Sarcoplasmic Reticulum

 The Sarcoplasmic reticulum (SR) is the Equivalent of the ER (Endoplasmic Reticulum), only specialized to Smooth and Striated (Cardiac and Skeletal) Muscle cells, It controls the release of calcium ions into the cell, thus being a main part in the control of Muscle contraction. In smooth muscle cells, the SR is much less developed.