acetylcholine, Ester of choline and acetic acid, a neurotransmitter active at many nerve synapses and at the motor end plate of vertebrate voluntary muscles. It affects several of the body’s systems, including the cardiovascular system (decreases heart rate and contraction strength, dilates blood vessels), gastrointestinal system (increases peristalsis in the stomach and amplitude of digestive contractions), and urinary system (decreases bladder capacity, increases voluntary voiding pressure). It also affects the respiratory system and stimulates secretion by all glands that receive parasympathetic nerve impulses (see autonomic nervous system). It is important in memory and learning and is deficient in the brains of those with late-stage Alzheimer disease.