T-tubules or transverse tubules run perpendicular to the long axis of the fibres in the myofibrils of skeletal muscle and are invaginations in the sarcolemma. The tubules enter the sarcoplasm and extend over the sarcoplasmic reticulum at the junctions of the A and I bands. On either side of the T-tubules, the area that is associated with regions of the sarcoplasmic reticulum is known as the terminal cisternae. The T-tubules and the terminal cisternae do not touch, however, but each T-tubule is associated with two terminal cisternae, and the three structures together are known as a triad.
When the T- tubule is depolarised due to an incoming action potential in the sarcolemma, the impulse is carried down the t-tubules and opens the gates in the sarcoplasmic reticulum on the associated terminal cisternae, releasing calcium ions. Calcium ions flow into the cytoplasm where the actin and myosin filaments are. The calcium ions bind to the troponin-tropomyosin molecules located on the actin filaments and the power stroke is able to occur.