In a longitudinal section of skeletal muscle, which component functions as the primary contractile element?

In a longitudinal section of skeletal muscle, which component functions as the primary contractile element?

• A. Nucleus
• B. Endomysium
• C. Perimysium
• D. Myofibril

Answer: (D)

The force that actually causes contraction comes from the myofibrils inside each muscle fiber. Myofibrils are long, parallel threads packed with repeating units called sarcomeres, where actin and myosin slide past one another to shorten the fiber. In a longitudinal view, the striations reflect this sarcomeric arrangement along the length of the myofibril, showing where contraction occurs. The nucleus is simply part of the muscle fiber and supports protein synthesis, not the contractile process. The endomysium and perimysium are connective tissues that wrap individual fibers and fascicles and help transmit force, but they do not contract. Because the myofibril contains the contractile machinery responsible for shortening, it is the primary contractile element.

The force that actually causes contraction comes from the myofibrils inside each muscle fiber. Myofibrils are long, parallel threads packed with repeating units called sarcomeres, where actin and myosin slide past one another to shorten the fiber. In a longitudinal view, the striations reflect this sarcomeric arrangement along the length of the myofibril, showing where contraction occurs. The nucleus is simply part of the muscle fiber and supports protein synthesis, not the contractile process. The endomysium and perimysium are connective tissues that wrap individual fibers and fascicles and help transmit force, but they do not contract. Because the myofibril contains the contractile machinery responsible for shortening, it is the primary contractile element.