[An analysis of mediator secretion in the active zone of the motor nerve ending]

The topography of transmitter release sites at the motor-nerve terminal of the cutaneous-pectoris frog muscle has been determined using three extracellular electrodes. It is shown that release sites are united in groups arranged transversally to the nerve endings and reflecting the transmitter release in the active zones (AZ) of the nerve terminal. The quantitative analysis of revealed groups has permitted concluding that the maximal level of secretion is at the centre of AZ, decreasing to the edge and aside from AZ. At the low extracellular Ca2+ concentration all the AZ take part in the spontaneous release process, while in the evoked one--only some of AZ. Advantages of the three-microelectrode method over the two-microelectrode one are analyzed. It is found that the transmitter secretion in spatially isolated AZ leads to the polymodality in uniquantal signal amplitude distribution at extracellular recording. The role of AZ in the transmitter release process is discussed.