Baroreceptor modulation of inspiratory termination by intercostal nerve stimulation

The influence of carotid sinus baroreceptor activity on the termination of inspiration by intercostal nerve stimulation was assessed in decerebrate cats which were paralyzed, artificially ventilated and bilaterally vagotomized. The carotid sinus was vascularly isolated bilaterally so that carotid sinus pressure could be precisely controlled independent of the systemic arterial pressure. Inspiratory activity was monitored by a phrenic neurogram. The threshold current at which electrical stimulation of the T6 intercostal nerve terminated phrenic neural activity was determined by stimulating the T6 intercostal nerve at 10 different delays from the onset of phrenic discharge. An increase in mean carotid sinus pressure from 100 to 150 mm Hg decreased the threshold current required to terminate inspiration at each delay. Similarly, a decrease in mean carotid sinus pressure from 100 to 50 mm Hg increased the threshold current. Alteration of the systemic arterial pressure (independent of the carotid sinus pressure) by norepinephrine administration did not alter the threshold current. These results demonstrate that alterations in carotid sinus baroreceptor activity have a distinct effect on the mechanisms governing inspiratory termination.