Restoration of motor unit properties and fiber type distribution in reinnervated axolotl skeletal muscle.

The contractile properties of functionally isolated motor units and the muscle fiber type distribution of reinnervated iliotibialis posterior muscles were examined in axolotls (Ambystoma mexicanum) 7 to 12 months after complete transection of the hind limb nerve trunks. Motor units were continuously distributed with respect to size and contractile speeds and there was a positive correlation between motor unit size and twitch:tetanus ratio. The degree of overlap between motor units was positively correlated with motor unit size and inversely correlated with similarity of contractile speeds. In addition, the muscle fiber type distribution within the reinnervated muscles, as revealed by immunocytochemical localization of tonic, slow, and fast myosin isoforms, was indistinguishable from that in the contralateral unoperated muscles, with no evidence of fiber type grouping. Thus, the motor organization after reinnervation was very similar to that of normal muscles examined previously. The only difference found was that a small number of motor units contracted unusually rapidly in reinnervated muscles. The absence of muscle fiber type redistribution and the essentially normal motor unit characteristics suggest that the original pattern of synaptic connections was reestablished following reinnervation. These observations are consistent with the returning motor neurons selectively reinnervating their original muscle fiber types.