Which statement best describes myofibrils?

• A. They are extracellular networks between fibers
• B. They are longitudinally oriented and extend the full length of the muscle fiber
• C. They wrap the entire muscle fiber
• D. They are the individual actin and myosin molecules

Answer: (B)

Myofibrils are the longitudinal, rod‑like contractile elements that run the full length of a skeletal muscle fiber. They are intracellular structures packed inside the muscle fiber, and each myofibril contains many sarcomeres arranged end to end, which gives the fiber its striated appearance and the ability to contract along its length. When sarcomeres shorten, the entire myofibril shortens, pulling on the surrounding structures and producing overall muscle fiber contraction. They are not extracellular networks between fibers, nor do they wrap the entire fiber (that wrapping is the role of connective tissue sheaths around the fiber). They are also not individual actin and myosin molecules; those filaments are the components inside the sarcomeres of the myofibrils. Hence, being longitudinally oriented and extending the full length of the muscle fiber best describes myofibrils.