Airway occlusion and genioglossus activity. Obstructive sleep apnea is characterized by airway occlusions that block nasal airflow (A, upper trace), while abdominal respiratory movements continue as illustrated by the abdominal movements (A, second trace) and esophageal pressure changes (B, upper trace). Continuous respiratory activity is also characterized by respiratory rhythmic diaphragmatic EMG (A, third trace, EMG D). While respiratory rhythmic activity continues in the abdomen, genioglossus activity either decreases during the airway occlusion (B, lower trace, EMG G), or the phasic genioglossus EMG becomes increasingly tonic (A, lower trace, EMG G). The termination of the airway occlusion is characterized by a burst of genioglossus activity (A,B). Figure modified from: A: Praud et al. 1988, B: Remmers et al. 1978.