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A sarcomere (Greek ''sarx'' "flesh", ''meros'' "part") is the basic unit of striated muscle tissue. Skeletal muscles are composed of tubular muscle cells (myocytes called muscle fibers) which are formed in a process known as myogenesis. Muscle fibers are composed of tubular myofibrils. Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as dark and light bands. Sarcomeres are composed of long, fibrous proteins as filaments that slide past each other when a muscle contracts or relaxes. Two of the important proteins are myosin, which forms the thick filament, and actin, which forms the thin filament. Myosin has a long, fibrous tail and a globular head, which binds to actin. The myosin head also binds to ATP, which is the source of energy for muscle movement. Myosin can only bind to actin when the binding sites on actin are exposed by calcium ions. Actin molecules are bound to the Z line, which forms the borders of the sarcomere. Other bands appear when the sarcomere is relaxed.〔 〕 A muscle fiber from a biceps muscle may contain 100,000 sarcomeres.〔Assuming that the length of biceps is 20 cm and the length of sarcomere is 2 micrometer, there are 100,000 sarcomeres along the length of biceps.〕 The myofibrils of smooth muscle cells are not arranged into sarcomeres. ==Bands== 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, or Z bodies). In electron micrographs of cross-striated muscle, the Z-line (from the German "Zwischenscheibe", ''the disc in between'' the I bands) appears as a series of dark lines. * Surrounding the Z-line is the region of the I-band (for ''isotropic''). I-band is the zone of thin filaments that is not superimposed by thick filaments. * Following the I-band is the A-band (for ''anisotropic''). Named for their properties under a polarizing microscope. An A-band contains the entire length of a single thick filament. * Within the A-band is a paler region called the H-zone (from the German "heller", ''brighter''). Named for their lighter appearance under a polarization microscope. H-band is the zone of the thick filaments that is not superimposed by the thin filaments. * Inside the H-zone is a thin M-line (from the German "Mittelscheibe", the disc in the ''middle'' of the sarcomere) formed of cross-connecting elements of the cytoskeleton. The relationship between the proteins and the regions of the sarcomere are as follows: * ''Actin'' filaments, the thin filaments, are the major component of the I-band and extend into the A-band. * ''Myosin'' filaments, the thick filaments, are bipolar and extend throughout the A-band. They are cross-linked at the centre by the M-band. * The giant protein ''titin'' (connectin) extends from the Z-line of the sarcomere, where it binds to the thick filament (myosin) system, to the M-band, where it is thought to interact with the thick filaments. Titin (and its splice isoforms) is the biggest single highly elasticated protein found in nature. It provides binding sites for numerous proteins and is thought to play an important role as sarcomeric ruler and as blueprint for the assembly of the sarcomere. * Another giant protein, nebulin, is hypothesised to extend along the thin filaments and the entire I-Band. Similar to titin, it is thought to act as a molecular ruler along for thin filament assembly. * Several proteins important for the stability of the sarcomeric structure are found in the Z-line as well as in the M-band of the sarcomere. * Actin filaments and titin molecules are cross-linked in the Z-disc via the Z-line protein alpha-actinin. * The M-band proteins myomesin as well as C-protein crosslink the thick filament system (myosins) and the M-band part of titin (the elastic filaments). * The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for the muscle contraction (sliding filament model).〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sarcomere」の詳細全文を読む スポンサード リンク
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