A chemical substance ("messenger") produced in and released by one neuron that carries a nervous impulse across a synapse (the small gap between the axon and dendrite of communicating neurons). They relay nervous impulses among neurons and between neurons and other types of cells, such as in muscle and glandular tissues. Neurotransmitters can excite or inhibit another neuron or receptor organ. There are more than 300 known neurotransmitters. A few of the more common ones are acetylcholine, dopamine, norepinephrine, and serotonin.