Characteristics of transmitter release at regenerating frog neuromuscular junctions.

1. A study was made of the onset of transmission and the characteristics of transmitter release from regenerating nerve terminals in frog muscle fibres.2. Soon after transmission had been restored, some junctions were found which responded to nerve stimulation with only subthreshold end-plate potentials.3. The evoked transmitter release had a non-linear dependence on the external calcium concentration, like that seen at normal junctions.4. The synaptic delay was only slightly longer than normal, and the amplitudes of single quantum potentials evoked by nerve stimulation seemed to have a normal distribution.5. The mean amplitude of the spontaneous miniature end-plate potentials was often substantially smaller than the mean amplitude of the evoked quantal potentials at a given end-plate. Some of these small spontaneous potentials were due to transmitter release from the axon terminal. Possible explanations for this discrepancy in size of spontaneous and evoked potentials are discussed. Two to three weeks after reinnervation began, the amplitude of the spontaneous miniature end-plate potentials returned to normal.