The normal electrical conduction in the heart allows the impulse that is generated by the sinoatrial node (SA node) of the heart to be propagated to, and stimulate, the myocardium. It is the ordered, rhythmic stimulation of the myocardium during the cardiac cycle that allows efficient contraction of the heart. Electrical signals arising in the SA node (located in the right atrium) stimulate the atria to contract and travel to the atrioventricular node (AV node), which is located in the interatrial septum. After a delay, the stimulus diverges and is conducted through the left and right bundle of His to the respective Purkinje fibers for each side of the heart, as well as to the endocardium at the apex of the heart, then finally to the ventricular epicardium.
Purkinje fibers differ in numerous ways from working cardiomyocytes. In general, Purkinje cells are larger than cardiomyocytes and are distinguishable histologically. The transverse tubules that are present in cardiomyocytes are absent in Purkinje cells. The primary role of Purkinje cells is rapid conduction of the electrical impulse. For this reason, Purkinje cells have fewer myofibrils and therefore they appear lighter than cardiomyocytes under the microscope. On the other hand, the amount of glycogen in Purkinje fiber is much higher than that in cardiomyocytes. Glycogen can be metabolized anaerobically which may make Purkinje cells more resistance to hypoxia than working myocardial cells, although some evidence suggests that the opposite is true.
The conduction velocity of electrical impulses is much higher in Purkinje fibers (2–3 m/s) than in cardiomyocytes (0.3–0.4 m/s). The fast propagation is partially due to the differences in the gap junctions of these cells. The amount of Cx40, a connexin that forms high conductance channels, is at least three fold greater in Purkinje fibers than in cardiomyocytes. As part of the conduction system, the Purkinje cells have potential automaticity, which is normally suppressed by the faster pacemaker activity of the sinoatrial node. Cardiomyocytes usually do not have automaticity.
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