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.