Changes in the Asynchronicity of Transmitter Release at the Neuromuscular Junction during Prolonged High-Frequency Stimulation

In experiments on neuromuscular junctions in the frog m. cutaneous-pectoris, changes in the intensity and asynchronicity of transmitter release during high-frequency (10 and 50 sec^-1) rhythmic stimulation of the motor nerve were investigated using extracellular recording. At low extracellular Ca²⁺ concentrations, rhythmic stimulation resulted in a gradual enlargement of the quantum content of end-plate currents (EPC), the so-called facilitation. The latter phenomenon was accompanied by an increase in the average value and variance of synaptic delays of single-quantum EPC, a shift of the main mode of their distribution towards greater values, and an increase in the latency of the nerve ending responses. The above-described changes reduce the magnitude of facilitation in the neuromuscular synapse.