Under vagal activation, what happens to funny channels and calcium channels, and what is the net effect on pacemaker slope?

• A. Decreases funny channels and opens Ca++ channels later, slope of phase 4 decreases
• B. Increases funny channels and opens Ca++ channels earlier, slope of phase 4 increases
• C. No change to funny or Ca++ channels
• D. Increases Ca++ entry with no effect on funny channels

Answer: (A)

Vagal (parasympathetic) activation slows heart rate by dampening the mechanisms that depolarize the SA nodal cell during diastole. Acetylcholine acting on M2 receptors reduces the funny current (If), which normally helps push the membrane toward threshold during diastole. It also decreases calcium entry by limiting voltage-gated Ca2+ channel activity, delaying their activation. The combination makes the diastolic depolarization slope (phase 4) shallower, so the pacemaker fires more slowly. This aligns with the idea that If is reduced and Ca2+ channel activation is delayed, producing a slower phase 4 slope.