The endings of the fibers in the neuroma could be excited by both mechanical and chemical stimuli. From 0 to 26% (mean, 13%) of these fibers had mechanosensitive endings, some of which were located in the muscle/facia tissue outside the neuroma itself. Some fibers were excited by direct application of chemicals to their endings in the neuroma; 3.0% of A- and C-fibers responded to bradykinin, 2.0% to histamine, and 2.8% to adrenaline. There was no systemic variation in the percentages of mechano- or chemosensitive fibers with neuroma age. 4. The C-fiber action potentials showed a continuing decrease in conduction velocities over the 9 wk after nerve transection. More than 4 wk after transection, the conduction velocity of neuroma fibers was 88% that of C-fibers of normal saphenous nerve. 5. We conclude that fibers in a cutaneous nerve neuroma have some sensory capabilities similar to those in normal nerves terminating in the skin. This could be because they are retained after the nerve is transected or because they are initially lost but then regenerate. However, the numbers are restricted, probably because the fibers remain isolated from factors produced by their target skin tissue that are necessary for development and maintenance of sensory functions.