A deep invagination of the sarcolemma, specifically at the junction of the A and I bands. It appears as a small, narrow tube (tubule) that runs transversely across the myofibril of striated muscle and extends over the surface of the sarcolemma.
The entire region of the striated muscle cell must contract simultaneously. Since a striated muscle fiber is relatively large, the narrow, small tubules (T-tubules) permit rapid depolarization of the membrane. The tubules are studded with a large number of L-type calcium channels that serve as passageway for the rapid transmission of electrical impulses to the interior of the muscle fiber. These impulses, or action potentials, trigger contraction of the muscle fiber.