Observations on the elimination of polyneuronal innervation in developing mammalian skeletal muscle.

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Abstract
The mechanism responsible for the elimination of polyneuronal innervation in developing rat soleus muscles was studied electrophysiologically and histologically. Initially all the axons contacting a single end-plate have simple bulbous terminals. As elimination proceeds 1 axon develops terminal branches while the other terminals remain bulbous and may be seen in contact with, or a short distance away from, the end-plate. The branched terminal probably remains in contact with the muscle fiber while the other terminals withdraw. At a time when polyneuronal innervation can no longer be detected electrophysiologically, the histological technique still shows the end-plates contacted by more than 1 nerve terminal. The effect of activity on the disappearance of polyneuronal innervation was examined. Activity was increased by electrical stimulation of the right sciatic nerve. This procedure also produced reflex activity in the contralateral limb. In both cases polyneuronal innervation was eliminated more rapidly in the active muscles. The finding that proteolytic enzymes are released from muscles treated with acetylcholine (ACh) and the observation of the more rapid elimination of supernumerary terminals at the end-plates of active muscles, lead to the suggestion that superfluous nerve-muscle contacts are removed by the proteolytic enzymes in response to neuromuscular activity. The selective stabilization of only 1 of the terminals is discussed in the light of these results.