Regeneration of cutaneous afferent unmyelinated (C) fibers after transection.

The cutaneous receptive properties of C-fiber units were studied 1-8 mo after transection and repair of the rabbit's great auricular nerve. The proportions of C-fiber units that could be excited by the cutaneous stimuli known to excite normal C-fiber afferent elements of the same nerve increased with time of recovery and approached the normal range within 5 mo after transection. No evidence was obtained that suggests that any specific receptor type regenerated more rapidly than others. No differences were established between the cutaneous receptive properties of normal and regenerated low-threshold mechanoreceptors or specific thermoreceptors. However, the properties of regenerated polymodal nociceptors were different from those of normal polymodal nociceptors. Mechanical thresholds for regenerated polymodal nociceptors were greater than normal at 2 mo after transection but not later, and nearly 20% of regenerated polymodal units had heat thresholds lower than normal. It is argued that the abnormal properties of some regenerated polymodal nociceptors could explain, in part, the elevated thresholds to noxious mechanical stimulation and the thermal hyperalgesia reported during recovery from nerve transection in man.