Which statement about muscle myofibrils is true?

• A. They are the outermost connective tissue around a muscle
• B. They are short, segment-like structures within sarcomeres
• C. They are the thick and thin filaments that slide past one another during contraction
• D. They are longitudinally oriented and extend the full length of the muscle fiber cell

Answer: (D)

Muscle myofibrils are the long, rod‑like elements that run the length of a muscle fiber. They are organized in parallel and extend from one end of the cell to the other, so they are longitudinally oriented along the fiber. Each myofibril contains repeating contractile units called sarcomeres, where thick (myosin) and thin (actin) filaments interact to produce contraction by sliding past one another. The outermost connective tissue wrapping a whole muscle is the epimysium, not a myofibril. A sarcomere is the segment within a myofibril, and the thick and thin filaments are components inside the sarcomere, not the myofibril itself. Thus, the true statement is that myofibrils extend the full length of the muscle fiber and run along its length.