Neurotransmitter

A neurotransmitter is the chemical a neuron uses to communicate. When an electrical impulse (action potential) reaches the end of a neuron, it triggers the release of neurotransmitter molecules stored in vesicles at the axon terminal. The molecules diffuse across the synapse — the tiny gap between cells — and bind to receptors on the next cell, passing the signal along.

What happens next depends on the transmitter and the receptor. Excitatory signals, such as those carried by glutamate, make the receiving neuron more likely to fire; inhibitory signals, such as those carried by GABA, make it less likely. The signal ends when the neurotransmitter is broken down by enzymes, reabsorbed into the sending neuron through reuptake, or diffuses away.

The major neurotransmitters each have signature roles: acetylcholine drives muscle contraction and memory, dopamine supports reward and movement, serotonin influences mood, appetite, and sleep, and norepinephrine mediates arousal and the stress response. Imbalances in these systems are linked to conditions such as Parkinson's disease (dopamine), depression (serotonin and norepinephrine), and myasthenia gravis (acetylcholine signaling) — and most psychoactive drugs work by boosting, blocking, or mimicking a neurotransmitter.

Neurotransmitters are core testable content across several exams. USMLE Step 1 covers their synthesis, pathways, and receptor pharmacology; the MCAT tests them in the biological bases of behavior and in the mechanisms of consciousness-altering drugs; and AP Psychology tests neurons, synaptic transmission, and the links between neurotransmitter systems and psychological disorders.