Fig. 1. Protocols. Protocol 1: After surgery, we determined the ED⁵⁰(median effective dose) of propofol or isoflurane to compare the effects of equianesthetic doses of these anesthetics on genioglossus activity and breathing. In isoflurane-anesthetized animals, we subsequently measured these variables at two additional dose levels,  i.e. , 1.49 ± 0.01 (ED50) (n = 12) and either 2% or 2.25% (n = 6 each). Protocol 2: We evaluated the effects of different interventions on the dose–response relation of isoflurane at the genioglossus muscle and breathing. In a subset of 6 chronically instrumented animals, we compared the effect of light isoflurane anesthesia between wakefulness and light anesthesia (1% isoflurane). Protocol 3: We evaluated during propofol anesthesia the effects on genioglossus muscle electromyogram of conditions that increased respiratory drive (hypoxia and hypercapnia) to flow rate values that were similar to those observed during isoflurane anesthesia. CO²= carbon dioxide; KF = Kölliker-Fuse nucleus; NP = negative pharyngeal pressure application.