The nature of electrical propagation in cardiac muscle

It has long been appreciated that cardiac muscle is composed of individual cells connected by low-resistance connections, but most concepts of cardiac impulse conduction have been based on a simplified model of propagation assuming continuously uniform intracellular resistivity in the direction of propagation. In this article we describe the development of the application of the theory of continuous media to propagation in cardiac muscle and review some of the successes achieved with this theory. New evidence is cited that propagation in cardiac muscle often displays a discontinuous nature. We consider the hypothesis that this previously unrecognized aspect of propagation can be explained by discontinuities in axial resistance related to known structural complexities of cardiac muscle. A major implication is that the combination of discontinuities of effective axial resistivity at several size levels can produce a wide variety of complex abnormalities of propagation, including most currently known cardiac conduction disturbances that have been considered to require spatial nonuniformity of membrane properties.