W. Jonathan Lederer
Different cardiac cells serve different and very specialized functions, but all are electrically active. The heart's electrical signal normally originates in a group of cells high in the right atrium that depolarize spontaneously; it then spreads throughout the heart from cell to cell (Fig. 21-1). As this action potential propagates through the heart—sometimes carried by cells that form specialized conducting pathways and sometimes by the very cells that generate the force of contraction—it assumes different appearances within the different cardiac cells (Fig. 21-2). Based on the speed of the upstroke, we can characterize action potentials as either slow (sinoatrial and atrioventricular nodes) or fast (atrial myocytes, Purkinje fibers, and ventricular myocytes).
FIGURE 21-1 Conduction pathways through the heart. A section through the long axis of the heart is shown.
FIGURE 21-2 Cardiac action potentials. The distinctive shapes of action potentials at five sites along the spread of excitation are shown.
Because the excitation of cardiac myocytes triggers contraction—a process called excitation-contraction coupling (see p. 229)—the propagation of action potentials must be carefully timed to synchronize ventricular contraction and thereby optimize the ejection of blood. This chapter focuses on the membrane currents responsible for the generation and transmission of action potentials in heart tissue. We also examine how to record the heart's electrical flow by placing electrodes on the surface of the body to create one of the simplest and yet one of the most useful diagnostic tools available to the clinician—the electrocardiogram.