What is sarcoplasmic reticulum?

Sarcoplasmic reticulum is smooth endoplasmic reticulum found in myocytes (muscle cells). Sarcoplasmic reticulum stores calcium ions and pumps them out into the sarcoplasm (cytoplasm of myocytes) when the muscle fiber is stimulated. After their release from the sarcoplasmic reticulum, calcium ions interact with contractile proteins that utilize ATP to shorten the muscle fiber. The sarcoplasmic reticulum plays a major role in excitation and contraction reactions.