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Acetylcholine is a neurotransmitter, a chemical messenger that transmits signals between nerve cells and also between nerve cells and muscles. It is the primary neurotransmitter of the parasympathetic nervous system and also plays a role in the sympathetic nervous system. Acetylcholine is synthesized in the presynaptic neuron from choline and acetyl-CoA. It is then stored in vesicles and released into the synaptic cleft upon stimulation. Once in the synaptic cleft, acetylcholine binds to specific receptors on the postsynaptic membrane, either excitatory or inhibitory, depending on the receptor type. It is then rapidly broken down by the enzyme acetylcholinesterase into choline and acetate, which are then reabsorbed by the presynaptic neuron or cleared from the synapse. This breakdown is crucial for terminating the signal and allowing for subsequent neurotransmission.

Acetylcholine has a wide range of functions in the body. In the peripheral nervous system, it is responsible for muscle contraction at the neuromuscular junction, slowing of the heart rate, and stimulation of digestive processes. In the central nervous system, acetylcholine is involved in learning, memory, attention, and arousal. Disorders affecting acetylcholine signaling are associated with various neurological and psychiatric conditions, including Alzheimer's disease, myasthenia gravis, and Parkinson's disease. Therapies targeting acetylcholine pathways are used to treat these conditions.