Sacroplasmic reticulum

The Sarcoplasmic Reticulum (SR) is present in all types of muscle cell (cardiac, smooth and skeletal) and is a form of modified endoplasmic reticulum . It contains an essential store of Ca²⁺ ions; when the SR is stimulated these ions are released to initiate a contraction in the muscle. In other words, the sarcoplasmic reticulum regulates the intracellular concentration of calcium ions.

In cardiac and skeletal muscle cells the SR surrounds the myofibril and release their Ca²⁺ ions at areas known as terminal cisternae. Terminal cisternae are the part of the sarcoplasmic reticulum nearest T-tubules. Skeletal and cardiac sarcoplasmic reticulum are similar in this way; however cardiac cells SRs are undeveloped and less dense in comparison to skeletal cells.

Smooth muscle cells also contain a sarcoplasmic reticulum that extends throughout the cell, in contrast to the other muscle types though, smooth muscles do not contain T-tubules and instead have junctional regions of the sarcoplasmic reticulum that abut areas of the sarcolemma or caveolae. Furthermore, Ca²⁺ ions are not only released from the SR in smooth muscle cells but also come from the extracellular fluid during muscular contraction.