Under normal physiologic conditions, the dominant pacemaker cells of the heart lie within the
sinoatrial node; in adults, these pacemaker cells fire at rates between 60 to 100 beats per minute
(i.e., faster than cells in any other cardiac region, See Figure 3). Even at rest, modulation by the
autonomic nervous system dominates, with the primary drive from the parasympathetics; at rest or
during sleep, the sinoatrial nodal rate decreases to about 75 beats per minute (bpm) or even slower.
In addition to pacemaker cells of the sinoatrial node, other cells within the conduction system are
capable of developing autorhythmicity, specifically those within the atrioventricular node (junction
region) and His-Purkinje system. Yet, rhythms generated within these cells are in a much lower range
(25 to 55 bpm), hence not altering the intrinsic atrial rates (Figure 2). These lower rate rhythms
are commonly referred to as ventricular escape rhythms and are important for patient survival, since
they maintain some degree of cardiac output in situations when the sinoatrial and/or atrioventricular
nodes are functioning inappropriately (e.g., in a patient with atrial fibrillation). Note that the
various populations of pacemaker myocytes (i.e., in the sinoatrial and atrioventricular nodes) elicit
so-called slow type action potentials (slow response action potential; see below).