Patterns of motor innervation in the pectoral muscle of adult Xenopus laevis: evidence for possible synaptic remodelling.

1. An anatomical and electrophysiological study was performed on the pectoral muscle of Xenopus laevis. 2. Silver-impregnated preparations revealed immature endings, collateral and terminal sprouting and signs of synaptic regression. 3. Twenty percent of the fibres received a dual innervation from two different nerves. The synapses of 25% of these fibres are formed in close vicinity. 4. Some of the singly innervated and most of the dually innervated end-plates generated only subthreshold electrical activity. Synaptic efficacy in dually innervated muscle fibres with closely spaced or distant endings was, on the average, one third and two thirds, respectively, of that obtained in singly innervated fibres. 5. Fibres with subthreshold electrical activity displayed normal ACh sensitivity. 6. The existence of non-transmitting synapses, of dually innervated end-plate sites and of morphological signs of the sprouting of new endings and the degeneration of old ones suggests that synaptic remodelling may occur in normal adult muscles.