Genesis of cardiac arrhythmias.

Activity of Automatic Cells The normal rhythm of the mammalian heart results from spontaneous excitation of cells in the sinoatrial node. These cells possess the property of automaticity.' The transmembrane potential of working muscle fibers in the atria or ventricles demonstrates a rapid depolarization on excitation (phase 0), a period of variable duration during which the cell repolarizes (phases 1, 2, and 3), and then a stable resting potential (phase 4), which persists until the next propagated impulse arrives and causes excitation. In contrast, in cells of the sinoatrial node, repolarization is not followed by a period during which the transmembrane potential is stable. Instead, immediately after the end of repolarization the membrane potential begins to decrease slowly. This slow depolarization during phase 4 lowers the transmembrane potential toward the threshold potential, the value of transmembrane potential at which excitation occurs. If the slow depolarization attains the threshold potential, excitation occurs and the cell develops an action potential which then propagates to excite adjacent cells and, normally, the rest of the heart.2 All cells which demonstrate this slow diastolic depolarization are said to be automatic. This mechanism for spontaneous firing has been called the normal automatic mechanism to differentiate it from other