Acetylcholine

Acetylcholine (ACh), chemical in the body of animals that functions as a neurotransmitter, sending electrical impulses across synapses between nerve cells, and from motor neurons to muscle cells, causing the muscle cells to contract. As an electric impulse reaches the nerve ending, the nerve cell releases acetylcholine, which passes across the synapse and bonds chemically with a receptor molecule in the membrane of the neighboring nerve cell. The bonding of acetylcholine to the receptor molecule alters the polarity and permeability of the membrane so that the nerve impulse can be transmitted. The effect of acetylcholine can be neutralized by an enzyme, such as cholinesterase, which decomposes acetylcholine through the process of hydrolysis. When acetylcholine is decomposed, the muscle relaxes.

See Brain; Neurophysiology.