Synapse formation and elimination during growth of the pectoral muscle in Xenopus laevis.

1. Synapse formation and synapse elimination were studied in the pectoral muscle of Xenopus laevis. 2. Histology showed that fibres were not added during postmetamorphic growth. Most fibres were innervated at two widely separated junctions and this number did not change as frogs grew. 3. Intracellular recording revealed that fibres with two junctions could be mononeuronally innervated, or innervated in one of three different polyneuronal patterns. A growth-related shift in innervation pattern was observed, with the polyneuronal patterns replaced by mononeuronal innervation. 4. Endplate potentials (EPPs) evoked by low-frequency nerve stimulation were simultaneously measured at both junctions on individual fibres. For each fibre, the ratio of EPP amplitudes (smaller/larger) was calculated. When the two junctions were innervated by different motoneurones (A-B), the median EPP ratio was smaller than when the two junctions were innervated by the same motoneurone (A-A), although the difference was not significant. 5. The difference in the ratio of EPP amplitudes became significant, however, if junctions were conditioned by a train of fifty stimuli at 10 Hz. Immediately after such a train, EPP ratios for A-B fibres were significantly smaller than ratios for A-A fibres. This difference was due to greater synaptic depression at one of the junctions on A-B fibres. 6. We concluded that enhanced depression of the EPP upon repetitive stimulation is a physiological correlate of the competition that underlies synapse elimination.