system
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human
What electrically isolates the top of the heart (atria) from the bottom (ventricles)?

I am an EMT and I have an interest in 12 lead EKGs.  I know the basics about cardiac conduction (SA node generates a p wave that propagates through the atria and arrives at the AV node.  There is a delay in the AV node and the QRS complex propagates through the ventricles via the bundle of His, and Purkinje fibers.  

I also know there are literally hundreds of arrhythmias and many are due to another conduction path from the top to the bottom of the heart, bypassing the AV node (ex. Wolf-Parkinson-White) and that allows atrial p waves to have a back door into the ventricles (some times fatally).

So thinking of this electrically, there is supposed to be a high impedance (in ohms) from the top to the bottom of the heart, with the exception of the AV node.   My question is, what structure in the heart provides the high electrical impedance and what is the morphology that causes a spot or path to have a low electrical impedance?
gpt
It's not so much that there is a specific structure that presents high impedance to the electrical signals, as it is that cardiac tissue (like any other tissue) presents high impedance *but* cardiac tissue has specific conductivity pathways that it permits. The conductive pathways arise from the gap junction in cardiac intercalated discs, which permits action potentials to propagate from one cardiomyocyte to another. 

Which makes sense, because even outside the atrioventricular wall, cardiac conduction is not omnidirectional; it proceeds in a very specific order at very specific speeds, which are controlled by the arrangement of cardiomyocytes, their interconnections through intercalated discs, and by variations in physiology of different types of cardiomyocytes.