Terminal cisternae are discrete regions within the muscle cell. They 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. When excitation of the membrane arrives at the
T-tubule nearest the
muscle fiber, a
dihydropyridine channel (
DHP channel) is activated. This is similar to a
voltage-gated calcium channel, but is not actually an
ionotropic channel. Instead, it serves to activate
ryanodine, which will let
calcium ions pass into the sarcoplasmic reticulum, and triggers calcium release to the muscle fiber itself. A T-tubule surrounded by two terminal cisternae is called a
triad. The terminal cisternae, along with the transverse tubules, are the mechanisms of transduction from a
nervous impulse to an actual
muscle contraction. == See also ==