Sarcoplasmic Reticulum

The sarcoplasmic reticulum, or SR, is a specialized form of the endoplasmic reticulum in muscle cells that stores, releases, and reabsorbs calcium ions. By controlling calcium concentration around the contractile machinery, it plays a central role in switching muscle contraction on and off .^1,2

In skeletal muscle fibers, the sarcoplasmic reticulum forms an extensive network of membrane-bound tubules and enlarged regions called terminal cisternae around the myofibrils. A transverse tubule, or T-tubule, positioned between two terminal cisternae forms a structure called a triad. This close arrangement allows electrical activity traveling along the muscle membrane and T-tubules to rapidly trigger calcium release from the SR .^1,2

During excitation-contraction coupling in skeletal muscle, activation of voltage-sensitive proteins in the T-tubule membrane leads to opening of ryanodine receptors in the SR membrane. Calcium then moves from the SR into the cytoplasm, where it activates the contractile machinery. After stimulation ends, SERCA pumps use energy from ATP to transport calcium back into the SR. Lowering cytoplasmic calcium allows the muscle fiber to relax .^1,2

The sarcoplasmic reticulum should not be confused with the T-tubule system. T-tubules are inward extensions of the muscle-cell plasma membrane that conduct electrical signals into the fiber, whereas the sarcoplasmic reticulum is a separate intracellular membrane system that serves as the major regulated calcium store for contraction .^1,2