Final answer:
The depolarization of the junctional folds causes the voltage-gated sodium channels to open.
Step-by-step explanation:
The depolarization of the junctional folds causes the voltage-gated sodium channels to open. As the membrane depolarizes, these ion channels are triggered to open, allowing sodium ions to enter the muscle fiber. This influx of sodium ions initiates an action potential, which rapidly spreads along the entire membrane to trigger excitation-contraction coupling.