Phase 0 of the cardiac action potential is primarily due to the influx of which ion?

• A. Sodium
• B. Calcium
• C. Potassium
• D. Chloride

Answer: (A)

Phase 0 represents the rapid upstroke of the cardiac action potential and is driven mainly by the opening of fast voltage-gated sodium channels. The inward sodium current quickly depolarizes the cell from a negative resting potential toward positive values, producing the steep rise seen in ventricular and atrial muscle. This Na+ influx creates the sharp phase 0 slope. Afterward, phase 1 involves a brief outward potassium current, phase 2 is the plateau with calcium entry balancing potassium, and phase 3 is repolarization from potassium efflux. Chloride doesn’t play a major role in initiating phase 0. In pacemaker tissue, phase 0 can be driven more by calcium entry, but for typical ventricular/atrial cells, sodium influx is the primary driver of the rapid upstroke.