Synaptic vesicle release at the neuromuscular junction (NMJ) is highly reliable and is vital to the success of synaptic transmission. We examined synaptic vesicle number, distribution, and release at individual type-identified rat diaphragm NMJ. Three-dimensional reconstructions of electron microscopy images were used to obtain novel measurements of active zone distribution and the number of docked synaptic vesicles. Diaphragm musclephrenic nerve preparations were used to perform electrophysiological measurements of the decline in quantal content (QC) during repetitive phrenic nerve stimulation. The number of synaptic vesicles available for release vastly exceeds those released with a single stimulus, thus reflecting a relatively low probability of release for individual docked vesicles and at each active zone. There are two components that describe the decline in QC resulting from repetitive stimulation: a rapid phase (<0.5 s) and a delayed phase (<2.5 s). Differences in the initial rapid decline in QC were evident across type-identified presynaptic terminals (fiber type classification based on myosin heavy chain composition). At terminals innervating type IIx and/or IIb fibers, the initial decline in QC during repetitive stimulation matched the predicted depletion of docked synaptic vesicles. In contrast, at terminals innervating type I or IIa fibers, a faster than predicted decline in QC with repetitive stimulation suggests that a decrease in the probability of release at these terminals plays a role in addition to depletion of docked vesicles. Differences in QC decline likely reflect fiber-type specific differences in activation history and correspond with well-described differences in neuromuscular transmission across muscle fiber types.
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