Question

The H zone is the region in the center of the sarcomere composed of the lighter bands on either side of and including the M line. In this region, the myosin filaments are centered on the M line, and there are no overlapping actin filaments. Therefore, a cross-section through this region would reveal only myosin. What is the H zone?

1) The region in the center of the sarcomere where myosin filaments are centered on the M line and there are no overlapping actin filaments
2) The region in the center of the sarcomere where actin filaments are centered on the M line and there are no overlapping myosin filaments
3) The region in the center of the sarcomere where both myosin and actin filaments are centered on the M line
4) The region in the center of the sarcomere where myosin and actin filaments overlap
5) The region in the center of the sarcomere where only actin filaments are present

Answer 1

The H zone is the central region of a sarcomere where only myosin thick filaments are present and centered on the M line, with no overlapping actin thin filaments.

Step-by-step explanation:

The H zone is a specific area within a skeletal muscle's sarcomere, which is the functional contractile unit of the muscle cell. This zone is highlighted by the presence of only myosin thick filaments, centered on the M line, with no overlapping actin thin filaments. When a muscle contracts, the distance between Z discs on either side of the sarcomere is reduced. The H zone, being a part of the A band, does not change in length, but appears to get smaller as the overlapping region of thin and thick filaments increases due to actin filaments sliding over the myosin. As such, the correct answer to the question is 1) The region in the center of the sarcomere where myosin filaments are centered on the M line and there are no overlapping actin filaments.