AV conduction delay

[aspiringmd1015]

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was going through cvs physiology, looks like my book isnt explaining it that well, so i understand why SA node is the pacemaker, bc of a faster rate of phase 4 depolarization(inward na current), but because the AV node has a normal delay to allow filling to occur and corresponds to the P-R interval, is this delay because of the slower phase 4 of depolarization(which makes the SA node the pacemaker and not the AV node) or is the av node physiologic delay because of the rate of rise of phase 0(inward Calcium) because i think my book mentions the delay is because of phase 0.

 

[thehundredthone]

See if this helps: http://www.cvphysiology.com/Arrhythmias/A003.htm

 

[Transposony]

The delay at AV node is mainly due to small diameter of the conducting fibers. Think of it as a "Bottleneck".
Sympathetic / Vagal tone helps control the rate.
Under normal circumstances AV (and SA) node is predominantly under parasympathetic control.