Pace of Heart Contractions
why is there a slight delay in the passage of electrical
activity at the atrioventricular node?
This is to allow sufficient time for the atria to finish contraction and for
blood to flow from the atria into the ventricles. From the SA node (the
pacemaker) the impulse spreads over the atria and causes them to contract.
From the SA the impulse goes to the AV node. Here the fibers narrow, similar
to traffic trying to squeeze from four lanes down to two, and this causes the
impulse to slow down. Once the impulse has made it through to the bundle of
His, the conduction is rapid once again and the entire ventricular myocardium
undergoes depolarization and contracts simulataneously.
(From Tortora, 7th ed.)
Look up Purkinje fibers. These move the impulse through the heart quickly,
but at the node there are no such fibers and the impulse is delayed a few
hundredths of a second. This delay allows time for the atrium to force
blood into the ventriculars, before the ventriculars contract.
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Update: June 2012