In a sarcomere, which region is characterized by the presence of only thick (myosin) filaments?

• A. Z discs
• B. H zone
• C. A bands
• D. I band

Answer: (B)

Understanding how a sarcomere is organized by filament types helps you see why this region is the one with only thick filaments. Thick myosin filaments are arranged in the center of the sarcomere, anchored at the M line, while thin actin filaments extend from the Z discs toward the center. The central part of the A band where actin does not overlap the thick filaments is the region that contains only thick filaments—the H zone. This area appears lighter under a microscope because there are no actin filaments there. So, the H zone is the portion of the sarcomere with exclusively thick (myosin) filaments. During contraction, actin slides inward, and the H zone shortens as more overlap occurs between thick and thin filaments.