Ongoing activity in neuroma afferents bearing retrograde sprouts.

Electrophysiological recordings were made from axons teased from the sciatic nerve 17-34 mm (mean 26.8 mm) central to a chronic nerve-end neuroma in adult rats. 23 fibers (2% of those sampled) appeared to have had a sprout(s) that grew in the retrograde (central) direction for at least this distance. Nine of the 23 (39%) carried spontaneous ongoing discharge. The parent fiber was myelinated (an A-fiber) in most instances, but unmyelinated (a C-fiber) in some. Thus, following nerve injury, a subset of afferent axons undergo retrograde sprouting, and many of them fire spontaneously. These contribute, along with the afferents whose trapped ends terminate at the injury site, to the ectopic afferent barrage generated in neuromas.