Acetylcholine is a chemical messenger that carries signals across the synapses of the nervous system. It was the first neurotransmitter to be identified, and it remains one of the most versatile.

At the neuromuscular junction it triggers muscle contraction. In the brain it supports attention, learning, and memory; the progressive loss of acetylcholine-producing neurons is a hallmark of Alzheimer’s disease. In the autonomic nervous system it slows the heart and stimulates digestion. Different receptor types—nicotinic and muscarinic—allow the same molecule to produce different effects depending on the tissue.

Drugs that increase acetylcholine levels, such as those used in early Alzheimer’s treatment, can modestly improve cognitive symptoms. Drugs that block it, including some antihistamines and antidepressants, can cause dry mouth, blurred vision, and memory fog. The toxin produced by Clostridium botulinum prevents the release of acetylcholine and thereby paralyzes muscle; in controlled doses it is used medically and cosmetically.

Acetylcholine illustrates how a single chemical can serve both the simplest motor act and the most complex mental operations.