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Updated: Aug 18, 2026

Historical View and Physiology Demonstration at the NMJ of the Crayfish Opener Muscle
Published on: November 9, 2009
Localization of active sites in the neuromuscular junction of the frog
Abstract:
Spontaneous quantal discharges at the frog neuromuscular junction were simultaneously monitored with three glass microelectrodes. Two extracellular electrodes were located at the synaptic cleft a few microns apart from each other so that each quantal discharge occurring nearby evoked signals in both electrodes. From the observed ratios of the signal amplitudes it is inferred that quantal discharges exclusively occur at discrete sites. The distance between active sites is in the same range as that between "active zones" seen from electron micrographs.
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When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated sodium channels. Sodium ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated calcium channels to open.