Since Keith and Flack’s anatomical discovery of the Sino-Atrial Node (SAN), the primary pacemaker of the human heart, the question of how such a small SAN structure can pace the entire heart has remained for a large part unanswered. There is a paucity of information regarding physiological function of the SAN in normal and diseased human heart at the cellular, tissue, and organ levels. Recent advances in optical mapping technology have made it possible to unambiguously resolve the origin of excitation and conduction within the animal and human atrial pacemaker complexes including SAN. The combination of high-resolution optical mapping and histological structural analysis reveal that the human SAN and atrioventricular (AVN) are functionally insulated from the surrounding atrial myocardium, except for several critical conduction pathways. Indeed, the atrial pacemaker clusters including SAN and AVN require anatomical (fibrosis, fat, and blood vessels) and/or functional barriers (paucity of connexins), in order to protect them from the hyperpolarizing influence of the surrounding atrium. The presence of conduction barriers and pathways may help explain how a small cluster of pacemaker cells in the atrial pacemaker complex manages to depolarize different, widely-distributed areas of the right atria as evidenced functionally by exit points and breakthroughs. The autonomic nervous system and humoral factors can further regulate conduction through these pathways, affecting pacemaker automaticity and ultimately heart rate. Moreover, the conduction barriers and multiple pathways can form substrates for reentrant activity and thus lead to atrial flutter and fibrillation.