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). An action potential arrives at the synaptic knob and causes depolarisation. The depolarisation of the synaptic membrane causes the voltage dependent calcium channels to open. This leads to a quick rise of the intracellular calcium concentration. An increase in intracellular calcium results in the exocytosis of vesicles containing neurotransmitters. The released neurotransmitters diffuse into the synaptic cleft And then can associate and disassociate in a concentration dependent basis with receptors
that are localized on the postsynaptic cell. These receptors can be G-protein coupled receptors (4) or receptor-operated ion channels (5) that when stimulated by the neurotransmitter can exert their effect (either directly or indirectly) on the opening of ion channels. By inducing opening of these channels, the postsynaptic membrane can depolarise. Depending on whether this depolarisation reaches the threshold, the signal can be passed over the axon of the postsynaptic cell. Or neurotransmitters can bind to receptors on the presynaptic cell The action of the neurotransmitter in the synapse can be terminated by reuptake into the presynaptic terminal. Or via intercellular metabolism (or simply diffusing away from the synapse.)
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.