What is the role of the terminal cisterna of the sarcoplasmic reticulum in excitation-contraction coupling?

• A. They store Ca2+ and release it via ryanodine receptors to trigger contraction.
• B. They generate ATP for contraction.
• C. They synthesize actin.
• D. They absorb calcium from the cytoplasm.

Answer: (A)

The terminal cisternae act as a Ca2+ storage and release site in skeletal muscle. When an action potential reaches the muscle fiber, the voltage sensors in the T-tubule interact with ryanodine receptors on the terminal cisternae, opening them and flooding the cytoplasm with Ca2+. That surge of Ca2+ binds to troponin C, moves tropomyosin away from actin’s myosin-binding sites, and enables cross-bridge cycling that produces contraction. After contraction, Ca2+ is pumped back into the SR by SERCA pumps to relax the muscle. Other options don’t fit this role: ATP is produced mainly by mitochondria, actin is synthesized by ribosomes, and while the SR does take up Ca2+ to resequester it, the key step in triggering contraction is Ca2+ release from the terminal cisternae.