sarcoplasmic reticulum The specialized endoplasmic reticulum found in the fibres of voluntary 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.

sarcoplasmic reticulum A network of membranes that run both longitudinally and transversely across striated muscle cells. Apparently, the longitudinal channels are derived from the endoplasmic reticulum and are not in direct communication with those running transversely (T-system), which are derived from the cell membrane. The function of the sarcoplasmic reticulum is to act as a store for calcium ions (Ca⁺⁺) involved in the contractile process.