Figure 8.

CO₂ responses of phrenic nerve and XII nerve activities in hyperoxic decerebrate dogs. Upper: Example of the effects of CO₂, measured as end-tidal ETCO₂ (mmHg), on the moving-time averages of phrenic nerve activity (PNG, top trace) and XII nerve activity (XII, 2^nd trace). Lower Left: plot of normalized peak activity vs. ETCO₂ associated with the example from the upper traces. Nonlinear regression was used to fit both nerve activity plots and to aid in extrapolating the apneic thresholds. A hyperbolic function of the form: Y=(X-φ)³/[(X-φ)³+(X_o-φ)³)] was used, where Y is peak activity, X is ETCO₂, φ is ETCO₂-axis intercept and X_o is the ETCO₂ level for 50% of maximum peak activity. The response curve for XII activity is shifted to the right and shallower with a greater linear range compared to the phrenic activity. The apneic thresholds are ~38 and ~42 mmHg for the phrenic and XII activities, respectively. An ETCO₂ of ~52 mmHg produced a peak XII activity of 50% of maximum. Lower center: XII nerve activity vs. ETCO₂ plots for five dogs (thin lines) and group mean curve (thick line). In three of five animals there was no XII activity below an ETCO₂ of 42 mmHg. Lower right: mean curves ± 1 SE bands of both activities. The XII nerve activity curve was consistently right shifted and shallower than the phrenic response curve. ETCO₂ levels of 65–70 mmHg were required to produce maximal XII activity. ETCO₂ levels required for 50% of maximum ranged from 41–55 mmHg (center plots).