Which ion primarily drives the rapid depolarization phase (phase 0) of ventricular action potentials?

• A. Potassium (K+)
• B. Sodium (Na+)
• C. Calcium (Ca2+)
• D. Chloride (Cl-)

Answer: (B)

The rapid upstroke of ventricular action potentials is driven by the fast inward sodium current. When the cell reaches threshold, voltage-gated sodium channels open quickly, allowing a large influx of Na+ into the cell. This sudden entry rises the membrane potential rapidly from a negative resting value toward a positive level, producing the steep depolarization that defines phase 0 and sets the pace for conduction through the ventricles.

Potassium currents mainly handle repolarization later, returning the membrane potential toward its resting value, while calcium entry through L-type channels sustains the plateau phase (phase 2) by opposing K+ efflux. Chloride currents don’t play the main role in driving the initial depolarization in ventricular cells. In nodal tissue, the upstroke is slower and more calcium-dependent, but in ventricular myocytes the fast Na+ current is the key driver.