What are muscle myofilaments?

• A. They are the thick and thin filaments that slide past one another during contraction
• B. They are bundles of muscle fibers within fascicles
• C. They are connective tissue around muscle fibers
• D. They are membranes surrounding sarcomeres

Answer: (B)

Muscle myofilaments are the thick and thin protein filaments inside each sarcomere that make up the contractile apparatus. The thick filaments are mainly myosin, and the thin filaments are actin (with regulatory proteins tropomyosin and troponin). When a muscle contracts, these filaments slide past one another in the sliding filament mechanism, shortening the sarcomere and generating force. This concept focuses on the actual contractile elements within the muscle fiber, not the higher-order organization or surrounding tissues. Bundles of muscle fibers within fascicles describe fascicles, connective tissue around fibers refers to endomysium/perimysium/epimysium, and membranes around sarcomeres aren’t the myofilaments themselves (sarcolemma or other membranes surround cells or organelles, while the sarcoplasmic reticulum surrounds myofibrils).