In skeletal muscle, which statement describes the primary trigger for Ca2+ release from the sarcoplasmic reticulum during excitation-contraction coupling?

• A. CICR from extracellular Ca2+ influx.
• B. Direct coupling between DHPR and RyR in T-tubules triggers SR Ca2+ release.
• C. Voltage-gated calcium channels on the sarcolemma release Ca2+ from extracellular space.
• D. Calcium release is not required for contraction.

Answer: (B)

During skeletal muscle excitation-contraction coupling, depolarization of the T-tubules activates the dihydropyridine receptor (DHPR), and in skeletal muscle this activation directly gates the ryanodine receptor (RyR1) on the sarcoplasmic reticulum. This physical coupling opens the SR calcium release channels, causing a rapid release of Ca2+ into the cytosol. The sudden Ca2+ rise binds troponin C, enabling cross-bridge cycling and contraction. This mechanism differs from cardiac muscle, where calcium influx through membrane channels triggers calcium release from the SR (calcium-induced calcium release). So the direct DHPR–RyR1 coupling is the primary trigger for SR Ca2+ release in skeletal muscle.