Define the following terms: I band, A band and H-zone

In striated muscle fibers such as skeletal and cardiac muscles, the basic contractile unit is the sarcomere, which is the region between two Z-lines. Each sarcomere contains two main types of filaments: thin filaments (actin) and thick filaments (myosin). These filaments are arranged in a specific, overlapping pattern that produces the characteristic alternating light and dark bands seen under the microscope. Based on the arrangement and degree of filament overlap, the sarcomere displays three main horizontal zones: I band, A band, and H-zone. Understanding these zones is essential for grasping how muscles contract and how structural changes are translated into mechanical force.

1. I Band (Isotropic Band):

The I band is the lighter region of the sarcomere that contains only thin filaments (actin). It does not have any thick filaments, and for this reason, it appears less dense under the microscope. The I band stretches from the end of one thick filament in a sarcomere to the beginning of a thick filament in the adjacent sarcomere. This band is bisected by the Z line (or Z disc), which serves as the point of anchorage for the thin filaments from two neighboring sarcomeres. During muscle contraction, the I band shortens significantly as the thin filaments slide over the thick filaments, bringing the Z lines closer together.

2. A Band (Anisotropic Band):

The A band is the dark band seen in a sarcomere and represents the entire length of the thick filaments (myosin). This region also includes areas where thick and thin filaments overlap, which contributes to its darker appearance. Unlike the I band, the A band remains unchanged in length during contraction because the thick filaments do not change their length. The central part of the A band contains only thick filaments (forming the H-zone), while the lateral parts include overlapping thin filaments.

3. H-Zone (Hensen's Zone):

The H-zone is the lighter central part of the A band where there is no overlap between thick and thin filaments. It contains only thick filaments (myosin) and is visible only when the muscle is in a relaxed state. During muscle contraction, as the actin filaments slide inwards, the H-zone becomes narrower or even disappears because thin filaments move into this region and begin to overlap with the thick filaments.






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