A neurotransmitter is a chemical messenger utilised to propagate an action potential (or inhibitory response) from one neurone to another. The messenger binds to a receptor on the postsynaptic terminal, resulting in the opening of ligand gated ion channels, often Na+. Common neurotransmitters include Acetyl Choline, Noradrenaline, Glutamate and Dopamine.
Certain poisons exert their effects by inhibiting neurotransmitter function, such as Sarin. Sarin works by inhibiting acetyl cholinesterase, the enzyme responsible for breaking down acetyl choline and returning its constituents to the presynaptic terminal.