When an action potential arrives at the axon terminal, it triggers the opening of calcium channels in the membrane of that neurone. This, in turn, triggers the release of the neurotransmitter from the vesicles it is stored in, into the synaptic cleft. The neurotransmitter then diffuses across the space between the two neurones and binds to specific receptors on the membrane of the post-synaptic neurone. The interaction between the neurotransmitter and receptor may either stimulate or inhibit the initiation of an action potential in the second neurone ie it may be excitatory or inhibitory.
There are two main types of receptor, one kind of receptor is directly linked to an ion channel in the postsynaptic membrane whilst the other type of receptor is not directly linked to an ion channel, but has its effects in the postsynaptic neurone by signalling the production of a second messenger. The second messenger can lead to many changes in the postsynaptic neurone including the opening of ion channels in the plasma membrane. These receptors enable more complex and long lasting effects to occur in the postsynaptic neurone.