Final answer:
The statement that depolarization of cardiac muscle occurs when Na+ ions enter the cell as voltage-gated Na+ ion channels open is true; this is essential for initiating an action potential and the subsequent contraction of the cardiac muscle fibers.
Step-by-step explanation:
Depolarization of cardiac muscle occurs when Na+ ions diffuse into the cell as voltage-gated Na+ ion channels open. This statement is true. During an action potential, after a nerve impulse opens these channels, Na+ enters the cell, and this influx causes depolarization by reducing the membrane potential. As the membrane depolarizes, excitation-contraction coupling is initiated, resulting in the contraction of the cardiac muscle fiber.
The process of depolarization is a critical component in the conduction of electrical impulses within the heart, leading to coordinated heartbeats. After depolarization, repolarization occurs when the membrane becomes impermeable to Na+, and K+ ions move out of the cell, helping to restore the resting potential.