Figure 165

Schematic representation of normal activation and impulse transmission through the His-Purkinje system with final entry into ventricular myocardium. Intracellular recording electrodes are placed in the proximal Purkinje network (P1), in the Purkinje branch on the right of the diagram (P2), and in ventricular myocardium (V). The inset to the right illustrates the membrane action-potential recordings from the respective microelectrodes. The action-potential duration, and thus the effective refractory period, is longest in the more distal portion of the Purkinje branch immediately before insertion into the ventricular myocardium. Under normal conditions, the impulses within the terminal Purkinje network conduct with relatively equal velocities so as to activate the ventricular myocardium in a uniform manner. The longer duration of the effective refractory period in the terminal Purkinje fiber prevents the impulse, traversing within ventricular myocardium, from reentering the Purkinje network in the retrograde direction. The many wave fronts of excitation invading the ventricular myocardium from multiple insertions of the Purkinje network will collide in the ventricular myocardium and terminate. The net result is a homogeneous and nearly simultaneous activation of the entire ventricular myocardium within 400 msec. The electrocardiographic tracing below illustrates a normal sinus rhythm in which there is a repetitive and coordinated activation of the entire heart. One conducted sinoatrial impulse entering the ventricle from the atrioventricular node distributes over the His-Purkinje system to elicit one QRS complex indicating depolarization of the ventricular myocardium.

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