The sinoatrial node (SAN) is a cluster of cells in the right atrium of the heart which spontaneously generate electrical impulses to set the heart’s rate and rhythm for pumping blood to the body. The individual cell (or even group of cells) which produce the very first electrical impulse within the SAN is known to change based on the extent of sympathetic or parasympathetic tone. This physical shift in the leading pacemaker site allows the heart to adapt to external inputs while maintaining its functional integrity. Still, it is not fully understand how and why the SAN dynamically shifts its leading pacemaker location. We are utilizing new experimental and analysis techniques to close the gaps of knowledge which exist between the structure of the SAN and its pacemaking function in various mammalian atrial tissues (in terms of both species and health status). The locational shifts of pacemaking regions are studied in depth for their distribution of pacemaker properties and density of parasympathetic and sympathetic nerves and receptors.