Which region of a sarcomere contains only thick filaments?

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

Answer: (B)

In a sarcomere, different regions reflect which filaments are present. The area in the middle around the M-line that contains only thick filaments is the H-zone. It has myosin filaments without overlapping actin, so there are no cross-bridge interactions there. The A-band, by contrast, includes the full length of the thick filaments and also the regions where they overlap with thin filaments. The I-band contains only thin filaments and spans to the Z-line, which marks the boundary between adjacent sarcomeres. During contraction, actin slides inward, causing the I-band and the H-zone to shorten, while the overall length of the A-band remains unchanged. Therefore, the region with only thick filaments is the H-zone.