Why are neurotransmitters released?

The presynaptic (before the synapse) neuron receives the action potential, which is an electric current (>-65 mv) propogated within and between individual neurons.

Ca++ ions enter the axon terminal (a section of the neuron) when voltage-gated calcium (Ca++) channels open.

Neurotransmitter-filled synaptic vesicles fuse with the presynaptic terminal when Ca++ is present, releasing transmitter molecules into the synaptic cleft, which is the space between the postsynaptic and presynaptic neurons.

A local excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP) is produced in the postsynaptic neuron by certain transmitter molecules binding to specific receptor molecules in the postsynaptic membrane. This binding can occur directly or indirectly and cause the postsynaptic membrane's ion channels to open.

The postsynaptic cell's IPSPs and EPSPs extend toward the axon hillock, where a neuron will fire an action potential if the depolarization there reaches the threshold (-65 mv).

Unused neurotransmitter is either reabsorbed by the synapse, inactivated by enzymes, or used as an autocrine signal—a signal that a neuron sends that directly affects itself—which lowers neurotransmitter release.

Neurotransmitters are released to transmit signals between neurons, facilitating communication in the nervous system.