Acetylcholine

Acetylcholine, abbreviated ACh, is a neurotransmitter built from choline and an acetyl group. Neurons that release it are called cholinergic. It is the transmitter at the neuromuscular junction, at the ganglia of both autonomic divisions, at the endings of parasympathetic nerves, and at many synapses in the brain, where it helps with memory and attention.

At the neuromuscular junction, ACh released by a motor neuron binds receptors on the muscle fiber's motor end-plate. These receptors are channels that let positive ions in, depolarizing the membrane and setting off an action potential that leads to contraction. The enzyme acetylcholinesterase, sitting in the synaptic cleft, breaks ACh down almost at once, so each nerve signal produces a brief, controlled twitch rather than a lasting one.

ACh acts on two receptor families. Nicotinic receptors, which nicotine also activates, are ion channels on skeletal muscle and on neurons in autonomic ganglia. Muscarinic receptors, named after a mushroom toxin, are G protein-coupled receptors on organs supplied by parasympathetic nerves, where ACh slows the heart and stimulates digestion. In myasthenia gravis, antibodies against ACh receptors weaken nerve-to-muscle signaling and cause muscle weakness.