In the steps of skeletal muscle contraction, what is the primary role of $C^2+$ released from the sarcoplasmic reticulum?

• A. It binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin
• B. It binds directly to myosin heads to provide the energy for the power stroke
• C. It prevents ATP from binding to myosin, maintaining the rigor state
• D. It activates acetylcholinesterase to terminate the signal at the neuromuscular junction

Answer: (A)

Verified step by step guidance

1. Understand that during skeletal muscle contraction, calcium ions (Ca^{2+}) are released from the sarcoplasmic reticulum into the cytoplasm of the muscle fiber.
2. Recognize that the primary role of Ca^{2+} is to bind to the regulatory protein troponin, which is located on the thin filament (actin).
3. When Ca^{2+} binds to troponin, it induces a conformational change that causes tropomyosin, another regulatory protein, to shift its position on the actin filament.
4. This movement of tropomyosin exposes the myosin-binding sites on actin, allowing the myosin heads to attach and initiate the cross-bridge cycle necessary for muscle contraction.
5. Note that Ca^{2+} does not bind directly to myosin heads, prevent ATP binding, or activate acetylcholinesterase; its key function is to regulate the exposure of binding sites on actin through troponin.