PHRENIC MOTONEURON DISCHARGE DURING SUSTAINED INSPIRATORY RESISTIVE LOADING

I determined whether prolonged inspiratory resistive loading (IRL) aff ects phrenic motoneuron discharge, independent of changes in chemical drive. In seven decerebrate spontaneously breathing cats, the discharg e pat terns of eight phrenic motoneurons from filaments of one phrenic nerve were monitored, along with the global activity of the contralat eral phrenic nerve, transdiaphragmatic pressure, and fractional end-ti dal CO2 levels, Discharge patterns during hyperoxic CO2 rebreathing an d breathing against an IRL (2,500-4,000 cmH(2)O . l(-1). s) were compa red. During IRL, transdiaphragmatic pressure increased and then either plateaued or decreased. At the highest fractional end-tidal CO2 commo n to both I uns, instantaneous discharge frequencies in six motoneuron s were greater during sustained IRL than during rebreathing, when comp ared at the same time after the onset of inspiration. These increased discharge frequencies suggest the presence of a load-induced nonchemic al drive to phrenic motoneurons from unidentified source(s).