ttubules

T-tubules, or transverse tubules, are narrow invaginations of the sarcolemma (the muscle cell plasma membrane) that extend into the interior of skeletal and cardiac muscle fibers. They form an extensive network that surrounds each myofibril and run transversely to the longitudinal axis of the fiber, facilitating rapid electrical signals from the surface to deeper regions of the cell.

In skeletal muscle, T-tubules align with pairs of terminal cisternae of the sarcoplasmic reticulum to form triads at the boundary between the A and I bands, near the Z-line. In cardiac muscle, T-tubules are generally larger and form dyads, closely associated with a single terminal cisterna, near the Z-line. This arrangement positions the calcium-release machinery in close proximity to the voltage-sensing membrane.

Functionally, T-tubules are essential for excitation-contraction coupling. An action potential traveling along the T-tubule activates voltage-gated L-type calcium channels (dihydropyridine receptors). In skeletal muscle, this triggers calcium release from the sarcoplasmic reticulum through ryanodine receptors largely via a mechanical interaction between the DHPR and RyR1. In cardiac muscle, calcium entry through DHPRs initiates calcium-induced calcium release via RyR2. The resulting rise in cytosolic calcium triggers actin–myosin interaction and contraction.

Overall, T-tubules increase the surface area of the sarcolemma, bring voltage sensors close to SR calcium release channels, and synchronize calcium signaling to ensure rapid, uniform muscle contraction. Disruption of T-tubule structure is associated with certain muscular and cardiac diseases.