The study demonstrates that atrial excitation is interrupted at the A-V ring due to progressive decreases in depolarization rate and amplitude in the adjacent musculature.
The nature of the spread of excitation of atrial musculature in the very vicinity of the A-V ring was studied by means of intracellular microelectrode. The records obtained from this region showed progressive decreases in depolarization rate and amplitude of the depolarization upstroke as the A-V ring was approached. These 2 factors, the decreases in the rate and amplitude, will give a less effect as a stimulus.The shape of the membrane action potential recorded from the cells in this region was similar with that from the A-V node and the diastolic slow depolarization was often recorded from a cell group localized in the region posteriorly to the coronary sinus orifice.The QRS complex of the unipolar lead electrogram led from the same point where the microelectrode was plunged was compared with the depolarization upstroke of the membrane action potential. The onset of the sharpest downward deflection of the intrinsic deflection was nearly synchronous with the onset of the sharp upstroke of the membrane action potential. The small initial slow component of the membrane action potential was surmised to reflect the potential change extrinsic to the cell penetrated.
Maretsugu Horiba (1965) studied this question.