The formation of synapses in reinnervated and cross-reinnervated striated muscle during development.

1. A study has been made of the formation of synapses in reinnervated and cross-reinnervated developing striated muscles which normally receive either a focal or distributed innervation, using histological, ultrastructural and electrophysiological techniques.2. The focally innervated mammalian tibialis anterior muscle, denervated soon after birth, was reinnervated at both the original end-plates as well as on the new muscle added during the period of denervation; but not on the muscle present at the time of denervation. Nearly all the synapses which had formed, other than at the original end-plates, disappeared by 6 weeks post-natal.3. The avian anterior latissimus dorsi muscle (ALD), which receives a distributed innervation, was denervated during the first week post-hatched, and became reinnervated both at the original synaptic sites as well as on the new muscle added during the period of denervation; all these synapses were spaced approximately 200 mum apart along the length of individual muscle cells.4. The myofibres of the ALD muscle cross-reinnervated at hatching with the superior brachialis nerve, which contains fast motor axons that normally form a focal innervation, were each focally innervated by a single ;en plaque' terminal; these synapses had the same electrical properties as normal synapses formed by fast motor axons.5. Many of the myofibres of the avian posterior latissimus dorsi (PLD), which normally receive a focal innervation, received a distributed innervation from ;en grappe' terminals when cross-reinnervated with the ALD nerve at hatching.6. It is suggested that during development the nerve type determines the pattern of synapses over an effector; this is achieved by the nerve, after forming the initial synaptic contact, making the rest of the muscle cell membrane refractory to further synapse formation for some distance, this distance being determined by the nerve type.