Acetylcholine Receptors in Muscle Fiber (Cells)

Author: Scott A. Sheffield MS

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Embedded in the sarcolemma (= fiber membrane) 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 image descriptionopen.

ACh not moving after the gates closed in connection between the terminal branch of the nerve axon and the muscle end plate
ACh being released after the gates opened in connection between the terminal branch of the nerve axon and the muscle end plate
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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.