ELECTRICAL ACTIVITY OF THE ATRIOVENTRICULAR CONDUCTING TISSUE OF THE TOAD, STUDIED BY A MINUTE SUCTION ELECTRODE

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Abstract
By means of an improved type of suction electrode with a fine tip, the characteristic action potential (A-V potential) was recorded from the A-V groove of the heart of toad. The features of A- V potential as distinct from those of the atrium and ventricle were as follows the rising velocity of upstroke was low, its amplitude was small (5 mV on the average as contrasted to 40 mV in the artrial or ventricular action potential) and the peak of spike was rounded. The A-V groove was not everywhere conductile, but the conduction was confined in 2, well circumscribed areas (about 1 mm x 1 mm in size) at the dorsal and ventral parts of the A-V groove. They were behind the A-V valve and in close proximity to Bidder''s ganglia. In the majority of cases, the dorsal pathway played a dominant role in the A-V conduction. When this pathway was artificially suppressed by septal nerve stimulation and topical application of acetyl-chloine the ventral pathway took over the role of A-V conduction. By stepwise shifting of the exploring suction electrode carefully from the atrium to the ventricle, it was possible to trace the temporal sequence of excitation along the A-V pathway. The propagation of excitation in the pathway was found to be uniform and the conduction velocity was surprisingly low (ca. 2 mm/sec at 15[degree]C). The normal A-V delay is thus due to an extremely slow but uniform conduction in the A-V conducting pathway. In antidromic conduction, the V-A conduction was also found to be uniform but its velocity was usually lower than in orthodromic conduction. The uniform conduction through the pathway was well maintained even when the A-V interval was markedly prolonged at low temperature. Increase in driving rate, however, changed the uniform conduction in the pathway into a decre-mental conduction. This is considered to be due to a slow recovery of excitability (long refractoriness) of fibers in the A-V pathway.

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