Porocytosis: a transient pore array secretes the neurotransmitter packet. 
The porocytosis hypothesis is based on the arrayed nature of synaptic vesicles which forms the anatomical functional unit of secretion.  The presynaptic array and the postsynaptic array of receptors form a synaptomere which is the unit of transmission.  A transient increase in calcium ions, triggered by an action potential, activates all pores of the array to pulse transmitter.  The array insures transmission while permitting a frequency dependent amount of secretion.  Therefore the amount of secretion is variable which permits plasticity.  Secretion from the array has the property of immediate synaptic plasticity whereas a change in array size would change synaptic strength.  The robust nature of the array insures fidelity of transmission, a frequency dependent dynamic signature of transmission giving the property of immediate plasticity; and, a change in array size yields a change in synaptic strength for long term reliability.