In the steps of skeletal muscle contraction, what is the primary role of calcium ions (Ca2+) in initiating contraction?

• A. Calcium enters the muscle fiber from the bloodstream to depolarize the sarcolemma and generate the action potential.
• B. Calcium directly hydrolyzes ATP to provide energy for the myosin power stroke.
• C. Calcium binds to myosin heads to detach them from actin at the end of the power stroke.
• D. Calcium binds to troponin, causing tropomyosin to move away from actin’s myosin-binding sites.

Answer: (D)

Verified step by step guidance

1. Understand that skeletal muscle contraction is regulated by the interaction between actin and myosin filaments, which is controlled by the position of tropomyosin on the actin filament.
2. Recognize that calcium ions (Ca2+) play a crucial role by binding to the regulatory protein troponin, which is attached to tropomyosin on the thin filament.
3. When Ca2+ binds to troponin, it induces a conformational change that causes tropomyosin to shift away from the myosin-binding sites on actin, exposing these sites.
4. This exposure allows the myosin heads to attach to actin, forming cross-bridges necessary for the contraction cycle to proceed.
5. Note that calcium does not enter the muscle fiber from the bloodstream to initiate the action potential, nor does it hydrolyze ATP or bind directly to myosin heads; its primary role is to regulate the accessibility of binding sites on actin through troponin.