The specialized endoplasmic reticulum found in the fibres of skeletal and cardiac muscle, which forms a network of membrane-lined cavities surrounding the contractile myofibrils that run through the fibres. The release of calcium ions from the sarcoplasmic reticulum into the cytosol following stimulation from an action potential (see transverse tubules) causes subsequent contraction of the myofibrils (see sarcomere). The calcium ions are immediately returned to the sarcoplasmic reticulum by calcium ion pumps in the membrane.