Re: why does nodal tissue cause slow conduction in a cardiac cycle of heart?

Dear Brendan,
	That's a good question. There are two basic reasons why conduction 
velocity is slowed in the AV node. First, because of a much smaller sodium 
current relative to cells in the atria or ventricles. As you know the 
upswing of the action potential in a cardiomyocyte is brought about by an 
influx of sodium into the cell. In AV node cells, this current is markedly 
smaller thus slowing the propagation velocity of the action potential along 
the length of the cell. The second reason is due to the structure of the AV 
node. Most of the cells in the atria and ventricle are connected 
electrically which allows the action potential to move from cell to cell. 
This is done via a structures known as gap junctions which allow the 
efficient transmission of the action potential from cell to cell. Gap 
junctions are formed by another class of ion channels (distinct from sodium 
channels for example) known as connexins. In some regions of the AV node, 
however, the number of gap junctions between cells is reduced resulting in 
poor electrical coupling. For these two basic reasons conduction of 
impulses is slower in the AV node relative to other regions of the 
myocardium. Hope this helps.

Terry