Which region of a sarcomere contains only thick filaments?

• A. I-band
• B. Z-disc
• C. M-line
• D. H-zone

Answer: (D)

The concept tested is identifying the center region of the sarcomere where thick filaments exist without overlapping thin filaments. Thick (myosin) filaments run through the middle and are anchored toward the center, while thin (actin) filaments extend from the Z-discs and overlap with the thick filaments over much of the sarcomere. The band that has thick filaments with no thin filament overlap is the H-zone, located within the A-band. This zone shortens as contraction pulls thin filaments inward, reducing the non-overlapping region. The I-band contains only thin filaments, the Z-disc marks the boundaries, and the M-line is the central anchor for thick filaments. Thus, the region containing only thick filaments is the H-zone.