The SA node is the normal primary (leading) pacemaker (Figure 1), having a higher intrinsic firing rate, than the AV node, ventricle, etc. (Table 6)
Following SA nodal firing, the wave of depolarization spreads over the atria (Figure 2), mainly through three specialized pathways, the internodal tracts (Table 5). At the AV junctional tissue and AV node, conduction rate drops precipitously due to presence of many small fibers and numerous junctions (Table 4). It then continues at a much increased velocity through the interventricular septum via the right and left bundle branches (of His), and out into the ventricular myocardium by way of the purkinje fibers. Direct conduction from atria to ventricles is prevented by a connective tissue ring at the level of the AV valves.
AV nodal conduction delay provides time for atrial emptying to the ventricles before ventricular contraction occurs, thus increasing pumping efficiency.
Note: Conduction from the atria to the ventricles via muscle tissue is prevented by connective tissue rings, therefore it has to go through the conduction pathway. In certain pathologies the atria don't delay, eg. Wolff-Parkinson-White syndrome.
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