The first degenerative changes seen after an axon has been divided are at the neuromuscular junction.
The onset of changes in the axon terminal depends on two things: the length of the distal stump of axon and the species of animal
The onset of end-plate failure is also species dependent, being much later larger mammals.
When degenerative changes have started, only 3-5 hours are required for complete disruption of the endplate, which is accomplished by clumping of synaptic vesicles, swelling of mitochondria, breaking up of cristae, glycogen bags appear in the axoplasm, and lysosomes become evident.
There is a latent period of spontaneous discharge and electrical stimulation of the axon results in normal neuromuscular transmission, after this period the neuromuscular function fails abruptly.
Silent synapses: neuromuscular junctions that are largely intact, but in which there is insufficient depolarization of the muscle fiber membranes to generate action potentials.
Changes in Muscle Fibers
Denervation atrophy – gradual wasting of muscle after a nerve to the muscle is severed, demonstrates that the muscle fibers are dependent upon the motoneuron for maintenance of their normal structure
Atrophy can be detected at about the 3rd day of denervation and is rapid during the ensuing 2 months
Denervation affects both red and white fibers equally.
Atrophy is the result of increased protein degradation and decreased protein synthesis
As early as the 2nd day after denervation, myonuclei become rounded, instead of narrow and elongated and the nucleoli become enlarged and more prominent
Many nuclei move into the centers of the fibers where they line up to form chains.
In late atrophy, all that remains of some fibers are chains or clumps of nuclei surrounded by thin cylinders of cytoplasm.
May result in the death of a fiber, and can be restricted to part of a fiber.