Location of pacemaker sites in the human sinus node

Filled circles represent the location of sites under autonomic control, while the open circles are "escape" sites that are not under the control of the autonomic nervous system and are capable of generating ectopic impulses. There is a predominant clustering of all sites along the sulcus terminalis and superior vena caval (SVC)-right atrial (RA) junction in the upper posterior right atrium; a few escape sites are also located in the left atrium, below the pulmonary vein (PV). Stimulation of the parasympathetic nervous system shifts the dominant pacemaker site to a more caudal region of the sinus complex, while sympathetic stimulation shifts the site to a more cranial location.