Question

Which of the following regions decreases in length during muscle contraction?

• A. 2 and 3
• B. 3 and 4
• C. 1, 2, and 4
• D. 2, 3, and 4

Answer 1

The correct option is D 2, 3, and 4

Region 1 is the A band that contains thick and thin filaments overlapping each other. Region 2 here is the I band which is formed only of thin actin filaments. Region 3 is the H zone, formed only of thick myosin filaments. Finally, region 4 on the diagram is the area between the Z lines, and is called a sarcomere.
During muscle contraction, the I band reduces in size as the overlap between the thick filaments and the thin filaments increases. The H zone also decreases in length because of the same reason. As the actin filaments come closer to the M-line, the Z lines come closer together, so the sarcomere also decreases in length. The A band is the only part of the sarcomere that does not change length during muscle contraction.