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Acetylcholine, often abbreviated as ACh, is a neurotransmitter, a chemical messenger that carries signals between nerve cells and other cells in the body. It plays a crucial role in various physiological processes, including muscle contraction, memory, learning, and attention. In the peripheral nervous system, acetylcholine is the primary neurotransmitter at the neuromuscular junction, where it binds to receptors on muscle fibers, causing them to contract. It is also involved in regulating heart rate and digestion.

Within the central nervous system, acetylcholine acts as a key player in cognitive functions. It is essential for forming new memories and retrieving existing ones. Furthermore, it contributes to alertness, arousal, and the ability to focus attention. Dysregulation of acetylcholine signaling is implicated in several neurological and psychiatric conditions, such as Alzheimer's disease, where a deficiency in acetylcholine is associated with memory loss and cognitive decline. Conversely, excessive acetylcholine can lead to muscle spasms and other complications.

The synthesis of acetylcholine occurs in the nerve terminal from choline and acetyl-CoA. It is then stored in synaptic vesicles and released into the synaptic cleft upon nerve impulse. Once it has transmitted its signal, acetylcholine is rapidly broken down by an enzyme called acetylcholinesterase into choline and acetate. This enzymatic degradation is vital for terminating the signal and allowing the nerve cell to prepare for subsequent transmissions. This breakdown also prevents overstimulation.