Most presynaptic terminals release small numbers of neurotransmitter containing vesicles even when action potentials are not present. This is stochastic and the probability of release (Pr) can be modified by numerous factors including the presence and speed of an action potential. These vesicles are released at synaptic active zones, areas of the axon terminal that have all of the machinery and conditions necessary to specialize in vesicle fusion with the plasma membrane. Until relatively recently, the prevailing hypothesis was that only one vesicle at a time is released from these active zones. However, research over the past several decades has added support for an additional mechanism of neurotransmitter vesicle release.