[Automatism: an intrinsic property of cardiac tissues]

Automaticity is an intrinsic property of some types of cardiac tissues, like the nodes and the conducting fibers. Automatic cells are characterized by the appearance, during diastole, of a slow depolarization which is capable of reaching threshold and generate an action potential. This depolarization is the so-called slow diastolic depolarization, or pacemaker potential. Tissues with automaticity do not have the same intrinsic firing frequency, which determines the existence of a true pacemaker; normally this role is assumed by the sinus node and several latent, or subsidiary pacemakers. The latter do not give any manifestation, because, since they have a slower firing frequency than that of the true pacemaker, the impulses generated by the latter will activate them before they can reach threshold. The ionic mechanisms that originate the pacemaker potential are just some of the processes that constitute the electrophysiological properties of the cardiac cell membranes. The mechanism includes the slow inactivation of an outward current combined with a constant inward current. Recently published evidence suggests that the pacemaker potential could be the resultant of an inward current that is slowly activated during the final part of the repolarization, but these data are still controversial and difficult to interpret. The factors that regulate heart rate modify the electrophysiological characteristics of the membrane by means of three major mechanisms: the slope of the pacemaker potential, the voltage of the threshold potential and, the maximum diastolic potential. Some of the most important factors that modulate heart rate include the neurotransmitters of the automatic nervous system, the plasmatic levels of some ions, like potassium and calcium, and changes in the intrinsic properties of the membrane, like the influence of heart rate on the activity of the sodium pump.