Biological Systems MCAT Practice Exam

Which phase of cardiac muscle cell excitation is characterized by the influx of sodium ions?

1. Phase 0

2. Phase 1

3. Phase 2

4. Phase 3

The correct answer is: Phase 0

The correct choice highlights Phase 0 of cardiac muscle cell excitation, which is the initial phase of the action potential in these cells. During Phase 0, there is a rapid depolarization caused by the opening of voltage-gated sodium channels in the cardiac cell membrane. When these channels open, sodium ions (Na+) rush into the cell, resulting in a swift increase in the intracellular sodium concentration. This influx of positive charge leads to a change in the membrane potential, propelling the cell toward a positive threshold and initiating the action potential. Understanding this particular phase is crucial, as it sets the stage for subsequent phases of cardiac muscle action potentials, which are characterized by other ion movements such as calcium and potassium. The distinct characteristics of Phase 0 play a vital role in the overall conduction of electrical signals in the heart, influencing heart rhythm and contractility.