The SA node is the normal pacemaker of the heart. The action potentials in the SA node are somewhat different from the action potentials in contractile cells. The most obvious difference is that the phase 4 resting potentials are unstable, as shown in Fig. 11. Note that the resting potential starts from its maximum value of about —60 mV, then slowly depolarizes until it reaches threshold and undergoes a regenerative action potential. On completion of the action potential, the cell again begins to depolarize and another action potential is initiated. This process occurs about 60-100 times each minute, resulting in the cardiac rhythm. The progressive depolarization of the phase 4 potential is known as the pacemaker potential. The ionic mechanism underlying the pacemaker potential is not fully understood, but it is thought to be due to an inward sodium leakage current.
Cells in the AV node also have pacemaker potentials but they are slower (only about 40 beat/min) and thus the SA node dominates with its faster rhythm. An isolated strip of Purkinje fibers will spontaneously generate action potentials with a frequency of about 25 action potentials per minute, a rate slower than that of either SA or AV nodal cells. The AV node and Purkinje fibers are called latent pacemakers because they will assume the pacemaker role should the signal from the SA node be interrupted. Atrial and ventricular cells have virtually no pacemaker activity.
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