MEDizzy - Excitation–contraction coupling is a process in which a

Feed Library Learning Discuss Leaderboard Journal

Musculoskeletal and Integumentary Systems

Excitation–contraction coupling is a process in which a muscle cell is stimulated to contract, and this process is followed by muscle cell relaxation. It involves the following events (not necessarily in this order): 1. Action potential 2. Ca2+ release into cytoplasm 3. Cross-bridge cycling 4. Cytoplasmic Ca2+ removal 5. Depolarization of T tubules Which of the following lists the correct sequence of events for excitation–contraction coupling in skeletal muscle tissue?

ExplanationAction potentials travel along the surface of the myocytes and into the T (transverse) tubules, depolarizing the muscle cell membrane. This depolarization opens voltage-sensitive L-type calcium channels located on the sarcolemma and triggers a subsequent release of calcium from the sarcoplasmic reticulum (SR) through ryanodine receptor (RyR) channels. Calcium released by the SR increases the intracellular calcium concentration. The free calcium binds to troponin-C on the thin (actin) filaments and induces a conformational change that shifts troponin-I and exposes a site on the actin molecule that binds to the myosin head, and results in ATP hydrolysis that produces a conformational change in the actin–myosin complex. The result of these changes is cross-bridge cycling, such that the actin and myosin filaments slide past each other, thereby shortening the sarcomere. Finally, calcium entry into the cell slows and calcium is re-sequestered by the SR to allow relaxation to occur. Small amounts of cytosolic calcium are also transported out of the cell by sodium–calcium exchange pumps.