Which unit contains actin and myosin and is responsible for contraction?

• A. M-line
• B. H-zone
• C. Z-line
• D. Sarcomere

Answer: (D)

The main idea is that the sarcomere is the basic contractile unit of skeletal muscle, and it is where actin and myosin interact to produce contraction. A sarcomere runs from one Z-line to the next and contains overlapping thin filaments (actin) and thick filaments (myosin). During contraction, calcium allows myosin heads to bind actin and pull the thin filaments toward the center of the sarcomere, shortening the sarcomere and generating force. The other parts—M-line, H-zone, and Z-line—are structural features that organize the sarcomere: the Z-lines define its boundaries, the M-line anchors the thick filaments, and the H-zone is the thick-filament–only region that narrows as contraction proceeds. So the unit that contains actin and myosin and drives contraction is the sarcomere.