The role of vagal afferent information during inspiration in determining phrenic motoneurone output.

Dogs were anaesthetised, paralysed and artifically ventilated using ramp inflations triggered by the onset of the 'integrated' phrenic signal. VT was initially adjusted to maintain a constant PaCO2 and the duration of inflation (TI(resp)) was adjusted to match the phrenic TI. With these control values of VT and TI(resp) a variety of experimental manoeuvres were performed to alter the pattern of volume information delivered during inspiration. These manoeuvres included: (1) alterations in VT while maintaining a constant TI(resp); (2) alterations in TI(resp) alone, with and without obstruction to expiration; (3) delays in the onset of inflation with and without a simultaneous reduction in TI(resp) and (4) the imposition of two inflations, each with the same reduced TI(resp) within the same inspiration. The results suggest that the pattern of vagally-mediated volume information during inspiration determines both the shape and the duration of phrenic motoneurone output. The importance of volume at inspiratory 'offswitch' as the sole determinant of TI is therefore denied. The rate of inflation was not found to contribute to the responses obtained.