Where do myosin and actin overlap somewhat?

• A. In the cytoplasm
• B. On the plasma membrane
• C. In the nucleus
• D. In the sarcomere

Answer: (D)

Within a skeletal muscle fiber, myosin thick filaments and actin thin filaments are organized into repeating units called sarcomeres. Actin extends from the Z-discs toward the center, while myosin sits in the middle. The region where these filaments overlap—the zone of overlap—resides inside the sarcomere, specifically in the A-band. This is the site where myosin heads bind to actin and pull, sliding the filaments past one another to shorten the sarcomere during contraction. The other locations aren’t where this contractile overlap is organized: the cytoplasm is just the general fluid inside the cell, the plasma membrane is the cell boundary, and the nucleus houses genetic material, not the arrangement of contractile proteins.