Functional properties of cutaneous A- and C-fibers 1-15 months after a nerve lesion

The functional properties of cutaneous afferent fibers were investigated 1-15 mo after nerve lesions, which allowed regeneration into denervated skin. After crushing or transection and resuturing the rat sural nerve, ongoing activity and responses to cold, heat, and mechanical stimuli presented to the denervated skin or to the nerve distal to the lesion were examined in 273 A-fibers and 211 C-fibers. Reinnervation of skin by A-fibers was largely complete by 1-4 mo after crushing but incomplete after transection and resuturing. A few A-fibers could be activated from the nerve trunk, even after 10-15 mo. Almost all regenerated A-fibers were mechanosensitive and about 6% were cold- or heat-sensitive. A few A-fibers had ongoing activity after nerve crush. Only 15-35% of C-fibers could be activated at 1-4 mo, but 60% were excited from the skin at 10-15 mo, when many also had receptive fields within the lesioned nerve. The remaining C-fibers had receptive fields only within the nerve trunk. Responses of both intraneural and intradermal endings of C-fibers could be classified into functional groups similar to those of C-fibers in control nerves to cutaneous stimuli. The frequency of afferent C-fibers with ongoing activity that were not highly cold sensitive was 45%. We conclude that the functional characteristics of afferent A- and C-fibers are expressed by regenerating nerve endings, even when they do not reinnervate their target tissue. The reinnervation of skin by afferent C-fibers is extremely slow and may never recover to normal.