Abstract
In a preliminary study undertaken to ascertain the changes produced in excitability of the excised turtle ventricle by rest and activity, 11 hearts were used from Chelydra serpentina and Emys sp. The Lapicque potentiometer and chronaximeter were used. The stimuli were applied by way of non-polarizable electrodes.
The results, confirming and extending those previously reported by Garrey and Ashman 1 and by Ashman and Hafkesbring, 2 were variable, but certain conclusions clearly emerge.
1. Excitability, expressed either as rheobase, chronaxie or least time is greatly influenced by activity.
2. In hearts in good physiological condition when excised (3 examples) there was a treppe in excitability, comparable to the treppe in contractility, and the treppe was demonstrable both in rheobase and “least time.” Our evidence shows, although less clearly, that there was a treppe in chronaxie. Expressed in terms of quantity of electrical energy required to excite the tissue, activity definitely increased the excitability.
3. In 6 other hearts, also in good or fair physiological condition, it was impossible to demonstrate a definite treppe in rheobase, yet in these the treppe in least time and probably, therefore, in chronaxie was definite or striking. Compared with the resting heart, after several responses the least effective duration of a current of constant strength may be decreased by 20 to 25%.
4. In 2 ventricles the manifestations of fatigue completely masked the treppe. In these the rheobase was markedly raised by activity, although the chronaxia was shortened. In terms of the quantity of energy required to excite the tissue, activity here decreased the excitability. This again illustrates a point previously stressed, 3 i e., that the chronaxie is not necessarily a measure of excitability.
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