Fig. 4. Schematic model explaining pharyngeal airway patency, showing soft-tissue volume inside bony enclosure and bony enclosure size on either side of fulcrum that represents magnitude of pharyngeal muscle contraction,  i.e. , neural mechanisms. Interaction between the anatomical balance and neural mechanisms determines pharyngeal airway size. Increased neural mechanisms can compensate the anatomical imbalance in obstructive sleep apnea (OSA) patients during wakefulness. When the neural mechanisms are suppressed during sleep and anesthesia, pharyngeal airway severely narrows because of the anatomical imbalance in OSA patients.