Muscle reinnervation--II. Sprouting, synapse formation and repression.

Extensor digitorum longus muscle is reinnervated by the regenerating neurites at the end-plate region; as soon as the contact is made, the rate of neurite elongation inside the cleft decreases about 1000-fold while interfibre growth and sprout formation proceed unchanged. Polyinnervation reaches the maximum level 7-10 days after reinnervation, then synaptic repression begins. The elimination of redundant innervation takes place when the biophysical properties of the muscle are again normal. There is no sign of either phagocytosis or degeneration, therefore the process of synaptic repression is probably due to retraction, as neurites do when in culture. The role of Schwann cells and nerve sheath in the process of maintenance is suggested.