The sarcoplasmic reticulum (SR) is a specialized organelle found in muscle cells, particularly in skeletal and cardiac muscle fibers. It plays a crucial role in regulating calcium ion (Ca2+) levels within the muscle cell cytosol, which is essential for muscle contraction and relaxation.The SR is composed of tubules and cisternae, which are flattened sacs arranged around each myofibril. The elongated tubules, known as longitudinal SR, primarily regulate Ca2+ levels within muscle cells. Additionally, the SR membrane contains Ca2+ ATPase pumps, which actively transport calcium ions from the cytosol into the SR lumen during muscle relaxation.