The phase 0 in cardiac muscle cells is caused due to the opening of fast ___________ channels

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

Answer: (B)

Phase 0 is the rapid upstroke of the cardiac myocyte action potential, driven by a large inward current from fast voltage-gated sodium channels. When the cell reaches threshold, these channels open quickly, allowing Na+ to flood into the cell and depolarize the membrane from a negative resting potential toward positive values. This swift influx creates the steep slope of phase 0. Potassium channels later contribute to repolarization, calcium channels open subsequently to sustain the plateau (phase 2), and chloride channels don’t play the primary role in this upstroke. In contrast, in pacemaker tissue the upstroke is mainly due to calcium entry, but in ventricular and atrial muscle, fast sodium channels are responsible for phase 0.