Terminal cisternae

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. The T-tubules plus adjacent terminal cisternae create a triad. In cardiac muscle, adjacent to the intercalated disks, one would find a diad which consists of a T-tubule adjacent to a single terminal cisterna.