CHAPTER 10 – Cardiac Action Potentials

This chapter reviews the physiological characteristics of the cardiac action potentials (AP), with emphasis on the major currents responsible for the various components or phases of the APs, and some pathophysiology of cardiac APs. APs in the heart are generated by the complex time-dependent interaction of several currents carried primarily by Na + , K + , and Ca 2+ ions. Some cardiac cells display automaticity because of the presence of a cyclical spontaneous depolarization called the pacemaker potential. Cells of the sinoatrial node normally exhibit the fastest spontaneous depolarization of automatic cells in the heart and are therefore, called the normal pacemaker cells. Automaticity in nodal cells is caused by the interaction of several currents, including a hyperpolarization-activated cation current, a small background Na + current, the delayed rectifier K + current, and Ca 2+ currents. The sympathetic and parasympathetic nerves play important and opposite roles in regulating the force of myocardial contraction and heart rate. Thus, sympathetic nerves stimulate the Ca 2+ current, which enhances the force of contraction of the heart. Parasympathetic nerves inhibit the Ca 2+ current, which decreases the force of contraction of the heart. Sympathetic nerves enhance the rate of spontaneous phase 4 depolarization in the sinoatrial node, resulting in an increased heart rate. Parasympathetic nerves decrease the rate of phase 4 depolarization, and also hyperpolarize sinoatrial node cells, resulting in a decreased heart rate.