Ultrastructural correlates of motor nerve regeneration in crayfish

Summary

Identified motor neurons innervating distal limb muscles in the crayfish claw have been examined by light and electron microscopy after surgical interruption. The functionally competent distal segments of such axons, a few weeks after the operation, show enlarged glial sheaths that contain occasional small satellite axonal profiles (cf. Nordlander and Singer, 1972); evidence is presented that such profiles can arise from sources other than the outgrowing central processes of motor axons. When functional connections are reestablished with the muscles in 100 days or less, multiple axon profiles are never seen in motor axon sheaths in the distal limb segments, though they may occur close to the operation site. In contrast, delayed regenerates (≥200 days) show multiple profiles all the way to the muscles, and the functional connections made are abnormal. We conclude that, although the multiple profiles may represent outgrowth filaments from the central motor axons, they do not normally reach the muscle in prompt regenerates. The results suggest that instead they fuse with the surviving distal axon segment.