Figure 5.

RTN photostimulation increases late-expiratory flow in conscious rats. A, Trajectory of respiratory flow (waveform average, 10–15 breaths) at rest and during respiratory pacing caused by burst photostimulation of RTN at various frequencies (no stimulation, 75, 85, and 95 min⁻¹). Traces at extreme right depict flow trajectory during continuous 20 Hz stimulus (blue trace) or during hypercapnia (8% CO₂; black trace) in the same rat. At rest (left excerpt) during quiet breathing (72 breaths · min⁻¹) expiratory flow was monotonically decremental, reflecting the passive recoil of the lungs and no active expiration. When breathing was entrained at 75, 85, and 95 breaths · min⁻¹ respectively (middle), a large positive peak in flow preceded inspiration. This peak was most prominent at lower frequencies of entrainment and tended to fuse with the early-expiratory peak at higher frequencies. During high-frequency continuous stimulation (20 Hz; right, blue) under hypercapnic conditions (8% inspired; right, black), the expiratory flow became monophasic, although active expiration must have been present. B, Original record showing the effect of a 1 s stimulus train (20 Hz, 3 ms pulse). This example illustrates the progressive change in the pattern of expiratory flow from a biphasic to a monophasic pattern during continuous high-frequency photostimulation. C, Tidal volume, early and late-expiratory volumes are color coded; early and late volumes were defined by dividing expiration (E) in two periods of equal duration. D, Group data (N = 6). During stimulation-induced respiratory pacing, the ratio between late-expiratory and early-expiratory volume was significantly increased relative to resting breathing (**p < 0.01, *p < 0.05 relative to rest; one-way ANOVA). The imbalance between late-expiratory and early-expiratory flow was most prominent at the lowest entrainment frequencies. When respiratory frequency was elevated by hypercapnia or continuous RTN stimulation at 20 Hz, the ratio between late-expiratory and early-expiratory volume was no different from that observed at rest.