What is the sliding filament theory?

• A. The process by which bones grow
• B. The way tendons attach to muscles
• C. The method energy production in muscle cells
• D. The mechanism of muscle contraction involving actin and myosin

Answer: (D)

The sliding filament theory shows how muscles contract at the cellular level by actin and myosin filaments sliding past each other to shorten the sarcomere. When a contraction is triggered, calcium ions released inside the muscle fiber move the troponin-tropomyosin complex away from the binding sites on actin. This allows myosin heads to form cross-bridges with actin. Using energy from ATP, the myosin heads pull (the power stroke), sliding the actin filaments toward the center of the sarcomere. The filaments slide past each other, so the sarcomere shortens and the muscle contracts, while the lengths of the individual filaments don’t change; the Z-lines move closer, the I-band and the H-zone shorten, and the A-band stays the same length.

This is not about bone growth, tendon attachment, or energy production processes independent of the contraction mechanism. Those aspects describe other parts of physiology, whereas the sliding filament theory specifically explains how actin and myosin interactions generate contraction.