Terminal cisternae

Skeletal muscle, with terminal cisterna labeled near bottom.

Terminal cisternae are enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules. These discrete regions within the muscle cell store calcium (increasing the capacity of the sarcoplasmic reticulum to release calcium) and release it when an action potential courses down the transverse tubules, eliciting muscle contraction. Because terminal cisternae ensure rapid calcium delivery, they are well developed in muscles that contract quickly, such as fast twitch skeletal muscle. Terminal cisterns then go on to release calcium, which binds to troponin. This releases tropomyosin, exposing active sites of the thin filament, actin.