Intracellular resistance and conduction in bullfrog atrium

The present study was undertaken to study the mechanism of transmission across the junctions between cells of cardiac muscle. Two cases, one with high resistance junction and the second with low resistance junctions, were modeled mathematically and each model was analyzed to predict the ratio of the transverse to longitudinal threshold field strengths. This ratio was measured experimentally for atrial trabeculae from Rana catesbiana and agreed well with the value predicted by the model in which the resistance of the membranes at the junctions is low. It can be inferred, therefore, that the resistance at the junctions between cells of bullfrog atrial muscle is low enough to permit the flow of sufficient current from the pre- to postjunctional cell to cause excitation of the latter. One parameter required in the analysis was the space constant, lambda. This was measured with the trabecula in a volume conductor using a large planar external electrode to produce a change in the transmembrane potential and a microelectrode to measure the spatial decrement of the electrotonic potential. The measurements of electrotonic decrement in 16 trabeculae are pooled to provide an estimate of the value of the space constant in frog atrial trabeculae: lambda= 0.863 +/- 0.041 mm, s.d., n=96.