It is estimated that an action potential will trigger the release of approximately 20% of an axon terminal's neurotransmitter load.
Quantal vesicles release their contents into the synapse by binding to the presynaptic membrane and combining their phospholipid bilayers.
Current research suggests that neurotransmitter release into neuromuscular junctions is signaled using a hierarchy of calcium ion channels and receptors in the presynaptic membrane, with different channels and receptors showing varying degrees of excitability in the presynaptic membrane.
Neurotransmitter receptors will either signal postsynaptic channels to "open" or "close" which will affect the rates that ions are able to cross the synaptic membrane.
[4] Such estimations cannot be reliably used in all synapses, but can be useful tools in developing the understanding of neurotransmitter release time courses in general.
As described above, the synaptic vesicle will remain fused to the presynaptic membrane after its neurotransmitter contents have been released into the synapse.
[1] The exact mechanism and signaling cascade which triggers synaptic vesicle recycling is still unknown.