SA node question, please help

[OphthLover]

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FirstAid states that SA node is a dominant pacemaker because of its slow phase of upstroke. But I remember doing a question from UWorld, which stated that SA node has the fastest firing rate among all conductive cells (perkinje, atria, ventricle, AV node, etc.).

Wouldn't these two statements contradict each other? How can SA node cells have the fastest firing rate while having a slow upstroke? Am I missing something here? Please provide me with some insight for those of you who were able to understand this concept. Thank you.

 

[AZ7]

It could mean slow phase of upstroke compared to the upstroke seen in AP of other myocytes ( Compare Phase 0 of Myocardial AP and Pacemaker AP) . Myocardial AP would be steeper for a rapid response, wheres Nodal AP would be gradual to allow for relaxation and adequate filling time.
Even the AV node has a similar slope of AP, but much slower i guess

 

[StuartLiu]

The SA node is the dominant pacemaker because it has the highest firing rate compared to other tissues with automaticity (AV note, Purkinje fibers...etc).
On the other hand, what make these pacemakers cells unique is that these cells have inward current (funny current, If) which make them capable of reaching threshold by themselves. After reaching threshold, pacemaker cells have Ca channel but NOT fast Na channel. Therefore they have slow upstroke (They don't have fast sodium channels).
Hope this answers your question.

 

[worldbeater]

The SA node is the dominant pacemaker because it has the highest firing rate compared to other tissues with automaticity (AV note, Purkinje fibers...etc).
On the other hand, what make these pacemakers cells unique is that these cells have inward current (funny current, If) which make them capable of reaching threshold by themselves. After reaching threshold, pacemaker cells have Ca channel but NOT fast Na channel. Therefore they have slow upstroke (They don't have fast sodium channels).
Hope this answers your question.

Question. Wouldn't the SA node also have the highest firing rate because it's higher up in the atrium (near the SVC - World Q) compared to the other components you mentioned, and the depolarization charge from that point and moves downwards?

SA node is the atrium -> atrium only uses calcium to depolarize (one of two areas on the body that does not use sodium to depolarize, other is thalamus) -> so this slow upstroke makes sense.

 

[zhopv10]

Slow upstroke but the rate is fastest, ya I believe that is correct, it slows at the AV node giving enough time for the ventricles to fill. Also, careful saying the atrium uses calcium, the pacemaker cells (AV, SA, etc) use calcium the muscle fibers in the atrium use sodium for their upstroke I believe.


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[worldbeater]

Also, careful saying the atrium uses calcium, the pacemaker cells (AV, SA, etc) use calcium the muscle fibers in the atrium use sodium for their upstroke I believe.

Yeah I've had this debate before on SDN, your technically probably correct and will use that during the clinical years..but for any practice question that involves atrium and depolarization (and if they manipulate it with any drugs), I mark calcium and get it right >90% of the time, so I am going to go in that direction.

 

[worldbeater]

it slows at the AV node giving enough time for the ventricles to fill.

Yeah, AV node is the "fail safe" of the heart, it protects the ventricle from any abnormal physiology like arrhythmias. That's why ventricular arrhythmias are more dangerous than atrial arrhythmias.