Answer
When skeletal muscle contracts, the sarcomere, which is the basic functional unit of a muscle fiber, undergoes changes in its structure. The sarcomere is made up of several bands and zones, including the A band, H zone, and I band. Here's what happens to these bands during muscle contraction:
1. A band: The A band represents the length of the thick myosin filaments and some overlapping thin actin filaments. During muscle contraction, the A band remains relatively constant in width. The myosin and actin filaments slide past each other, but the length of the A band doesn't change significantly.
2. H zone: The H zone is a region within the A band where only myosin filaments are present, and there is no overlap with actin filaments. As the muscle contracts, the H zone becomes narrower or may even disappear entirely. This is because the myosin filaments are pulling the actin filaments towards the center of the sarcomere during contraction, causing the H zone to contract along with it.
3. I band: The I band represents the region of the sarcomere where only actin filaments are present, and there is no overlap with myosin filaments. During muscle contraction, the I band also becomes narrower. This narrowing occurs because the actin filaments are pulled inward by the myosin filaments, overlapping with myosin in the A band, which reduces the width of the I band.
In summary, during skeletal muscle contraction, both the H zone and the I band become narrower or may disappear entirely due to the sliding of myosin and actin filaments, while the A band remains relatively constant in width because it represents the length of the myosin filaments. These structural changes are part of the sliding filament theory, which explains how muscle contraction occurs at the molecular level.
Work Step by Step
When skeletal muscle contracts, the sarcomere, which is the basic functional unit of a muscle fiber, undergoes changes in its structure. The sarcomere is made up of several bands and zones, including the A band, H zone, and I band. Here's what happens to these bands during muscle contraction:
1. A band: The A band represents the length of the thick myosin filaments and some overlapping thin actin filaments. During muscle contraction, the A band remains relatively constant in width. The myosin and actin filaments slide past each other, but the length of the A band doesn't change significantly.
2. H zone: The H zone is a region within the A band where only myosin filaments are present, and there is no overlap with actin filaments. As the muscle contracts, the H zone becomes narrower or may even disappear entirely. This is because the myosin filaments are pulling the actin filaments towards the center of the sarcomere during contraction, causing the H zone to contract along with it.
3. I band: The I band represents the region of the sarcomere where only actin filaments are present, and there is no overlap with myosin filaments. During muscle contraction, the I band also becomes narrower. This narrowing occurs because the actin filaments are pulled inward by the myosin filaments, overlapping with myosin in the A band, which reduces the width of the I band.
In summary, during skeletal muscle contraction, both the H zone and the I band become narrower or may disappear entirely due to the sliding of myosin and actin filaments, while the A band remains relatively constant in width because it represents the length of the myosin filaments. These structural changes are part of the sliding filament theory, which explains how muscle contraction occurs at the molecular level.
