Question

the spontaneous opening of na⁺ channels marks the beginning of ________ of a cardiac muscle cell. multiple choice afterpolarization repolarization isopolarization hyperpolarization depolarization

Explanation:

When sodium ($Na^+$) channels open spontaneously in a cardiac - muscle cell, sodium ions flow into the cell. This influx of positive ions causes the membrane potential to become less negative (more positive), which is the process of depolarization. Afterpolarization occurs after an action potential and is not the start event when $Na^+$ channels open. Repolarization is the return of the membrane potential to its resting state after depolarization. Isopolarization is not a standard term in this context. Hyperpolarization is when the membrane potential becomes more negative than the resting potential.

Answer:

E. depolarization