Physiological properties of junctions between nerve and muscle developing during salamander limb regeneration.

1. Physiological properties of developing nerve-muscle junctions were studied in regenerating limbs of adult salamanders. 2. During the period of synapse formation the muscle fibres had diameters of 4-10 mum, resting potentials of minus 90 to minus 100 mV and input resistances of 10-50 Momega. Some, but not all, pairs of adjacent muscle fibres were electrically coupled. 3. At the stage when muscle fibres could first be identified, some of them were not innervated, at least as determined by electrophysiological criteria. 4. During muscle innervation the neuromuscular synapses were encountered in several intermediate phases of maturity. (i) At the least mature junctions small spontaneous synaptic potentials occurred, but stimulation of the motor nerve trunk did not evoke synchronous transmitter release. (ii) At other junctions maximal nerve stimulation evoked only a single end-plate potential of low quantum content. (iii) More mature fibres received synaptic input from as many as four motor neurons, which could be distinguished by their discrete stimulus thresholds. 5. During this period of synapse development the fibres lacked an action potential but often showed a prolonged response to depolarization. 6. Fibres in normal adult muscles had from one to three synaptic inputs, were not electrically coupled, and responded to depolarization with an action potential.