Motor nerve terminal loss from degenerating muscle fibers.

The fate of nerve terminals following elimination of postsynaptic target cells was studied in living mouse muscle. Several days after muscle fiber damage, observations of previously identified neuromuscular junctions showed that motor nerve terminal branches had rapidly disappeared from degenerating muscle fibers. Following muscle fiber regeneration, loss of terminal branches ceased and nerve terminals regrew, reestablishing some of the original sites and adding new branches. The distribution of acetylcholine receptors reorganized in the regenerated muscle so that perfect alignment was reestablished with the newly configured nerve terminals. These results argue that the maintenance of the full complement of nerve terminal branches at a neuromuscular junction is dependent on the presence of a healthy muscle fiber. Similarly, regenerating muscle is dependent on the nerve terminal for the organization and maintenance of postsynaptic receptors.