Electrical circuit of the 20 × 10 model (20 parallel chains of 10 cells each) of a cardiac muscle bundle used for determining the cross-sectional profile of longitudinal propagation velocities. The simulated bundle was symmetrical at the top and bottom, including values of Rol and Ror and the presence of two grounds. All 20 chains were stimulated intracellularly and simultaneously by the 20 stimulators at the left end of the bundle using rectangular current pulses (0.25 nA, 0.25 ms). To prevent cluttering of the diagram, the Rgj resistors are shown only for chain A. Voltage markers were placed intracellularly only in cells 1, 5 and 10 of each chain so as to limit the total number of traces to 60. The variables were: (a) the number of gj-channels placed across the longitudinal cell-to-cell junctions in each chain (Rgj), (b) the longitudinal resistance of the interstitial fluid between the parallel chains (Rol2), and (c) the bundle termination resistance at the ends of the bundle (RBT).