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The action of acetylcholine on atrioventricular nodal fibers has been studied with the aid of intracellular microelectrodes. It has been found that during total atrioventricular dissociation no block of conduction occurs at the junction between the A-V node and the bundle of His or within fibers of the atrium. Failure of impulse transmission occurs in fibers in the atrial margin of the A-V node. The application of acetylcholine produces characteristic changes in the action potentials of fibers in the atrial margin of the A-V node: depolarization becomes slow, amplitude falls and notching and slurring appear in the upstrokes. These observations support the concept that transmission of excitation through the A-V node is decremental and that the excitatory process in nodal fibers consists of more than one excitatory event.
Cranefield et al. (Thu,) studied this question.