The beating of our heart is the coordinated action of more than a billion muscle cells. Most of the time, only the muscle cells from the larger heart chambers contract and relax. But when the heart needs to work harder, it relies on back-up from the atrial muscle cells deep within the heart’s smaller chambers (atria). The health of these atrial cells relies on specific concentrations of cellular calcium. Now, for the first time, scientists have produced a mathematical model of calcium activity within the atrial heart cell. Rüdiger Thul, an author of the study, said, "This new model provides clinically relevant insights into the initiation and propagation of sub-cellular calcium signals. Thus, for the first time we can manipulate cellular properties throughout a whole atrial muscle cell in order to deduce which conditions give rise to abnormalities. This has the potential to point to new treatments for heart disease and irregular heart beat."