T-tubule

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.