Ventricular depolarization intrinsic

The mechanical activity of the heart (contraction of the atria and ventricles) occurs as a result of the electrical activity of the heart. The heart possesses an intrinsic electrical conduction system (Fig. 6-1). Normal myocardial contraction cannot occur without proper and normal function of the heart s electrical conduction system. Electrical depolarization of the atria results in atrial contraction, and ventricular depolarization is... 

Fig. 3.11 Intrinsic cardiac depolarization can be recorded at the pacing lead electrode and measured by the pacemaker. This signal is called an electrogram. In this example, the electrogram is measured from the ventricular lead and is caused by ventricular depolarization. The signal has an amplitude of 8mV. This signal would not be seen by the pacemaker if the sensitivity is set to 10 mV. However, by lowering the sensitivity value to 5 mV (making the pacemaker more sensitive), the intrinsic depolarization would be seen and pacemaker timing cycles would be reset when appropriate.

Myocytes within the sinoatrial node possess the most rapid intrinsic rate of automaticity therefore, the sinoatrial node serves as the normal pacemaker of the heart. Specialized cells within the atria, atrioventricular (A-V) node, and His-Purkinje system are capable of spontaneous depolarization, albeit at a slower rate. The more rapid rate of depolarization of the sinoatrial nodal cells normally suppresses all of the other cells with the potential for automaticity. The other cells will become pacemakers when their own intrinsic rate of depolarization becomes greater than that of the sinoatrial node or when the pacemaker cells within the sinoatrial node are depressed. When impulses fail to conduct across the A-V node to excite the ventricular myocardium (heart... 

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