Transmembrane potential changes during stimulation in a bidomain model of the myocardium

The changes in the transmembrane potential in cardiac tissue due to a point source extracellular stimulus are studied to help clarify the underlying mechanisms of cardiac stimulation and defibrillation. The present calculations employ the bidomain model of the myocardium. The tissue is considered passive and under steady-state conditions. Membrane polarization throughout the tissue is determined by the relative degrees of anisotropy in the intracellular space (the collection of the interiors of all fibers, including myoplasm and intracellular junctions) and in the extracellular space (interstitium). The results show that the intracellular space has to have an anisotropy ratio twice (or more) that of the extracellular space in order that regions of suprathreshold depolarization appear in the tissue interior