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Acetylcholinesterase is an enzyme that hydrolyses the neurotransmitter acetylcholine to choline and acetate. Its activity helps terminate signalling at cholinergic synapses, including the neuromuscular junction. Chemically, it is a catalyst: the reaction consumes acetylcholine and water, while the enzyme can be reused.^[ref]

The distinction between removal of a messenger and inhibition of its receptor matters. Reducing acetylcholinesterase activity leaves more acetylcholine available to activate receptors; it does not directly substitute for the messenger or block every receptor. Thus the duration of a chemical signal depends both on its release and on its clearance. Measuring reaction-product accumulation in an enzyme assay tests the clearance machinery separately from a responding muscle or neuron.

A cholinergic synapse is a chemical synapse that uses acetylcholine as a neurotransmitter. Release of acetylcholine allows it to bind receptors on the receiving cell. The response depends on receptor type: a nicotinic acetylcholine receptor is an ion channel, whereas a muscarinic acetylcholine receptor signals through a G protein.^[ref]

At a neuromuscular junction, activation of nicotinic acetylcholine receptors helps trigger a skeletal-muscle action potential. Other cholinergic pathways can change a cell's activity more slowly through muscarinic acetylcholine receptors. Thus naming the transmitter does not completely specify the postsynaptic effect. Acetylcholinesterase helps terminate signaling by breaking down acetylcholine in the synaptic region.^[ref]