[Distribution and functioning of mediator release sites in the neuromuscular junction of the frog]

Two microelectrodes were used to record end plate potentials (EPPs) and miniature EPPs (MEPPs) in a single nerve-muscle synapse of frog sartorius muscle. Under conditions of extracellular recording on the scattering diagrams of MEPPs amplitude some groups of signals (fields) were revealed which reflect the transmitter secretion in spatially distinct sites of the nerve terminal. It was found that the release sites in the synapse were grouped in certain areas and have different probability of quantum transmitter secretion. The comparison of fields on the scatter diagrams of MEPPs amplitude and that of uniquantal EPPs at low [Ca]0 (0.2-0.4 mmol/l) showed that the probability of the evoked and spontaneous transmitter release is different in certain secretion sites and sometimes a release site does not take part in evoked secretion. Simultaneous recording of synaptic potentials by extra- and intracellular electrodes showed that the transmitter secretion in spatially distinct groups of release sites leads to the appearance of multimodality on amplitude distribution of intracellular MEPPs and EPPs.