What initiates Ca2+ release from the sarcoplasmic reticulum in skeletal muscle?

• A. Voltage-sensitive proteins in T tubules stimulate Ca2+ release from terminal cisternae of SR
• B. Direct diffusion of Ca2+ from extracellular space
• C. Binding of Ca2+ to troponin triggers release
• D. Activation of myosin ATPase

Answer: (A)

Depolarization of the T-tubule membrane triggers Ca2+ release from the sarcoplasmic reticulum. When an action potential travels along the muscle fiber, voltage-sensitive proteins in the T tubules (the dihydropyridine receptors) respond to the change in membrane potential and mechanically couple to ryanodine receptors on the SR’s terminal cisternae. This coupling opens the Ca2+ channels in the SR, releasing Ca2+ into the cytosol.

This Ca2+ release is what starts the contraction process by allowing Ca2+ to bind troponin and move tropomyosin away from actin’s binding sites, enabling cross-bridge cycling. The other options don’t initiate release: Ca2+ diffusion from outside the cell isn’t the trigger in skeletal muscle, Ca2+ binding to troponin happens after Ca2+ is released, and myosin ATPase activation occurs during contraction after Ca2+ has risen.