At higher intracellular Ca2+ concentrations, which sequence leads to contraction?

At higher intracellular Ca2+ concentrations, which sequence leads to contraction?

• A. Ca2+ binds to troponin; troponin moves tropomyosin away; myosin heads bind actin; sarcomere shortens
• B. Tropomyosin continues to block active sites
• C. ATP is consumed to detach cross-bridges
• D. Calcium is pumped back to SR

Answer: (A)

When Ca2+ rises in the cytosol, it binds to troponin C on the thin filament. This binding causes troponin to change shape and pull tropomyosin away from the myosin-binding sites on actin. With those sites exposed, energized myosin heads can attach to actin and form cross-bridges. The cross-bridge cycle then drives the power stroke, pulling actin toward the center of the sarcomere and shortening the muscle fiber. ATP is needed to energize the myosin heads and, later, to detach them so the cycle can continue if Ca2+ remains high. The other options describe blocking the sites (which prevents contraction), detaching cross-bridges (a step toward relaxation), or pumping Ca2+ back into the sarcoplasmic reticulum (relaxation), none of which initiate contraction.

When Ca2+ rises in the cytosol, it binds to troponin C on the thin filament. This binding causes troponin to change shape and pull tropomyosin away from the myosin-binding sites on actin. With those sites exposed, energized myosin heads can attach to actin and form cross-bridges. The cross-bridge cycle then drives the power stroke, pulling actin toward the center of the sarcomere and shortening the muscle fiber. ATP is needed to energize the myosin heads and, later, to detach them so the cycle can continue if Ca2+ remains high. The other options describe blocking the sites (which prevents contraction), detaching cross-bridges (a step toward relaxation), or pumping Ca2+ back into the sarcoplasmic reticulum (relaxation), none of which initiate contraction.