According to the sliding filament theory, which event occurs during muscle contraction?

• A. Myosin filaments shorten while actin filaments remain stationary.
• B. The sarcomere lengthens as actin and myosin filaments move apart.
• C. Actin filaments slide past myosin filaments, shortening the sarcomere.
• D. Calcium ions are released, causing the sarcomere to expand.

Answer: (C)

Verified step by step guidance

1. Understand the sliding filament theory: This theory explains the mechanism of muscle contraction at the molecular level. It involves the interaction between actin and myosin filaments within the sarcomere, the functional unit of a muscle fiber.
2. Identify the role of actin and myosin filaments: During contraction, actin filaments slide past myosin filaments. This sliding action shortens the sarcomere, which is the region between two Z-discs in a muscle fiber.
3. Recognize the role of calcium ions: Calcium ions are released from the sarcoplasmic reticulum in response to a nerve signal. These ions bind to troponin, causing a conformational change that moves tropomyosin away from the binding sites on actin, allowing myosin heads to attach to actin.
4. Understand the cross-bridge cycle: Myosin heads bind to actin, forming cross-bridges. Using energy from ATP hydrolysis, the myosin heads pivot, pulling the actin filaments toward the center of the sarcomere. This action is repeated in a cycle, leading to sarcomere shortening.
5. Clarify the incorrect options: Myosin filaments do not shorten; they remain stationary while actin filaments slide. The sarcomere does not lengthen during contraction; it shortens. Calcium ions facilitate contraction but do not cause the sarcomere to expand.