ABSTRACT
THE ELECTRICAL ACTIVITY OF THE HEART A distinctive feature of the generation of the heartbeat is the plasticity of the action potential (AP) and its pattern at the level of the different specialized cells within the heart (pacemaker cells, conduction system, and working myocytes). At the cellular level, the AP is generated by the coordinated actions of a large number of ion channels. The activation depends on the ion conductance activity of specifi c voltage-gated ion channels, which mediate rapid, voltage-dependent changes in ion permeability causing a change in the membrane potential. The different complements or densities of channels and pumps determine the differences in the AP characteristics within different cells populating the heart and different myocardial regions. Figure 4.1 shows the AP and the approximate time course of the depolarizing and repolarizing currents carried by the different ion channels in the working myocytes. The AP is classically divided into fi ve phases (Fig. 4.1).
