FIG. 17:  The schematic in this Figure shows the vagal activation that occurs in response to apnea.   Hypoxemia stimulates peripheral chemoreceptors, reeling in the nucleus tractus solitarius and causing sympathetic nervous activation, but also vagal activation, causing the bradycardia.  A hyperventilation response acts to inhibit both of these changes, but when patients have OSA they do not have a hyperventilation response, which means they have excessive sympathetic activation but in addition, vagal activation is disinhibited.