Electrical Events in Specialized Muscle Fibers of a Mammalian Right Atrium: Part II. A-V Node

Transmembrane potentials and the process of excitation propagation of the rabbit's and canine heart in the A-V node and its vicinity were studied by means of the microelectrode method. (1) In the vicinity of the A-V node, no action potential showing the slow diastolic depolarization was obtained. The configuration of the action potential recorded from this area exhibited a tracing similar to that of the action potential recorded from the atrial ordinary muscle. (2) It was inferred that the right and septal branches of the crista terminalis extended into the atrial tissue near the vicinity of the A-V node. In these areas the arrival times of the excitation through both branches of the crista terminalis were almost the same. (3) It was impossible to clarify whether the excitation through the crista terminalis activated the A-V node or whether it was activated by the excitation through some specialized muscle, in which the conduction rate was slower than that of the crista terminalis. (4) The configuration of the transmembrane potential of the A-V node was peculiar. The transmembrane potential recorded from the atrial tissue in the A-V node adjacent to the S-A node was characterized by a slow diastolic depolarization, a low resting potential, a very gentle upstroke of the depolarization phase and a lack of overshoot and plateau. When the penetration approached the tissue close to the His bundle, the action potential began to show a plateau and a rather rapid upstroke of the depolarization phase. (5) A marked delay was observed in the area between the atrial ordinary muscle close to the A-V node and the tissue in the A-V node adjacent to the S-A node. The conduction rate in this area was 0.03∼0.025M./sec.