ACTIVITY OF MEDULLARY RESPIRATORY NEURONS DURING VENTILATOR-INDUCED APNEA IN SLEEP AND WAKEFULNESS

Mechanical ventilation of cats in sleep and wakefulness causes apnea, often within two to three cycles of the ventilator, We recorded 137 me dullary respiratory neurons in four adult cats during eupnea and durin g apnea caused by mechanical ventilation. We hypothesized that the res idual activity of respiratory neurons during apnea might reveal its ca use(s). The results showed that residual activity depended on 1) the a mount of nonrespiratory inputs to the cell (cells with more nonrespira tory inputs had greater amounts of residual activity); 2) the cell typ e (expiratory cells had more residual activity than inspiratory cells) ; and 3) the state of consciousness (more residual activity in wakeful ness and rapid-eye-movement sleep than in non-rapid-eye-movement sleep ). None of the cells showed an activation during ventilation that coul d explain the apnea. Residual activity of approximately one-half of th e cells was modulated in phase with the ventilator. The strength of th is modulation was quantified by using an effect-size statistic and was found to be weak. The patterns of modulation did not support the idea that mechanoreceptors excite some respiratory cells that, in turn, in hibit others. Indeed, most cells, inspiratory and expiratory, discharg ed during the deflation-inflation transition of ventilation. Residual activity failed to reveal the cause of apnea but showed that during ap nea respiratory neurons act as if they were disinhibited and disfacili tated.