acetylcholine
Acetylcholine is one of the body's master messengers. Picture it as a chemical text message released by a nerve ending that tells the next cell what to do: contract, secrete, or relay the signal onward. It is the transmitter of choice across a remarkably wide range of connections.
It is synthesized from choline and acetyl-CoA by choline acetyltransferase, stored in vesicles, and released into the synapse when a nerve fires. It acts on two receptor families: nicotinic receptors (fast ligand-gated ion channels at ganglia, the neuromuscular junction, and parts of the brain) and muscarinic receptors (slower G-protein-coupled receptors at parasympathetic target organs). After acting, it is rapidly broken down by the enzyme acetylcholinesterase, which keeps signaling brief and precise.
Because acetylcholine sits at so many key junctions, drugs that imitate it, block its receptors, or stop its breakdown have powerful and varied effects. The same chemistry that lets a cholinesterase inhibitor treat Alzheimer's disease also makes nerve agents and certain insecticides deadly poisons.
At the neuromuscular junction, acetylcholine released by a motor nerve binds nicotinic receptors on muscle, triggering the contraction that lets you move.
The transmitter behind voluntary muscle movement.
Acetylcholine itself is almost never given as a drug, because it is destroyed within seconds by cholinesterase throughout the body. Drugs instead use longer-lasting analogues or target the enzyme.