THE GENESIS OF THE STEP OR NOTCH ON THE UPSTROKE OF THE ACTION POTENTIAL OBTAINED FROM THE ATRIOVENTRICULAR NODE

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Abstract
1. The level of the notch or the step on the upstroke of the action potential recorded through a microelectrode from the rabbit atrioventricular node was not constant. The change of this part induced by changing the extracellular potassium or sodium concentration did not show any simple relationship. 2. Stimulation rate was increased in normal Tyrode solution, in solution with a higher potassium concentration and in Tyrode solution after addition of acetylcholine. Under all of these conditions the action potential was variously decomposed. The fragmentation was so marked that both of two mechanisms, i. e. local graded response and electrotonic spread of the action potential of adjacent muscle fibers, must be considered. 3. A peculiar finding occasionally observed in the atrioventricular node was mentioned. Only the chief part of the action potential showed a successive delay, the firing level changed successively, but the step part remained unchanged. This finding suggested more clearly that the nature of the small potentials here is the electrotonic spread of the action potential stopped by a small area of block or of refractory state and that the excitation waves made a detour to reach the site of exploration. 4. The reason why only the nodal potential shows special features under such conditions is probably due to the sparsity of muscle fibers in the atrioventricular node. 5. The above-mentioned mechanism refers to relatively abnormal states of the atrioventricular node in which its action potential is apt to show the steplike potential.