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Hello everyone,

According to FA, Phase 4 of pacemaker action potential is called "slow diastolic depolarization" whereas Phase 0 is called "upstroke".
My question is are these "other names" for the phases or just an explanation of what happen during each?

Today while doing UWorld, I stumbled upon this question which I first thought to be "very easy", however due to the descriptions provided by FA for each of the pacemaker phases, I got the question wrong.

UWorld starts by showing a graphical representation of the pacemaker action potential and then asks "Verapamil administration would most likely have which of the following effects on these cells?"

I know that Verapamil, as a calcium channel blocker, would affect Phase 0 of pacemaker action potential as ca influx is responsible for this phase. However, UWorld chooses the answer "Slowed diastolic depolarization". I've to admit that all answers provided are clearly wrong, but the confusion comes from identifying Phase 4 as "diastolic depolarization" not Phase 0, which also occurs during diastole.
 

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Diastolic deplarization :pacemaker potential is completely in diastole

The whole cardiac pacemaker potential happens during diastole.
It consists of phases 4,0,3
The slow depolarization before the upstroke is phase 4.
It uses funny currents. This is just important for automaticity of the pacemaker cells. The Funny current channels are "HCN channels"
H for hyperpolarization activated (that is the membrane potential towards the end of sinoatrial action potential when it is between -70 to - 40mV, it gets activated) and
"CN" for "cyclic nucleotide": get this: funny current generation is increased by binding of cAMP to the HCN channels. cAMP production is increased by sympathetic stimulation.
Funny current is a slow inward Na current.
After mm potential is brought to -40 mV, the L-type Ca channels open.And there is an inward flow of Ca into the cell : causing a gradual upstroke.
(these currents are slower than the fast Na currents in other depolarization phases of AP in axons etc.)
 
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