G Gag Reflex DENIS D. BENSARD 1 ,KATHRYN M. BEAUCHAMP 2 1 Department of Acute Care Surgery Denver Health Medical Center, University of Colorado School of Medicine, Denver, CO, USA 2 Department of Neurosurgery, Denver Health Medical Center, University of Colorado School of Medicine, Denver, CO, USA Synonyms Pharyngeal reflex Definition The gag reflex is a reflex contraction of the back of the throat, elicited by touching the posterior pharyngeal wall, tonsillar area, or the base of the tongue. Stimulation results in a visible contraction of the pharyngeal wall. The gag reflex is a protective response that prevents oral contents from entering the throat except as part of normal swallowing and helps prevent choking. The afferent limb of the reflex is supplied by the glossopharyngeal nerve (cranial nerve IX), which inputs to the nucleus solitarius and the spinal trigeminal nucleus. The efferent limb is supplied by the vagus nerve (cranial nerve X) from the nucleus ambiguus. All of these are located in the medulla. The lower cranial nerves are involved in pharyngeal and laryngeal function as well as in movements of the neck and tongue. Damage to them can result in problems with speech and swallowing. These nerves are commonly affected by conditions that damage the medulla or cause bilateral damage to corticobulbar connections. This can create motor problems that affect tongue and pharynx movement as well as speech. Individual or combined lesions of the glossopharyngeal and vagus nerves may result in an impaired gag reflex. Characteristics Stimulation of the gag reflex produces a brisk and brief elevation of the soft palate and a bilateral contraction of the pharyngeal muscles evoked by touching the posterior pharyngeal wall. In an isolated glossopharyngeal nerve (sensory) lesion, there will be no response when the affected side is touched. In isolated vagal nerve damage, the soft palate will elevate and pull toward the intact side, regardless of the side of the pharynx that is touched. If both cranial nerve (CN) IX and CN X are damaged on one side, stimulation of the normal side elicits only a unilateral response, with deviation of the soft palate to that side. Touching the damaged side produces no response at all. Sensitivities to the gag reflex vary among individuals. The gag reflex may be under such strong voluntary control that stimulation causes very little or no response. In contrast, in very sensitive individuals a simple gag may progress to retching and vomiting. Absence of the gag reflex can be a symptom of severe medical conditions that result in damage to the glossopharyngeal nerve, the vagus nerve, or brain. Yet, up to one third of healthy people do not have a gag reflex. Moreover, with increasing age the gag reflex weakens without evident impairment. Individuals may learn to suppress the gag reflex, for example, sword swallowers. Others may learn to trigger the reflex, as observed in patients suffering from bulimia nervosa, who intentionally induce vomiting. To differentiate between involvement of the peripheral or brainstem portion of a cranial nerve pathway, it is important to consider whether there is additionally the involvement of the corticospinal or spinothalamic tracts of the cranial nerves that course through the brainstem and/or evidence of damage to cerebellar function. It is unusual for brain stem lesions to involve one or two cranial nerves in isolation, without also affecting the contiguous long-tracts or cerebellar system structures. Supranuclear motor pathways to the palate, and pharyngealand laryngeal musculature are bilateral. Therefore, unilateral lesions, even large strokes, rarely produce a persistent problem with lower cranial nerve function. Bilateral acute or subacute loss of hemispheric connections to the medullary nuclei causes difficulty with swallowing, phonating and, initially, a depressed gag reflex. In time, the gag reflex may become uncontrollably hyperactive, as do many other skeletal and autonomic reflexes when they are no longer under supranuclear control. Jean-Louis Vincent & Jesse B. Hall (eds.), Encyclopedia of Intensive Care Medicine, DOI 10.1007/978-3-642-00418-6, # Springer-Verlag Berlin Heidelberg 2012