Additional Evidence for High-Resistance Inter-calated Discs in the Myocardium

Parallel-fibered strips of cardiac (cat papillary) and skeletal (frog sartorius) muscles were mounted in air and action potentials were externally recorded from the middle of the strip. Electric fields, oriented longitudinally with respect to the long axis of the cells, were applied. A pair of stimulating electrodes was located at each end of the strip to test for propagation. Under conditions of blocked propagation produced by addition of isotonic sucrose solution, the cells near the middle of the strip could be tested for direct response to longitudinal fields. In long-celled skeletal muscle, the sucrose solution produced simultaneously failure of electrical and mechanical response to stimulation across the strip as well as failure of propagation; stimulation at ten times threshold failed to elicit responses. However, in short-celled cardiac muscle, the sucrose solution produced failure of propagation first; electrical and mechanical responses to longitudinal field stimulation continued without a change in threshold. From these experiments it may be concluded that transverse membranes of high resistance are present in cardiac muscle; thus, radial currents are forced to flow through each cell membrane leading to direct excitation.