On the fast track, the synaptic vesicles stay in close proximity to the plasma membrane. This state may correspond to vesicle attachment in part via loose SNARE complexes between synaptobrevin, syntaxin1, and SNAP-25. Entry of calcium could trigger a conformational change in synaptotagmin 1 that would allow further zippering of the SNARE complex into a tight state, which would lead to the hemifusion stage of lipid bilayer fusion followed by the opening of a lipid-made fusion pore. An unknown mechanism would prevent the SNARE complex from proceeding further, thus preventing the membrane anchors of synaptobrevin and syntaxin from disassociating. SNAPs and NSF might catalyze the reversion of tight complexes into incompletely disassembled SNARE complexes, and conformational changes in dynamin could close the lipidic fusion pore.
As in the slow track, the proton pump would restore the electrochemical gradient, allowing for the uptake of neurotransmitter into the vesicle, which can undergo further rounds of exocytosis.
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