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A T-tubule (or transverse tubule) is a deep invagination of the sarcolemma, which is the plasma membrane of skeletal muscle and cardiac muscle cells. These invaginations allow depolarization of the membrane to quickly penetrate to the interior of the cell. ==Structure== Each muscle fiber is surrounded by a sarcolemma (the muscle fiber's plasma membrane). Transverse tubules are invaginations of the sarcolemma. Though the invaginations are normally perpendicular to the length of the fiber, many T-tubules can lie axial to the long-axis of the fiber, both wrapping around and running alongside myofilament bundles. The size and arrangement of T-tubules vary depending on muscle type and species. T-tubules are thought to be rich in a number of proteins, including a large number of L-type calcium channels. In skeletal muscle cells, T-tubules are typically located at the junction overlap between the A and I bands of the sarcomere, and together with a ''pair'' of terminal cisternae (bulbous enlarged areas of the sarcoplasmic reticulum) it forms an arrangement called a triad. In cardiac muscle, the T-tubule membrane is normally opposed to a ''single'' terminal cisterna, which may wrap around the T-tubule. This occurs at the Z line and is called a diad. It is physiologically important for excitation-contraction coupling (see section below) that the T-tubules are positioned close to the terminal cisternae of the sarcoplasmic reticulum, as the triad or diad arrangement allows physical and functional contact by voltage-dependent L-type calcium channels. So, an action potential along the sarcolemma causes calcium channels to open in the terminal cisternae/sarcoplasmic reticulum, which enables calcium to move from the sarcoplasmic reticulum into cytoplasm and the intracellular calcium concentration to increase. 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「T-tubule」の詳細全文を読む スポンサード リンク
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