neurotransmitter

one of several chemical substances released in minute quantities by axon tips into the SYNAPSE to enable a NERVE IMPULSE to cross. It diffuses across the space and may depolarize the opposite MEMBRANE , allowing the production of an action potential. Outside the central nervous system, ACETYLCHOLINE is a major neurotransmitter, and NORADRENALINE is released in the SYMPATHETIC NERVOUS SYSTEM . Acetylcholine and noradrenaline also operate within the CENTRAL NERVOUS SYSTEM , as does DOPAMINE , amongst others.