Electrical Events in Specialized Muscle Fibers of a Mammalian Right Atrium

The electrical activities of the S-A node regions of the rabbits' and canine hearts were studied by means of the microelectrode technique.The entire area suspected to be the S-A node region was explored at intervals of 1mm.(1) The membrane action potential from the S-A node region was characterized by the presence of the slow diastolic depolarization.The features of the membrane action potentials from the S-A node region could be classified into 2 types.One type is characterized by a moderately sloped slow diastolic depolarization, a gentle upstroke of the action potential, and rather small membrane resting and action potentials. The transition from the slow diastolic depolarization to the upstroke of the action potential was smooth. (refered to as Type A)The other type is characterized by a very slightly sloped slow diastolic depolarization, a steep upstroke of the action potential, and by larger membrane resting and action potentials than those of Type A.The transition from the slow diastolic depolarization to the upstroke of the action potential was abrupt. (refered to as Type B)(2) Type A tracings were recorded from a narrow area in the venous wall. This area was about 4mm. in length and lay parallel to the crista terminalis at a distance of from 1 to 1.5mm.Type B tracings were recorded from the area surrounding the region exhibiting the Type A potential, but not necessarily from the area of the S-A ring bundle as described by Hoffman et al.The S-A ring bundle could not be observed during the present experiment.The true pacemaker was located in some part of the area exhibiting the Type A potential.(3) The conduction rate of excitation in the S-A node region in general was observed to be late (about 0.02M./sec.).The sinus activation was transmitted rapidly in only one direction from the true pacemaker to the crista terminalis (about 0.08M./sec.).