Terminal cisterna

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 cisternae then go on to release calcium, which binds to troponin. This releases tropomyosin, exposing active sites of the thin filament, actin.

There are several mechanisms directly linked to the terminal cisternae which facilitate Excitation-Contraction coupling, which include ryanodine (RYR) receptors, and a voltage gated calcium channel which help transfer the action potential into a calcium efflux of the Sarcoplasmic Reticulum.

A T-tubule surrounded by two terminal cisternae is referred to as a " triad" in physiology. As previously explained, the terminal cisternae along with the transverse tubules are the mechanisms of transduction from a nervous impulse to an actual muscle contraction.

...
Wikipedia