Which event occurs according to the sliding filament theory of muscle contraction?

• A. Actin and myosin filaments shorten during contraction.
• B. Calcium ions are released from the sarcoplasmic reticulum and bind to myosin.
• C. ATP is not required for the detachment of myosin heads from actin.
• D. Myosin heads bind to actin, forming cross-bridges.

Answer: (D)

Verified step by step guidance

1. Understand the sliding filament theory: This theory explains how muscles contract by the sliding of actin and myosin filaments past each other, rather than shortening themselves.
2. Identify the role of calcium ions: Calcium ions are released from the sarcoplasmic reticulum and bind to troponin, not myosin, which causes a conformational change that exposes binding sites on actin filaments.
3. Recognize the importance of ATP: ATP is crucial for muscle contraction as it provides the energy needed for myosin heads to detach from actin after forming cross-bridges.
4. Focus on cross-bridge formation: Myosin heads bind to actin filaments, forming cross-bridges, which is a key event in muscle contraction according to the sliding filament theory.
5. Clarify the misconception about filament shortening: Actin and myosin filaments do not shorten; they slide past each other, which leads to muscle contraction.