AEMCA Practice Exam 2025 - Free Practice Questions and Study Guide for Advanced Emergency Medical Care Assistant Certification.

During the plateau phase of the cardiac action potential, the significant characteristic is that sodium channels close, which halts further influx of sodium ions that initially contributed to rapid depolarization. Simultaneously, calcium ions enter the cardiac muscle cells through calcium channels. This influx of calcium balances out the declining sodium influx, resulting in a prolonged period where the membrane potential remains relatively stable, hence the name "plateau."

This plateau phase is crucial for the heart because it prevents premature contractions and allows sufficient time for the ventricles to contract and pump blood effectively. It is distinct from the rapid depolarization phase that occurs earlier in the action potential, where sodium channels open and allow a quick influx of sodium, leading to a sharp rise in membrane potential.

In comparison, the other options focus on different phases of the action potential or represent processes not characteristic of the plateau phase, like repolarization occurring, which happens after the plateau, and the establishment of resting potential, which occurs after repolarization is completed.