[Factors determining the duration of a single postsynaptic response in neuromuscular junctions]

The miniature end-plate currents (MEPCs) and acetylcholine-induced current fluctuations were compared in experiments on fast and slow muscle fibres of lamprey, frog, chicken and rat. Simultaneous analysis of MEPCs decay time constant and mean life-time of single ionic channels suggests that the former is the main but not the sole factor determining the MEPCs duration. Additional factors, in particular, the relative insufficiency of acetylcholinesterase activity (AChE) are more important for the generation of the usual MEPSs in slow muscle fibres or the "giant" MEPCs in fast ones. Low activity of AChE favours the prominent asynchrony of interaction between acetylcholine molecules and cholinoreceptors causing prolongation of the time course of synaptic response. The mechanisms of MEPCs decay shortening induced by alpha-bungarotoxin or d-tubocurarine and the nonuniformity of different active spots of junctions are discussed.