Thin Filaments in Skeletal Muscle Fibers
Acetylcholine Receptors in Muscle Fiber (Cells)
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 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.