A neurotransmitter is a chemical messenger that transmits signals across the synapse from one neuron to another or to a muscle or gland cell. It converts the electrical activity of the presynaptic cell into a chemical influence on the postsynaptic cell.

When an action potential reaches the axon terminal, vesicles release neurotransmitter into the synaptic cleft. The molecules bind to receptors on the receiving cell, opening ion channels or triggering intracellular cascades that either excite or inhibit the postsynaptic neuron. Excess transmitter is removed by reuptake into the presynaptic cell, by enzymatic breakdown, or by diffusion.

Dozens of neurotransmitters have been identified. Acetylcholine, dopamine, serotonin, norepinephrine, GABA, and glutamate are among the best studied. Many psychoactive drugs act by mimicking, blocking, or altering the availability of specific neurotransmitters. The precise mix of transmitters and receptors at each synapse determines the character of the signal that is passed.

The gap between neurons is bridged by neurotransmitter molecules.