acetylcholine

Chemical that serves as a neurotransmitter, communicating nerve impulses between the cells of the nervous system. It is largely associated with the transmission of impulses across the synapse (junction) between nerve and muscle cells, causing the muscles to contract.

ACh is produced in the synaptic knob (a swelling at the end of a nerve cell) and stored in vesicles until a nerve impulse triggers its discharge across the synapse. When the ACh reaches the membrane of the receiving cell it binds with a specific site and brings about depolarization - a reversal of the electric charge on either side of the membrane - causing a fresh impulse (in nerve cells) or a contraction (in muscle cells). Its action is shortlived because it is quickly destroyed by the enzyme cholinesterase.

Anticholinergic drugs have a number of uses in medicine to block the action of ACh, thereby disrupting the passage of nerve impulses and relaxing certain muscles, for example in premedication before surgery.

Postmortems on sufferers of Alzheimer's disease have shown a deficit of ACh. This has led to the hypothesis (1993) that loss of ACh contributes to memory loss. Therefore, inhibitors of the enzyme acetylcholine esterase (which breaks down ACh) are now available for Alzheimer's sufferers. They appear to alleviate symptoms but do not affect the progression of the disease.