How do the T tubules and sarcoplasmic reticulum interact at triads to initiate contraction?

At triads, an electrical impulse travels along the T tubule, which is a deep invagination of the sarcolemma. Integral proteins of the T tubule act as voltage sensors and, when stimulated, cause gated channels in the adjacent terminal cisterns of the sarcoplasmic reticulum to open. This releases Ca2+ into the sarcoplasm, and the rise in intracellular calcium provides the final trigger that initiates contraction.

The T tubules and sarcoplasmic reticulum (SR) interact at triads, where a T tubule runs between paired terminal cisterns of the SR. Because T tubules are continuations of the sarcolemma, nerve-initiated electrical impulses conducted along the sarcolemma pass down the T tubules to the deep regions of the muscle fiber. At each triad, integral proteins protrude from both the T tubule and the SR into the intermembrane space. The T tubule proteins act as voltage sensors: when the electrical impulse reaches them, they respond by causing the integral proteins of the SR, which form gated channels, to open. Calcium ions stored in the terminal cisterns then flow through these channels into the sarcoplasm. This rapid, coordinated release of calcium, signaled by the T tubule system, is what triggers the contractile machinery in every myofibril at nearly the same time.