Pacemaker action potential

Question 1

Where?

Answer 1

SA and AV node

Question 2

Phase 0

Answer 2

Upstroke;
Opening of voltage-gated Ca2+ channels.
(Fast voltage gated Na+ channels are permanently inactivated because of the less negative resting voltage of these cells.)

Results in a slow conduction velocity used by the AV node to prolong transmission from atria to ventricles

Question 3

Phase 2

Answer 3

Plateau is absent

Question 4

Phase 3

Answer 4

Inactivation of the Ca2+ channels

Activation of K+ channels, so high K+ efflux

Question 5

Phase 4

Answer 5

Slow diastolic depolarization:
membrane potential spontaneously depolarizes as Na+ conductance increases (If different from Ina in phase 0 of ventricular AP) Accounts for automaticity of SA and AV nodes.

Question 6

Clinical significance of phase 4

Answer 6

The slope of phase 4 in the SA node determines HR.
ACh/Adenosine decrease the rate of diastolic depolarization and low HR, while catecholamines increase depolarization and HR.

Sympathetic stimulation increases the chance that If channel are open and thus increase HR