In the normal heart during sinus rhythm, impulse formation is initiated in the sinoatrial (SA) node (Figure 1.1), and then spreads as a wave of depolarization over the atria until it reaches the atrioventricular (AV) node, which is near the junction of the interatrial and interventricular septa.
The AV node represents the sole pathway for conducting the impulse from the atria to the ventricles, except when it is bypassed by abnormal congenital pathways. Its unique purpose is to slow the rate of impulse conduction to give the atria time to finish emptying of blood and the ventricles time to finish filling with blood. This is necessary because mechanical contraction of cardiac muscle is normally slower than the rapid process of electrical depolarization (Figure 1.2).
The common bundle of His next conducts the impulse through the superior ventricular septum and then quickly divides into the right and left bundle branch (Figure 1.1). Perhaps because the left ventricle is bigger than the right ventricle, the left bundle splits into two hemibundles, one running anteriorly and superiorly out of the page toward the reader, and the other running posteriorly and inferiorly into the page, as if away from the reader.
Finally, the bundle branches divide numerous times into Purkinje fibers, which are the final pathway for conduction of the impulse to ventricular muscle. Once ventricular muscle is stimulated by the impulse traveling down the Purkinje fibers, it depolarizes outwardly from endocardium to epicardium.
Electrical impulses are conducted much more rapidly through the specialized conduction system of the heart described in the previous paragraphs than through typical cardiac muscle. This allows the electrical impulse to reach almost all ventricular muscles nearly simultaneously, thus, allowing for
coordinated contraction of the ventricles. There is a small, but important, difference in the sequence of activation of various portions of the ventricles, which will be discussed later.
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