Which region of the sarcomere contains the entire length of thick filaments?

• A. I Band
• B. A Band
• C. Z Disc
• D. M Line

Answer: (B)

The region that contains the entire length of thick filaments is the A band. Thick filaments run the full span of the A band, where they overlap with thin filaments. The I band, by contrast, contains only thin actin filaments and lacks thick filaments. The Z discs mark the boundaries between adjacent sarcomeres, while the M line sits at the center and helps anchor the thick filaments. In contraction, the sarcomere shortens because the thin filaments slide past the thick ones, reducing the I band and H zone, but the length of the thick filaments themselves—and thus the A band—stays the same.