The primary function of the SR is to regulate intracellular Ca2+ levels. During muscle contraction, an action potential travels along the sarcolemma and into the T-tubules, triggering the release of Ca2+ from the terminal cisternae of the SR into the cytoplasm. The increased Ca2+ concentration allows for the interaction between actin and myosin filaments, resulting in muscle contraction. After contraction, Ca2+ is actively pumped back into the SR by the SERCA (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase) pump, allowing the muscle to relax.
