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) allow 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 cell. These impulses, or action potentials, are the trigger for muscle fiber contraction.
Also called: transverse tubule