Temporal changes in effectiveness of a constant inspiratory-terminating vagal stimulus

When an electrical stimulus is applied to the cervical vagus during inspiration it results in earlier termination of that phase. We investigated the temporal behavior of the effect of such a stimulus in the interval between its application and inspiratory termination to gain insight into the central processing of these signals. In pentobarbital-anesthetized cats we determined the threshold for inspiratory termination by delivering stimulus trains of constant duration to one vagus nerve at different times after inspiratory onset. The threshold frequency was obtained when the other nerve was unstimulated and when it was stimulated using trains of constant frequency (conditioning stimulus) beginning soon after inspiratory onset. The difference in threshold between the two test indicated the effectiveness of the conditioning stimulus in reducing threshold for inspiratory termination. The effectiveness of the conditioning stimulus increased progressively, reached a peak value at approximately 0.8 s, and progressively declined thereafter. This behavior indicates substantial integrative and accommodative processing of inspiratory terminating vagal inputs.