What rod-like structures run parallel to the length of a muscle fiber and contain sarcomeres end-to-end?

• A. Myofibrils
• B. Mitochondria
• C. Nuclei
• D. Lysosomes

Answer: (A)

The contractile machinery in a skeletal muscle fiber is organized into myofibrils. These are long, rod-like elements that run parallel to the length of the muscle fiber, forming the main internal structure. Inside each myofibril, the repeating units called sarcomeres are lined up end-to-end, creating the characteristic striated pattern and enabling contraction as actin and myosin filaments slide past one another. Other organelles, like mitochondria (energy producers), nuclei (skeletal muscle fibers are multinucleated and have nuclei near the periphery), and lysosomes (digestive compartments), don’t form these contractile chains and don’t contain sarcomeres arranged end-to-end.