What is a sarcomere?

• A. Contractile units of a myofibril responsible for the striated appearance
• B. The outer covering of a muscle
• C. A nerve fiber
• D. A tendon

Answer: (A)

A sarcomere is the basic contractile unit of a skeletal muscle, a segment of a myofibril from one Z-disc to the next. It contains the thin actin filaments anchored to the Z-discs and the thick myosin filaments overlapping with actin in the central region. When the muscle contracts, myosin heads form cross-bridges with actin and pull the filaments toward the center, causing the sarcomere to shorten. This sliding filament mechanism, repeated along many sarcomeres in a muscle fiber, produces the overall shortening and force of the muscle. The organized arrangement of sarcomeres gives the characteristic striated appearance of skeletal muscle. The other options refer to different structures: the outer covering of a muscle is connective tissue such as fascia or epimysium; a nerve fiber is a neuron; a tendon is dense connective tissue that attaches muscle to bone.