A chemical that mediates the transmission of a nerve impulse across a synapse or a neuromuscular junction (i.e. excitation), or causes hyperpolarization of a postsynaptic cell making an action potential less likely (i.e. inhibition). Examples are adrenaline, noradrenaline, dopamine, and serotonin (in adrenergic nerves), acetylcholine (in cholinergic nerves), glutamate, gamma-aminobutyric acid, substance P, and nitric oxide. The neurotransmitter is released at the synaptic knob at the tip of the axon into the synaptic cleft. It diffuses across to the opposite membrane (the postsynaptic membrane), where it stimulates receptors and initiates the excitation or inhibition of the postsynaptic cell. At a neuromuscular junction, the neurotransmitter transmits impulses to the muscle-fibre membrane. A single neurotransmitter may exert either an excitatory or an inhibitory action, depending on which type of receptor occurs on the postsynaptic membrane. See also cotransmitter.

http://themedicalbiochemistrypage.org/nerves.php Summary of the main neurotransmitters, their biochemistry, and mode of action, from the Medical Biochemistry pages of Indiana University