Acetylcholine is a neurotransmitter, one of the chemicals that neurons use to communicate with each other.

In the body, acetylcholine is the primary neurotransmitter which nerves use to signal muscles to initiate or cease movement. Botulinism toxin, the substance that causes botulism, works by preventing the release of acetylcholine in the body; this can paralyze respiratory muscles, causing suffocation and death. The venom of the black widow spider causes an equally lethal overrelease of acetylcholine -- flooding the system.

In the brain, acetylcholine is produced in several locations including the basal forebrain. It may promote learning. Acetylcholine-producing cells in the basal forebrain are damaged in the early stages of Alzheimer's disease, which may contribute to the memory impairments which are an early symptom of the disease. The drugs tacrine (brand name Cognex) and donepezil (trade name Aricept), currently marketed as Alzheimer's drugs, are cholinesterase inhibitors, meaning that they increase the effectiveness of acetylcholine in the brain.