Factors Affecting the Recovery from Diving Bradycardia in the Frog

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Abstract
1. Recovery from diving bradycardia is not prevented by allowing a frog to surface into nitrogen or excess carbon dioxide. 2. Artificial respiration performed during submersion causes an immediate increase in heart rate whether the lungs are filled with air or nitrogen. The initial rapid increase in heart rate is prevented by bilateral vagotomy but is unaffected by section of the sympathetic connexion to the heart. 3. Spike activity in the pulmonary vagus increases as the lungs are filled. A burst of activity frequently occurs in the cardiac vagus during this period. The heart rate increases as the cardiac vagus becomes quiet. Activity continues in the pulmonary branch until deflation of the lungs. 4. The suggestion is made that the initial rapid increase in heart rate following artificial respiration or on surfacing is due to a decrease in parasympathetic inhibitory activity, by an interaction from either proprioceptors in the lungs or presso-receptors in the walls of the atria or veins.

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