Final answer:
The fundamental contractile unit of skeletal muscle is the sarcomere, composed of specific bands and zones such as Z lines, I band, A band, H zone, and M line. The letters in their names refer to various descriptive aspects, such as Z for Zigzag and I for Isotropic. During contraction, the sarcomere shortens, which involves the sliding of actin and myosin filaments, but the length of the A band remains constant.
Step-by-step explanation:
The fundamental contractile unit of skeletal muscle is the sarcomere. Within each sarcomere, you can identify distinct regions which are key to its contractile function. These include:
- The Z line (or Z disc): It defines the boundary of each sarcomere.
- The I band: It is the region that only contains thin actin filaments.
- The A band: This area spans the length of the thick myosin filaments, with some overlapping of the thin actin filaments.
- The H zone: Present in the middle of the A band, it contains only myosin filaments.
- The M line: Located in the center of the H zone, where myosin filaments are linked.
Each letter representing various parts of the sarcomere stands for:
- Z for Zigzag
- I for Isotropic
- A for Anisotropic
- H for Helle (German for bright)
- M for Mittel (German for middle)
During muscle contraction, several changes occur within these sarcomeric regions:
- The I band and the H zone get shorter as the actin and myosin filaments slide closer together.
- The Z lines move closer to each other as the sarcomere shortens.
- The A band remains the same length because the actual length of the myosin filaments doesn't change.
The sliding of actin over myosin filaments during contraction is powered by ATP and is regulated by the proteins troponin and tropomyosin, related to the sliding filament model of muscle contraction.