Excitability changes of the inspiratory "off-switch" mechanism tested by electrical stimulation in nucleus parabrachialis in the cat.

The time course of the excitability of the inspiratory "off-switch" mechanism with and without phasic vagal stretch receptor feedback has been studied in cats under light pentobarbitone anesthesia by electrical stimulation in the rostral pons using brief tetanic stimulation (300 Hz for 0.2 s). The threshold strength required just to elicit inspiratory "off-switch" was high early in inspiration and fell steeply with time. The threshold curves were steeper with than without phasic vagal feedback, and the difference reflects the phasic vagal contribution to the excitability of the inspiratory "off-switch" the absence of phasic vagal vagal feedback the time course of this threshold curve usually corresponded closely to that of the "integrated" phrenic activity at all PCO2 levels and body temperatures tested indicating that the "integrated" phrenic activity can be used as an index of the centrally generated inspiratory activity. In response to a rise in PCO2 both the rate of change of excitability of the inspiratory "off-switch" mechanism and its initial threshold level was increased. Changes in body temperature caused no change in the initial threshold but produced marked changes in the rate of rise of the "off-switch" excitability; Following an "augmented breath" the inspiratory "off-switch" threshold was markedly reduced