It is released by motor nerve cells and, when it binds to acetylcholine receptors of muscle fibers, stimulates those fibers to contract. Acetylcholine is also used in the central nervous system. The glands that receive impulses from the parasympathetic part of the autonomic nervous system are also stimulated in the same way. This is why an increase in acetylcholine causes a decreased heart rate and increased production of saliva.
Normally, the acetylcholine is quickly removed after having performed its action; this is done by the enzyme acetylcholinesterase[?] which converts acetylcholine into choline and acetic acid. The devastating effects of nerve agents are due to their inhibition of this enzyme, resulting in continuing stimulation of the muscles and glands. Since a shortage of acetylcholine in the brain has been associated with Alzheimer's disease, some drugs that inhibit acetylcholinesterase are used in the treatment of that disease.
Botulism toxin acts by suppressing the release of acetylcholine. Nicotine acts by increasing the activity of certain acetylcholine receptors, as does muscarine. Conversely, Atropine and scopolamine act by blocking these receptors.
The disease myasthenia gravis, characterized by general weakness and fatigue, occurs when the body mistakenly produces antibodies against acetylcholine receptors, and thus inhibits proper acetylcholine signal transmission.