Origination and Transmission of Impulse in the Right Auricle

(1) The origination and transmission of stimulus in right atrium of the canine or rabbit heart were studied by means of intracellular microelectrode method.(2) The area showing pacemaker potential pattern was localized in a area anterolaterally to the basal part of superior vena cava. The length was about 4.0mm and the breadth was also about 4.0mm. The true pacemaker potential pattern was obtained from slender zone within this area.(3) The spread of activation in this area was more or less radially in direction and almost constant in velocity except one direction along the zone toward crista terminalis showing true pacemaker potential. In this direction, the conduction rate was rather high and it was about 0.08M./sec.(4) After reaching crista terminalis, the activation was conducted with more accelerated velocity toward postero-inferior rand of the coronary sinus orifice and atrial septum through the right and septal branches of crista termi-nalis respectively. The conduction rate was about 1.5M./sec. which was the highest in the atrial musculature.(5) In the atrial free wall, the activation coming along through the crista terminalis was transmitted to mm. pectinati for farther conduction toward the A-V ring. The atrial roof musculature was activated by stimulus coming through a nearby m. pectinatus. The conduction rate through m. pectinatus was greater than that through atrial roof but was less than that through crista terminalis.(6) By cutting the crista terminalis or injecting osmic acid solution into this musculature, the stimulus conduction to distal portions of crista terminalis was markedly delayed. It was supposed that both branches of the crista termi-nalis played the most important role in stimulus conduction in the right atrium.(7) It was supposed that the musculature in the vicinity of the A-V node was activated by the stimulus coming along through 2 routes, one through the right branch and the other through the septal branch of the crista terminalis. At an area near the boundary of septal musculature and the A-V node, the conduction rate of activation wave showed marked decrease. A-V node was verified by its typical pattern of the membrane action potential.(8) The conduction rate of the activation wave in the area close to A-V ring became progressively decreased as the ring was approached and the activation was not transmitted beyond the ring. With this progressive de-crease in the conduction rate, the membrane action potential also gradually decreased in the steepness of the sharp depolarization upstroke. It was speculated that the changes in membrane action potential were an important cause for the constant blockade of stimulus conduction between atrial and ventricular musculature.