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Embedded in the motor end plate of the muscle fiber are receptors for acetylcholine (ACh), the neurotransmitter that is released by the terminal branches of a motor neuron.
An enlargement of the receptors reveals that they are composed of five (5) polypeptide subunits, which together form a gated, water-filled ion channel in the lipid bilayer. Normally, the gates are closed and ions are prevented from passing through the channel. Two of the receptor's subunits have binding sites for ACh molecules. When both binding sites are occupied by ACh, the gated subunits change shape (= conformation) and the channel opens.
ACh receptors are also known as chemical-gated or ligand-gated channels. Ligand is used because the term refers to one molecule binding to another (larger) molecule. While it is open, sodium (= Na+) and potassium (= K+) ions diffuse through the channel.
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