Localization of active sites in the neuromuscular junction of the frog.

Spontaneous quantal discharges at the frog neuromuscular junction were simultaneously monitored with three glass microelectrodes. Two extracellular electrodes were located at the synaptic cleft a few microns apart from each other so that each quantal discharge occurring nearby evoked signals in both electrodes. From the observed ratios of the signal amplitudes it is inferred that quantal discharges exclusively occur at discrete sites. The distance between active sites is in the same range as that between "active zones" seen from electron micrographs.