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A T-tubule (or transverse tubule) is a deep invagination of the sarcolemma, which is the plasma membrane, only found in skeletal and cardiac muscle cells. These invaginations allow depolarization of the membrane to quickly penetrate to the interior of the cell.


Each muscle fiber is surrounded by a sarcolemma (the muscle fiber's plasma membrane) which invaginates perpendicular to the length of the fiber to form what is called a T-tubule. At these invaginations the sarcolemma is studded with a large number of L-type calcium channels. <br /><br />

In skeletal muscle cells specifically, T-tubule invaginations 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. <br />Cardiac muscle has a similar structure; instead of a triad there is a diad, which is composed of a T-tubule and a single terminal cisterna, and it occurs at the Z line. <br /><br />

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......
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