In the sliding filament model of skeletal muscle contraction, what is the primary role of Ca$2^{+}$ released from the sarcoplasmic reticulum?

• A. It binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.
• B. It directly provides the energy for the power stroke by being hydrolyzed in place of ATP.
• C. It binds to myosin heads to trigger their detachment from actin at the end of the power stroke.
• D. It opens voltage-gated Na${+}^{}$ channels on the sarcolemma to initiate the muscle action potential.

Answer: (A)

Verified step by step guidance

1. Understand that in the sliding filament model, 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 (Ca^{2+}) are released from the sarcoplasmic reticulum into the cytoplasm of the muscle cell in response to a muscle action potential.
3. Know that Ca^{2+} binds specifically to the protein troponin, which is attached to tropomyosin on the actin filament.
4. When Ca^{2+} binds to troponin, it causes a conformational change that moves tropomyosin away from the myosin-binding sites on actin, thereby exposing these sites.
5. This exposure allows myosin heads to attach to actin, enabling the cross-bridge cycle and muscle contraction to proceed.