neurotransmitter
A neurotransmitter is the chemical word one nerve cell uses to speak to the next. Where two neurons nearly touch — a junction called a synapse — the sending cell releases a puff of these molecules across the tiny gap, and the receiving cell 'hears' them through receptors on its surface. It is how the nervous system passes messages from cell to cell.
More precisely, an electrical signal reaching a nerve ending triggers the release of neurotransmitter from storage vesicles into the synaptic cleft. The molecules diffuse across and bind receptors on the next cell, either exciting or inhibiting it. The signal is then switched off by reuptake back into the cell, by enzymatic breakdown, or by diffusion away. Common examples include glutamate, GABA, dopamine, serotonin, norepinephrine, and acetylcholine.
This lifecycle — synthesis, storage, release, receptor binding, and termination — gives CNS drugs many points of attack. A drug can mimic a neurotransmitter, block its receptor, slow its breakdown, or block its reuptake, and almost every psychiatric and neurological medicine works by nudging one of these steps.
An important caveat: a single neurotransmitter is rarely tied to a single function or feeling. The same molecule, acting through different receptor subtypes and brain regions, can produce opposite effects, which is why 'fixing' a chemical imbalance is more subtle than it sounds.
Mapping a few drugs onto one synapse: an SSRI blocks serotonin reuptake, an MAO inhibitor blocks its breakdown, and an antipsychotic blocks the dopamine receptor — three different steps, three different effects.
Different drugs target different stages of the same synaptic cycle.
The neurotransmitter lifecycle is the master map of CNS pharmacology: name any psychiatric drug and you can usually place it at synthesis, release, the receptor, reuptake, or breakdown.