Firewalls in Atrial Myocytes

Atrial myocoytes play a prominent role in the generation of heart beats. Their contraction is controlled by Calcium signals that emerge at the cellular periphery and then proceed centripetally to engage the force-generating myofilaments. Experiments have demonstrated that these initial signals need to overcome a barrier just below the cell membrane before they move inward. Since atrial myoctes lack transverse tubules that transmit external signals to the cell interior as e.g. in ventricular myocytes, such a firewall represents a crucial determinant of atrial dynamics. For instances, it allows atrial myocytes to fine tune their responses to a wide range of vital stimuli. Here, we present a computationally advantageous model to investigate the mechanisms that give rise to these graded centripetal signals. Our framework takes into account the three dimensional organisation of atrial myocytes, especially the spatially restricted release of Calcium from internal storage compartments. We employ a fire-diffuse-fire (FDF) model to examine the spatio-temporal patterns and to probe the dependence of wave propagation on physiologically relevant parameters. Mimicking an excitable medium, the FDF approach reflects the significance of noise in intracellullar Calcium dynamics. The explicit construction of the corresponding Green's function allows for a detailed analysis.