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

• A. Na+ channel inactivation or blockade
• B. Ca++ channel inactivation or blockade
• C. K+ channel opening
• D. Increased sodium current

Answer: (B)

In AV nodal cells, the phase 0 upstroke is driven mainly by calcium entry through L-type calcium channels, not by sodium. The slope of this upstroke sets how fast the impulse spreads through the node. If calcium channels inactivate or are blocked, the inward Ca2+ current during phase 0 is reduced, the upstroke becomes slower, and conduction through the AV node slows. This is why decreased phase 0 slope decreases AV nodal conduction velocity.

Sodium channel activity is not the primary determinant of the AV nodal upstroke, so Na+ channel inactivation or blockade has less effect on AV nodal conduction. Opening potassium channels mainly affects repolarization, and increasing sodium current would speed up the upstroke rather than slow it.