Myelinated afferent fiber types that become spontaneously active and mechanosensitive following nerve transection in the rat.

It is difficult to know which afferent types preferentially develop ectopic firing characteristics following nerve injury because axotomy disconnects the sensory receptor ending from the remainder of the afferent neuron. We compared the prevalence of ectopic firing originating in nerve-end neuromas of nerves serving muscle and skin in the rat. Spontaneous firing was much more prevalent in the injured medial gastrocnemius nerve, a hindlimb muscle nerve, than in the saphenous and sural nerves which primarily innervate hindlimb skin. Ectopic mechanosensitivity, on the other hand, was more prominent in neuromas of the cutaneous nerves. In neuromas of the facial nerve, a cranial nerve which serves striated muscles of the face, there was no spontaneous discharge and very little ectopic mechanosensitivity. We conclude that the development of spontaneous ectopic discharge and ectopic mechanosensitivity depends on the type of myelinated afferent fiber involved.