| 1. |
Stimulation to left and right vagi caused an almost equal amount of inhibitory, and occasionally excitatory, effects on pacemaker activity. Both inhibitory and excitatory effects were abolished by atropine. Vagal stimulation hyperpolarized the resting membrane potential of pacemaker fibers in the sino-atrial valve, but did not change their action potential profile.
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| 2. |
The atrial action potential showed a prominent decrease in the action potential amplitude and duration in response to vagal stimulation. The atrial region surrounding the sino-atrial valve was more sensitive to right vagal stimulation.
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| 3. |
The fibers in the atrio-ventricular ring muscle were less sensitive to vagal stimulation than the atrial fibers. Some fibers showed a decrease in the action potential amplitude and duration by vagal stimulation, and other fibers showed a decrease in the amplitude, but a prolongation of the duration as the result of a slowing of the rate of upstroke. The atrial-ventricular conduction delay or block by vagal stimulation may depend on these properties of the action potential of the atrio-ventricular ring muscle.
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| 4. |
The sino-atrial conduction block is explained by the fact that the atrial fibers are more sensitive to vagal stimulation than pacemaker fibers.
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| 5. |
The possible pathways for the sino-ventricular conduction during vagal stimulation are discussed.
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