In non-nodal cells, decreased phase 0 slope decreases conduction velocity due to which mechanism?

• A. Na+ channel inactivation or blockade
• B. Ca++ channel inactivation
• C. Increased potassium conductance
• D. Parasympathetic activation

Answer: (A)

The rapid upstroke (phase 0) of the action potential in non-nodal cardiac cells is driven mainly by a large inward Na+ current through fast voltage-gated Na+ channels. The steepness of this upstroke, dV/dt, directly determines conduction velocity: a steeper slope means faster propagation. When Na+ channels become inactivated or blocked, the available inward Na+ current during phase 0 is reduced, so the upstroke is less steep and conduction velocity slows across the myocardium.

Ca2+ channel inactivation would primarily affect tissues where phase 0 relies on Ca2+ influx (such as nodal tissue), so it doesn’t explain the slowed upstroke in non-nodal cells. Increased potassium conductance tends to alter repolarization and resting potential rather than the rapid depolarization phase. Parasympathetic activation mainly modulates conduction in atrial tissue and the AV node through other pathways and does not specifically reduce the phase 0 slope in non-nodal myocardium.

So the mechanism that best accounts for decreased phase 0 slope leading to slower conduction in non-nodal cells is reduced inward Na+ current from Na+ channel inactivation or blockade.