Plasticity of neuromuscular junction architectures in rat slow and fast muscle fibers following temporary denervation and reinnervation processes.

We evaluated the effects of brief, temporary denervation caused by ischiadic nerve-freezing on the processes of degeneration and regeneration of ultrastructural features in neuromuscular junction (NMJ) architecture in different types of rat skeletal muscle fibers. Nerve terminal (NT) area was decreased significantly 12 h after nerve freezing in both fast-twitch (FT) and slow-twitch (ST) fibers. One day after nerve freezing, some terminal axons were absent; decrease in NT area was remarkable in ST fibers, and there was retraction of Schwann cells and perineural epithelial cells. Fiber type-specific differences were observed in pattern of decrease in NT area between 24 h and 7 days after nerve freezing (there was significantly more decrease in FT fibers). The primary synaptic cleft became shallow, and the secondary junctional folds shorter and wider, but the basement lamina filling the subneural apparatus was unaltered. The number of secondary junctional folds decreased gradually between 6 h and 14 days after nerve freezing in both types of fiber. In control muscle fibers, synaptic vesicle density (SVD) per terminal area was significantly higher in FT fibers. The SVD densities decreased following nerve freezing-induced destruction of NMJs, and were minimal 3 days in FT fibers or 7 days ST fibers after nerve freezing. At 3 weeks, regeneration of both FT and ST fibers was well advanced, and all parameters had recovered to control values in FT fibers 28 days after nerve freezing. Severe degradation of the ultrastructural features in NMJs occurred due to temporary denervation during muscle fiber degeneration processes, and these structural changes were all reversible and fiber type-specific.