Neurotransmitter

A neurotransmitter is a signaling molecule made by neurons and stored in tiny sacs, or vesicles, inside the axon terminal. When an action potential arrives, the vesicles release their contents into the synaptic cleft, and the molecules bind receptor proteins on the receiving cell. Each receptor recognizes only its own transmitter, much as a lock accepts only one key.

The effect depends on the receptor as much as on the transmitter. Ionotropic receptors are ion channels that open at once and give fast responses, while metabotropic receptors act through G proteins and second messengers, producing slower and longer-lasting changes. A transmitter's action ends when it diffuses away, is broken down by an enzyme, or is pumped back into the cell that released it, a step called reuptake.

The main groups are acetylcholine; amino acids such as glutamate, which is generally excitatory, and GABA and glycine, which are inhibitory; biogenic amines such as dopamine, serotonin and norepinephrine; and neuropeptides such as the endorphins. Many medicines and drugs of abuse work by changing these signals, for example by blocking reuptake or by mimicking a natural transmitter.