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The pharyngeal reflex or gag reflex (also known as a laryngeal spasm) is a reflex contraction of the back of the throat, evoked by touching the roof of your mouth, the back of your tongue, the area around your tonsils and the back of your throat. It, along with other aero digestive reflexes such as reflexive pharyngeal swallowing, prevents something from entering the throat except as part of normal swallowing and helps prevent choking.
In a reflex arc, a series of physiological steps occur very rapidly to produce a reflex. Generally a sensory receptor receives an environmental stimulus, in this case from objects reaching nerves in the back of your throat, and sends a message via an afferent nerve to the central nervous system (CNS). The CNS receives this message and sends an appropriate response via an efferent nerve (also known as a motor neuron) to effector cells located in the same initial area that can then carry out the appropriate response.
The afferent limb of this reflex is supplied by the glossopharyngeal nerve (cranial nerve IX), which synapses on the nucleus solitarius and the spinal trigeminal nucleus, both nuclei in the medulla oblongata in the brain stem. The efferent limb is supplied by the vagus nerve (cranial nerve X) from the nucleus ambiguus, also located in the medulla in the brain stem.
Suppression and activationEdit
Swallowing unusually large objects or placing objects in the back of the mouth may cause the pharyngeal reflex. Some people, for instance sword swallowers, have learned how to suppress it. In contrast, triggering the reflex is sometimes done intentionally to induce vomiting, for example by those who suffer from bulimia nervosa.
According to one study, one in three people lack a gag reflex. However, on the other end of the spectrum are people with a hypersensitive gag reflex. This hypersensitivity can lead to a variety of issues, from swallowing a pill or large bites of food to visiting the dentist. Hypersensitivity is generally a conditioned response, usually occurring following a previous experience. There are a variety of ways to desensitize one’s hypersensitivity, from relaxation to numbing the mouth and throat to training your soft palate to get used to being touched. Of course, the effectiveness of these techniques varies with the person.
Absence of the gag reflex and pharyngeal sensation can be a symptom of a number of severe medical conditions, such as damage to the glossopharyngeal nerve, the vagus nerve, or brain death. At one point, it was thought that a lack of the gag reflex in stroke patients was a good predictor for dysphagia (difficulty with swallowing) or laryngeal aspiration (food or drink entering the larynx), and was therefore commonly checked for. However, in one study, 37% of healthy people did not have a gag reflex, yet all subjects except for one still retained an intact pharyngeal sensation. These results suggest that the muscles that control the gag reflex remain independent of those that control normal swallowing. Since this reflex is commonly not found in healthy people, its predictive value in determining the risk for swallowing disorders is severely limited. Pharyngeal sensation, on the other hand, as seen by this study, is rarely absent, and could prove better at predicting future problems with swallowing. 
Reflexive Pharyngeal SwallowEdit
Closely related to the gag reflex, in which food or other foreign substances are forced back out of the pharynx, swallowing generally pushes food through the digestive system into the stomach. This reflex in particular functions as a protective system for the upper respiratory tract as it not only forces the glottis to close, thereby preventing any substances getting into the airways, but also clears the pharynx of any residual substances by a swallow.
This particular reflex is simply one of several aero digestive reflexes such as the reflexive pharyngeal swallow, the pharyngoglottal closure reflex, in which no swallowing occurs yet the glottis still closes, and the pharyngo-upper esophageal sphincter contractile reflex, occurring mainly during gastroesophageal reflux episodes. All either forcibly close the glottis or allow the pharynx to remove particles into the digestive tract that may have been forced back up by both this tract and the upper respiratory tract. These reflexes can also protect the airways from any food or liquids that may spill over from the hypopharynx. The hypopharynx is the bottom part of pharynx, and can be considered the first area where the digestive tract splits from the airways. However, if the maximum capacity of fluids that the hypopharynx can safely hold is exceeded, then this excess fluid spills into the larynx and from there into the lungs. Therefore these reflexes prevent levels reaching this maximum volume.
Since both the digestive system and the respiratory system are connected by the pharynx, there are many problems and diseases that occur when the body is unable to regulate passage of food and air into the appropriate tracts. Perhaps the most preventable cause of damage to these reflexes originates from smoking. One study has shown that, when compared to non-smokers, the threshold volumes (in other words the lowest volume at which one of these reflexes is triggered) for both the pharyngo-upper esophageal sphincter contractile reflex and reflexive pharyngeal swallowing is increased.
- ↑ Medical Neurosciences.
- ↑ 2.0 2.1 Wilson, Tracy V. How Sword Swallowing Works. HowStuffWorks.
- ↑ 3.0 3.1 (February 1995). Pharyngeal sensation and gag reflex in healthy subjects. Lancet 345 (8948): 487–488.
- ↑ Dua, Kulwinder, Sri Naveen Surapaneni, Shiko Kuribayashi, Muhammed Hafeezullah, Reza Shaker (5 2011). Pharyngeal airway protective reflexes are triggered before the maximum volume of fluid that the hypopharynx can safely hold is exceeded. Am J Physiol Gastrointest Liver Physiol 301 (2): G197-202.
- ↑ Dua, K, E Bardan, J Ren, Z Sui, R Shaker (1998). Effect of chronic and acute cigarette smoking on the pharyngo-upper oesophageal sphincter contractile reflex and reflexive pharyngeal swallow. Gut: An International Journal of Gastroenterology and Hepatology 43: 537-541.
Nervous system physiology: neurophysiology - reflex
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