Ultrastructure of nerve terminals and muscle fibers in denervated crayfish muscle

Summary

Neuromuscular junctions, muscle fibers, and intramuscular nerve branches in crayfish opener muscles denervated for periods up to 368 days were examined with the electron microscope. The majority of the denervated muscles responded to stimulation of the isolated distal part of the motor axon with electrical activity and contractions. The neuromuscular junctions and muscle fibers appeared normal in such preparations: synapses and synaptic vesicles were present in the nerve terminals, and the organization of filaments and tubules in the muscle fibers was not distinguishable from that in normal muscles.

In some preparations denervated for a long time, and in others denervated by tying the motor axon close to the muscle, the motor axons showed physiological and morphological evidence of degeneration. Synapses, when present in such preparations, showed few synaptic vesicles, which were often clumped. Muscle fibers showed signs of degeneration. These observations suggest that isolated distal motor axon segments can remain alive for at least a year, retaining normal structure. As long as the motor axon is viable, the muscle remains normal in appearance, but if the axon degenerates, the muscle becomes abnormal. A trophic interdependence between nerve and muscle is probably involved.