Location and functioning of transmitter release sites in frog neuromuscular junction

Abstract

End-plate potentials (EPP) and miniature EPP (MEPP) were recorded in a single neuromuscular synapse of the frog sartorius muscle by means of two microelectrodes with a resistance of 0.5–2.0 Mθ. Groups of signals (fields), reflecting transmitter secretion in spatially distinct release sites were identified by extracellular recording on MEPP amplitude scatter diagrams. Release sites in the nerve ending were found to be unevenly distributed, to be grouped in certain areas, and to differ in their probability of secretion of a quantum of transmitter. Comparison of fields on MEPP and uniquantal EPP amplitude scatter diagrams in solution with low Ca++ concentration (0.2–0.4 mM) showed that ability to induce evoked and spontaneous transmitter release at the release site differs, and that sometimes a release site does not participate in evoked secretion. The results of simultaneous recording of synaptic potentials using extra- and intracellular electrodes indicate that transmitter secretion in spatially distinct groups of release sites leads to the appearance of polymodality in the distribution of amplitudes of intracellularly recorded MEPP and uniquantal EPP.