Figure 4. Effect of altering the mixture of O₂ and CO₂ on the inspired gas levels of an anesthetized rat and on the EMG activity recorded from specific tongue muscles and the diaphragm. Tongue force (upper first trace) is measured with a transducer, and retraction is denoted by a downward direction. EMG activity is shown (lower trace) as both the original signal (HG EMG, GG EMG) and the integrated level (HG iEMG, GG iEMG). The rat breathing room air (NORMOXIA) demonstrates rhythmic EMG activity from the hyoglossus retruding muscle (HG), the genioglossus protruding muscle (GG), and the primary respiratory muscle, the diaphragm (DIA iEMG). Modifying the inspired gas mixture to 10% O₂ in 90% N₂ (HYPOXIA) induces hypoxia and increases rhythmic EMG activity in both tongue muscles. Tracheal occlusion is induced (vertical dotted line) to prevent further respiration. Breathing only O₂ (HYPEROXIA) to induce hyperoxia has minimal effect on the tongue muscles even after tracheal occlusion. Breathing O₂ enriched with CO₂ (HYPEROXIC HYPERCAPNIA) increases the EMG activity in both tongue muscles and induces more tongue retraction. Tracheal occlusion enhances this hypercapnic response. (Adapted with permission from Fregosi and Fuller [1997])