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Embedded in the sarcolemma at the neuromuscular junction are (nicotinic) receptors for acetylcholine (ACh). This is the neurotransmitter that is released by the terminal branches of a motor neuron.
An enlargement of an ACh receptor reveals that it is composed of five (5) polypeptide subunits. Together, the subunits form a channel that spans the width of the sarcolemma. Inside the receptor (= channel) are gates, which are usually closed. Two of the receptor's subunits have binding sites for ACh. When ACh occupies both sites, the subunits change shape (= conformation) and the gates open. While open, the receptor (= channel) forms a temporary, water-filled pore through the sarcolemma, which allows sodium ions (= Na+) to enter a muscle fiber and potassium ions (= K+) to exit.
Nicotinic 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.
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