A chemical substance which transmits the action of a NERVE to a cell (see CELLS). It is released from nerve-endings and transmits the impulse across synapses (see SYNAPSE) to other nerves. In the central nervous system the substances acting as neurotransmitters include ACETYLCHOLINE, NORADRENALINE, DOPAMINE and SEROTONIN. The main transmitter in the peripheral system is acetylcholine, while for the sympathetic system it is noradrenaline. In recent years a new group of neurotransmitters called neuropeptides has been identified, comprising large protein molecules. One of the best-known is that of endorphins, which the brain uses to control pain. (See also NEURON(E); NERVOUS SYSTEM; PAIN.)