Release is quantal

The Neurotransmitter is secreted in discrete packets or quanta. Each of the quantum symbolizes the release of the contents of a single vesicle, around 4000 molecules of transmitter. At CNS synapses this causes a miniature postsynaptic potential, either excitatory or inhibitory, due to the release of transmitter from the vesicle acting on 30–100 postsynaptic receptors underneath the active zone. The Excitatory and inhibitory postsynaptic potentials symbolize the summation of multiple miniature postsynaptic potential generated by an action potential invading numerous active zones concurrently. This occurs either as axons branch to form numerous discrete terminals or as some terminals have more than one active zone.

The active zone of lots of CNS synapses looks to have only one release site. This is termed as the one vesicle or one quantum hypothesis. Though, transmitter release does not occur every time an action potential arrives at the presynaptic terminal. Separateactive zones behave in an all-or-none fashion since an action potential will either trigger the release of the single quantum or not. The proportion of achievements will reflect the probability of release. At central synapses the probabilities of release vary between various sites and at least at some synapses it also depends on the latest history of the synapse.