|MadSci Network: Anatomy|
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
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