When an action potential arrives at the synapse, the electrical signal of the action potential is transformed into a biochemical signal. This transformation is necessary in order to have the signal cross the small space between two neurons (20 nm).
The neuronal cell body communicates the signal for neurotransmitter release via an action potential.
Association with post-synaptic receptors is based on an electrochemical gradient.
Extra info: Association with post-synaptic receptors is usually based on the concentration of neurotransmitters present in the synapse.
In the presence of enough enzymes, extra-cellular metabolism is the fastest way to clear neurotransmitters from the synapse.
Extra info: Presynaptic reuptake appears always to be the most efficient manner in which to clear a synapse of neurotransmitter.