What are the major neurotransmitters and their functions?

Key neurotransmitters include acetylcholine, which enables muscle action, learning, and memory and is depleted in Alzheimer's disease. Dopamine influences movement, motivation, reward, and pleasure and is implicated in Parkinson's disease and schizophrenia. Serotonin regulates mood, sleep, appetite, and is linked to depression when deficient. Norepinephrine affects alertness, arousal, and the stress response. GABA, or gamma-aminobutyric acid, is the main inhibitory neurotransmitter that reduces neural activity and is targeted by anti-anxiety medications.