neurotransmitter
n. any of a large number of chemicals that can be released by neurons to mediate transmission of nerve signals across the junctions (synapses) between neurons. When triggered by a nerve impulse, the neurotransmitter is released from the terminal button (see axon), travels across the synaptic cleft, and binds to and reacts with receptor molecules in the postsynaptic membrane. Neurotransmitters include amines (e.g., norepinephrine, serotonin) and amino acids (e.g., glutamate, glycine). Some neurotransmitters can be categorized as generally excitatory (e.g., glutamate, glysine) or generally inhibitory (e.g. gamma-aminobutyric acid). Excitatory neurotransmitters exert a facilitatory or activating downstream effect on postsynaptic neurons. That is, they depolarize the postsynaptic neurons, resulting in a greater likelihood of an action potential.
Inhibitory
neurotransmitters hyperpolarize the postsynaptic neurons—thereby making the intracellular space more negative, and hence, requiring a greater positive charge to overcome—resulting in a smaller likelihood of an action potential. However, some neurotransmitters have both excitatory and inhibitory receptors; these include acetylcholine and dopamine. Also called chemical transmitter; synaptic transmitter.