neurotransmitter

A neurotransmitter is a tiny chemical messenger that one nerve cell releases to talk to the next. Brain cells (neurons) don't actually touch each other — there is a tiny gap between them called a synapse. An electrical signal can't simply jump across that gap, so when the signal reaches the end of one neuron, the cell squirts out a puff of these chemical molecules. They drift across the gap like a note passed across a narrow stream, carrying the message to the cell on the other side.

On the receiving cell, the message is read by special docking spots called receptors, each shaped to catch one kind of neurotransmitter — a lock-and-key fit. When the chemical clicks into its receptor, it tells the receiving neuron to either fire its own signal (excite) or stay quiet (calm down). Familiar examples include dopamine, serotonin, and acetylcholine. Because they steer nearly every thought, feeling, and movement, neurotransmitters are also where many medicines act: drugs for depression, pain, and Parkinson's disease all work by nudging these chemical messages up or down.