What causes the release of calcium ions into the sarcoplasm from the terminal cisterns?

Excitation-contraction coupling

Explanation:

The T-tubules are located between the terminal cistern adjacent to the RS, forming a triad consisting of two terminal cisterns and the T-tubule. The RS-tubule-T junctions and their associations with myofibrils develop in a series of consecutive steps, where the formation of junctions between the two membrane systems is done concurrently, initiating molecular changes in both membrane systems.

When the potential for nervous action reaches the muscular membrane, it undergoes depolarization that is transmitted to the depths of the cell by the membranes of the tubular-T system. When the action potential of the tubules is detected by a dihydropyridine receptor located at the junction of the tubule with the sarcoplasmic reticulum cistern, a mechanism not yet elucidated produces the opening of the receptors to ryanodine, which function as channels for the calcium. When these channels open, as the concentration of calcium ions within the RS is much higher than in the cell's cytosol, these ions leave their deposit to begin the process of interaction between thick and thin filaments, muscle contraction.

This series of events is called the 'excitation-contraction coupling' (e-c) and several proteins specifically located in the RS-tubule-T junction have essential roles in them. The dihydropyridine receptor (DHPR) in the T-tubule detects the voltage across the membrane and its activation causes the release of Ca2 + from the RS, while the RyR / Ca2 + release channel is located in the junction RS and is responsible for the release of Ca2 + from its storage locations. Both RyR and DHPR are necessary for proper muscle development, although none of them are necessary for T-RS anchoring or the search and / or association of CLQ and tradin in the binding RS.

i believe the correct answer is d