what are h zones?

Question

In anatomy and physiology, what are h zones? Are they only in relaxed fiber?

Answer 1

H zone (′āch ′zōn)
(histology) The central portion of an A band in a sarcomere; characterized by the presence of myosin filaments.

A sarcomere is the basic unit of a cross striated muscle's myofibril. Sarcomeres are multi-protein complexes composed of three different filament systems.

The thick filament system is composed of myosin protein.
The thin filaments are assembled by actin monomers.
The elastic filament system is composed of the giant protein titin (also called connectin).
A muscle cell, from a biceps, may contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into sarcomeres

The sarcomeres are what give skeletal and cardiac muscles their striated appearance. A sarcomere is defined as the segment between two neighbouring Z-lines (or Z-discs). In electron micrographs of cross striated muscle the Z-line (from the German "Zwischen", between the I bands) appears as a series of dark lines. Surrounding the Z-line is the region of the I-band (for isotropic). Following the I-band is the A-band (for anisotropic). Within the A-band is a paler region called the H-band (from the German "Heller", bright). Finally, inside the H-band is a thin M-line (from the German "Mittel", middle of the sarcomere). A-bands and I-bands were named for their properties under a polarizing microscope.

Answer 2

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Answer 3

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