Every voluntary movement you make starts with one small chemical messenger
Whenever you lift a finger, motor neurons squirt acetylcholine onto your muscle fibres, opening channels that let sodium rush in and trigger contraction. The same molecule helps wake the brain, focus attention and lay down memories. Nerve agents and many venoms kill by throwing this single system into chaos.
The name spells out the chemistry, since the molecule joins choline to acetic acid as an ester. Nerve cells build it from choline and acetyl-CoA using the enzyme choline acetyltransferase. Its charged ammonium group stops it slipping through fatty cell membranes, so acetylcholine given from outside stays outside cells and is thought not to cross the blood-brain barrier.
The signal must also be switched off fast. An enzyme called acetylcholinesterase, packed into the gap between nerve and muscle, breaks the molecule into choline and acetate within the synapse. Some neurotoxins block that enzyme, so acetylcholine piles up, the muscles for breathing seize and the heart can stop. Sarin and various plant, animal and bacterial toxins attack the same junction by overactivating or silencing muscles.
Its receptors come in two families named after the chemicals that switch on one but not the other. Nicotinic receptors respond to nicotine, whose addictiveness comes from acting on them in the brain; muscarinic receptors respond to muscarine from the fly agaric mushroom. Drugs acting on muscarinic receptors show how dose matters: atropine is poisonous in large amounts but used in small ones for certain heart and eye problems, while scopolamine can cause delirium, hallucinations and amnesia.
Beyond muscles, acetylcholine is the main transmitter of the parasympathetic nervous system, one branch of the network running involuntary body functions. In the brain, pathways from the basal forebrain to the cortex and hippocampus support thinking and memory; blocking acetylcholine there in animals produces forgetfulness resembling amnesia. It promotes REM sleep, and damage to these circuits is linked to the memory loss of Alzheimer's disease. Remarkably, it is ancient: the amoeba Acanthamoeba uses it as a growth signal, and its enzymes trace back to early single-celled life.
Source: Acetylcholine
