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.ransverse 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.