Electrical impedance of cardiac muscle.

I T HAS long been held that transmission of excitation from cell to cell in cardiac muscle is effected by local-circuit current. Recent electron microscopy" has shown that the myocardium is not a morphological syncytium; the intercalated discs are actually cell boundaries. If the resistances of the disc membranes are high, then local-circuit current transmission is unlikely, and the major possibility remaining would be that of chemical junction transmission. Several lines of evidence have led to a questioning of the validity of the electrical theory of transmission." The present report deals with an effort to determine the resistance and the capacitance of the transverse membranes by measurement of tissue impedance in the direction of fiber orientation, before and after reduction of the interspace ion concentration. This work is an extension, with impedance measurements, of a previous report which presented D.C. resistance measurements. The passive current flow paths through a muscle strip may be lumped into two major parallel paths: (a) the path through the extracellular fluid (ecf), and (b) the path through the cell membranes and intracellular fluid (icf). The resistance of the extracellular path depends on the interspace volume relative to the cell volume, and the resistance of the cellular path depends on the number and on the resistance of the cell membranes and on the myoplasmic resistance.