[Features of the postsynaptic potentials and ion currents in synapses of fast and slow rat muscle fibers].

The time course of spontaneous miniature end-plate potentials (MEPP) and currents (MEPC) was compared in fast and slow muscle fibres using extracellular focal recording or voltage-clamp technique. The mean values of the rise and decline half-time of MEPP and MEPC in slow fibres was 1.3-1.4 times as much as those in fast ones. A smaller difference was observed after AChE inhibition. Decay of the end-plate currents remained generally exponential and its rate depended on the membrane potential level. Distribution of different muscle fibres according to the time course of MEPP and MEPC in fast and slow muscles correlates with the content of the different fibre types identified on other properties. The possible factors controlling the time course of postsynaptic responses (AChE activity, the length of diffusional pathways, differences in the membrane passive electrical properties) and their significance in synapses of different types are discussed.