How do sodium ions affect a cardiac cell's resting potential?

Sodium ions play a crucial role in influencing a cardiac cell's resting potential, primarily through their effect on membrane potential during depolarization. The resting potential of a cardiac cell is typically around -90 millivolts, largely determined by the concentration gradients of sodium, potassium, and other ions across the cell membrane.

Upon activation, sodium channels open, allowing an influx of sodium ions into the cell, which leads to a rapid increase in the internal charge of the cell. This influx causes depolarization, moving the membrane potential toward a more positive value. This is crucial in generating an action potential, which is necessary for the contractile function of the heart.

The other choices, while they touch on aspects of cell physiology, do not accurately describe the specific role of sodium ions in this context. For instance, while sodium plays a role in maintaining a stable environment (which might pertain to fluid balance or osmotic conditions), it is primarily through the alteration of resting potential and generation of action potentials that sodium has its most immediate and critical impact in cardiac cells.