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A myofibril (also known as a muscle fibril or sarcostyle) is a basic rod-like organelle of a muscle cell. are composed of long, tubular cells known as muscle fibers, and these cells contain many chains of myofibrils. (2025). 9780815344643 ISBN 9780815344643 Each myofibril has a diameter of 1–2 . They are created during embryogenesis in a process known as myogenesis.
Myofibrils are composed of long proteins including actin, myosin, and titin, and other proteins that hold them together. These proteins are organized into Thick filaments, thin filaments, and elastic filament , which repeat along the length of the myofibril in sections or units of contraction called . Muscles contract by sliding the thick myosin, and thin actin myofilaments along each other.
The protein complex composed of actin and myosin is sometimes referred to as actomyosin.
In striated skeletal muscle and cardiac muscle muscle tissue the actin and myosin filaments each have a specific and constant length on the order of a few micrometers, far less than the length of the elongated muscle cell (a few millimeters in the case of human skeletal muscle cells). The filaments are organized into repeated subunits along the length of the myofibril. These subunits are called that are around three μm in length. The muscle cell is nearly filled with myofibrils running parallel to each other on the long axis of the cell. The sarcomeric subunits of one myofibril are in nearly perfect alignment with those of the myofibrils next to it. This alignment gives the cell its striped or striated appearance. Exposed muscle cells at certain angles, such as in meat cuts, can show structural coloration or iridescence due to this periodic alignment of the fibrils and sarcomeres.
The I bands appear lighter because these regions of the sarcomere mainly contain the thin actin filaments, whose smaller diameter allows the passage of light between them. The A band, on the other hand, contains mostly myosin filaments whose larger diameter restricts the passage of light. A stands for anisotropic and I for isotropic, referring to the optical properties of living muscle as demonstrated with polarized light microscopy.
The parts of the A band that abut the I bands are occupied by both actin and myosin filaments (where they interdigitate as described above). Also within the A band is a relatively brighter central region called the H-zone (from the German helle, meaning bright) in which there is no actin/myosin overlap when the muscle is in a relaxed state. Finally, the H-zone is bisected by a dark central line called the M-line (from the German mittel meaning middle).
When a nerve impulse arrives, Ca2+ ions cause troponin to change shape; this moves the troponin + tropomyosin complex away, leaving the myosin binding sites open.
The myosin head now binds to the actin myofilament. Energy in the head of the myosin myofilament moves the head, which slides the actin past; hence ADP is released.
ATP presents itself (as the presence of the calcium ions activates the myosin's ATPase), and the myosin heads disconnect from the actin to grab the ATP. The ATP is then broken down into ADP and phosphate. Energy is released and stored in the myosin head to utilize for later movement. The myosin heads now return to their upright relaxed position. If calcium is present, the process is repeated.
When a muscle contracts, the actin is pulled along myosin toward the center of the sarcomere until the actin and myosin filaments are completely overlapped. The H zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. Thus when the muscle is fully contracted, the H zone is no longer visible. Note that the actin and myosin filaments themselves do not change length, but instead slide past each other. This is known as the sliding filament theory of muscle contraction.Marieb, E. N., Hoehn, K., & Hoehn, F. (2007). Human Anatomy & Physiology. (7th ed., pp. 284–87). San Francisco, California: Benjamin-Cummings Pub Co.
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