Reduction of multiaxonal innervation at the neuromuscular junction of the rat during development.

1. A study was made of the functional and structural changes that occur during the decrease in multiaxonal innervation of neonate rat muscle fibres 2‐14 days after birth. 2. In day 8 to day 14 animals there was a constant daily loss in the average number of functionally transmitting axons/muscle fibre measured electrophysiologically. An investigation of synaptic transmission during this period revealed that the loss of functional contact from the supernumerary axons was not preceded by any sign of failing terminal conduction or a gradual decrease in transmission efficacy but rather appeared to occur abruptly. 3. Neonate end‐plates showing signs of abnormal ultrastructure were observed during the period of synapse elimination. Some axon terminals had a high cytoplasmic density and condensation of synaptic vesicles. Signs of Schwann cell encroachment into the synaptic cleft were readily found and large areas of post‐junctional membrane apposed only by Schwann cell were evident. 4. It is suggested that the mechanics of the process of synapse elimination in neonates is similar to that occurring during degeneration in the denervated adult. Transmission failure occurs abruptly at the supernumerary endings and they are disposed of by the Schwann cell.