Right atria from excised beating dog hearts were immersed in Tyrode solution with the endocardial surface of the anterior wall exposed. Glass microelectrodes were used to impale fibers in different anatomical areas of this preparation. Fibers with action potentials similar in contour to those of ventricular Purkinje fibers were found along the caval border of the crista terminalis in the area of the posterior internodal tract. The action potential of these fibers (plateau fibers) had a resting potential of -85 to -95 mv, a sharp spiked overshoot, a long plateau phase, and inherent diastolic depolarization. The maximum velocity of the action potential upstroke of plateau fibers was consistently greater than that of simultaneously recorded fibers which lacked a plateau phase (regular fibers). Epinephrine or isoproterenol produced an increase in both rate and magnitude of diastolic depolarization of plateau fibers and on occasion converted them to true pacemakers. Acetylcholine accelerated repolarization of plateau fibers with disappearance of the plateau. Increases in extracellular concentration of potassium ions from 2.7 to 10.8 mM rendered regular atrial fibers inexcitable, but plateau fibers continued to show action potentials. Plateau atrial fibers possess several characteristics exhibited by specialized conducting and impulse generating fibers. The possibility that these fibers constitute the posterior internodal tract and function in preferential conduction of excitation to the A-V node was discussed.
ADDITIONAL KEY WORDShigh extracellular potassium concentration posterior internodal tract epinephrine diastolic depolarization acetylcholine latent pacemaker activity isoproterenol• While it is generally accepted that fibers of the sJnoatrial (S-A) node serve as the pacemakers in the normal mammalian heart, the manner in which excitation spreads from this node through the atria and to the atrioventricular (A-V) node is not completely settled. Lewis et al. (1) concluded from studies using surface electrodes that excitation spreads radially from the S-A node through a syncytium composed of homogeneous tissue. The concept of radial spread of the impulse was challenged by Eyster and Meek (2-5) who presented evidence for spread of excitation from the S-A node to the From the Department of Physiology, University of Wisconsin Medical School, Madison, Wisconsin 53706.This investigation was supported in part by grants from the Wisconsin Heart Association and U. S. Public Health Service Grants 5-T1-HE53705-08 and 2-T1-HE5540-06.Accepted for publication June 27, 1968.CiratlM 'on esearch, Vol. XXIII, September 1968 A-V node along a preferential pathway. Also evidence for rapid spread of the impulse from the right to the left atrium was given by Bachmann (6) who described a special bundle of tissue linking the atria ventrally. Both anatomical and electrophysiological evidence has accumulated to support the contentions of Eyster and Meek for a preferential internodal pathway. In a recent review, Robb and Petri (7...