Passive electrical properties of the atrio-ventricular node.

The passive electrical properties of the atrioventricular node and of the atrial muscle of the rabbit heart were determined. To polarize the cells, a small suction electrode was used. The space constant of the atrio-ventricular node was found to be small (430 micron) compared to other cardiac fibers. The small value of gamma in nodal cells is due to a high intracellular resistance (ra, 40.9 +/- 9 M omega/cm) which is higher than in atrial muscle (9.6 +/- 2.2 M omega/cm). The input resistance of cells of the N layer was found equals to 880 K omega while in the right atrium was 320 K omega. The time constant of nodal cells in the AN layer was 3.4 ms, in the N layer 9 ms and in the atrium 5 ms. Assuming a specific membrane capacity (Cm) of nodal and atrial fibers of 1 micron F/cm2, Rm was found equals to 9.000 omega cm2 in N layer. 3.400 omega cm2 in AN layer and 3.800 omega cm2 in the right atrium. Acetylcholine (5 microgram/ml) reduced the space constant of the atrio-ventricular node by 38% and the time constant of nodal cells by 33%. The delay of impulse conduction in the A-V node semms then related to a high intracellular resistance along the pathway of conduction.