What triggers the release of calcium ions in skeletal muscle during excitation-contraction coupling?

• A. An action potential triggers calcium release from the sarcoplasmic reticulum via ryanodine receptors following depolarization of the transverse tubules.
• B. Calcium is released during contraction solely by mechanical stretch of the muscle.
• C. Calcium release is spontaneous and not linked to depolarization.
• D. Calcium is released from the mitochondria during contraction.

Answer: (A)

In skeletal muscle, the trigger for calcium release during excitation-contraction coupling is the depolarization of the muscle membrane that travels into the transverse tubules. This depolarization activates voltage-sensing dihydropyridine receptors in the T-tubule membrane, which are mechanically linked to ryanodine receptor type 1 channels on the adjacent sarcoplasmic reticulum. When these links respond to the voltage change, the RyR1 channels open and Ca2+ floods into the cytosol. That rise in Ca2+ enables actin–myosin cross-bridge cycling by binding to troponin C, allowing contraction to proceed.

This mechanism is distinct from a purely stretch-activated or spontaneous release, and from Ca2+ release that would originate from mitochondria; the skeletal muscle trigger is the electrical signal leading to SR Ca2+ release via the DHPR–RyR1 coupling.