The sarcomeres is the main contractile unit of muscle fiber in the skeletal muscle. Each sarcomere is composed of protein filaments (myofilaments) that include mainly the thick filaments called myosin, and thin filaments called actin. The bundles of myofilaments are called myofibrils.
Sarcomere gives the striated appearance to skeletal and cardiac muscles and was firstly explained by Van Leeuwenhoek. A sarcomeres is between two Z lines and surrounding the area of Z line the region of the I-band is present. After the region of I-band there is an area of A-band and A-band contains the entire length of the thick filament. Within the A-band there is an area of H-zone which is paler in color and the H-band has no actin in it. Inside H-band there is another thin line called M-line which is formed by cross-connecting components of the cytoskeleton. The sarcomere is fibrous proteins that work as filaments to help in the movement of muscles during contraction and relaxation. When a muscle contracts or relaxes sarcomere slide past each other which help in facilitating these movements.
Structure of Sarcomeres
The structure of the sarcomeres is traditionally described with dark and light bands visible under the microscope. This banding pattern in sarcomeres is due mainly to the arrangement of thick and thin myofilaments in each unit. These markings include:
- A bands (or anisotropic bands) – dark bands that contain whole thick filaments (myosin).
- I bands (or isotropic bands) – light bands that contain only the thin filaments (actin) and are located between the two thick filaments.
- Z disc – is an area that traverses the I bands and marks the point of the connection between the two neighboring actin filaments. That said, the sarcomere can also be described as the structure between the two Z discs.
- M line – marks the middle of the sarcomere and contains the protein called myomesin.
- H zone – is the area between the M line and Z disc. The H zone contains only myosin.
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