Pharyngeal reflex

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The pharyngeal reflex or gag reflex is a reflex muscular contraction of the back of the throat, evoked by touching the roof of the mouth, back of the tongue, area around the tonsils, uvula, and back of the throat. It, along with other aerodigestive reflexes such as reflexive pharyngeal swallowing, prevents objects in the oral cavity from entering the throat except as part of normal swallowing and helps prevent choking, and is a form of coughing. The pharyngeal reflex is different from the laryngeal spasm, which is a reflex muscular contraction of the vocal cords.

Contents

Reflex arc

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 the 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. [1]

In the case of the pharyngeal reflex:

The gag reflex involves a brisk and brief elevation of the soft palate and bilateral contraction of pharyngeal muscles evoked by touching the posterior pharyngeal wall. Touching the soft palate can lead to a similar reflex response. However, in that case, the sensory limb of the reflex is the CN V (trigeminal nerve). In very sensitive individuals, much more of the brain stem may be involved; a simple gag may enlarge to retching and vomiting in some.

Suppression and activation

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. [1] In contrast, triggering the reflex is sometimes done intentionally to induce vomiting, by those who have bulimia nervosa.

According to one study, one in three people lack a gag reflex. [2] However, on the other end of the spectrum are people with a hypersensitive gag reflex. This hypersensitivity can lead to issues in various situations, such as swallowing a pill or large bites of food, or 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 one's soft palate to get used to being touched.[ citation needed ]

Anti-nausea medicines, sedatives, local and general anaesthetics, herbal remedies, behavioural therapies, acupressure, acupuncture, laser, and prosthetic devices can be used to manage exaggerated gag reflex during dental treatment. Studies showed with very low‐certainty evidence that acupuncture and laser at the P6 point (located in the wrist) reduced gagging without sedation. However, there was no difference when practiced with sedation. Therefore, more research needs to be carried out regarding these interventions. [3]

Absence

In certain cases, 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.

In unilateral (one-sided) glossopharyngeal nerve (CN IX - sensory component) damage, there will be no gag response when touching the pharyngeal wall on the same side of the damaged nerve. With one-sided vagal nerve (CN X - motor component) damage, the soft palate will elevate and pull toward the intact side regardless of the side of the pharynx that is touched. This is because the sensory component is intact on both sides, but only the motor nerves supplying one side of the soft palatine and pharyngeal muscles is working, therefore the contraction of the muscles in the reflex is asymmetrical. If both CN IX and X are damaged on one side (not uncommon), stimulation of the normal side elicits only a unilateral response, with deviation of the soft palate to that side; no consensual response is seen. Touching the damaged side produces no response at all.

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. [2]

Reflexive pharyngeal swallow

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 aerodigestive reflexes, including also the pharyngoglottal closure reflex (in which no swallowing occurs, yet the glottis still closes) and the pharyngo-upper esophageal sphincter contractile reflex, which occurs 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 the 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. [4]

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 (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. [5]

See also

Related Research Articles

<span class="mw-page-title-main">Vagus nerve</span> Cranial nerve X, for visceral innervation

The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that carries sensory fibers that create a pathway that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right vagus nerves—but they are typically referred to collectively as a single subsystem. The vagus is the longest nerve of the autonomic nervous system in the human body and comprises both sensory and motor fibers. The sensory fibers originate from neurons of the nodose ganglion, whereas the motor fibers come from neurons of the dorsal motor nucleus of the vagus and the nucleus ambiguus. The vagus was also historically called the pneumogastric nerve.

<span class="mw-page-title-main">Larynx</span> Voice box, an organ in the neck of amphibians, reptiles, and mammals

The larynx, commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about 4–5 centimeters in diameter. The larynx houses the vocal cords, and manipulates pitch and volume, which is essential for phonation. It is situated just below where the tract of the pharynx splits into the trachea and the esophagus. The word 'larynx' comes from the Ancient Greek word lárunx ʻlarynx, gullet, throat.ʼ

<span class="mw-page-title-main">Tongue</span> Muscular organ in the mouth of most vertebrates

The tongue is a muscular organ in the mouth of a typical tetrapod. It manipulates food for chewing and swallowing as part of the digestive process, and is the primary organ of taste. The tongue's upper surface (dorsum) is covered by taste buds housed in numerous lingual papillae. It is sensitive and kept moist by saliva and is richly supplied with nerves and blood vessels. The tongue also serves as a natural means of cleaning the teeth. A major function of the tongue is the enabling of speech in humans and vocalization in other animals.

Swallowing, also called deglutition or inglutition in scientific contexts, is the process in the body of a human or other animal that allows for a substance to pass from the mouth, to the pharynx, and into the esophagus, while shutting the epiglottis. Swallowing is an important part of eating and drinking. If the process fails and the material goes through the trachea, then choking or pulmonary aspiration can occur. In the human body the automatic temporary closing of the epiglottis is controlled by the swallowing reflex.

<span class="mw-page-title-main">Medulla oblongata</span> Structure of the brain stem

The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involuntary) functions, ranging from vomiting to sneezing. The medulla contains the cardiac, respiratory, vomiting and vasomotor centers, and therefore deals with the autonomic functions of breathing, heart rate and blood pressure as well as the sleep–wake cycle.

<span class="mw-page-title-main">Glossopharyngeal nerve</span> Cranial nerve IX, for the tongue and pharynx

The glossopharyngeal nerve, also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior to the vagus nerve. Being a mixed nerve (sensorimotor), it carries afferent sensory and efferent motor information. The motor division of the glossopharyngeal nerve is derived from the basal plate of the embryonic medulla oblongata, whereas the sensory division originates from the cranial neural crest.

<span class="mw-page-title-main">Soft palate</span> Flexible part of maxilla

The soft palate is, in mammals, the soft tissue constituting the back of the roof of the mouth. The soft palate is part of the palate of the mouth; the other part is the hard palate. The soft palate is distinguished from the hard palate at the front of the mouth in that it does not contain bone.

<span class="mw-page-title-main">Epiglottis</span> Leaf-shaped flap in the throat that prevents food from entering the windpipe and the lungs

The epiglottis is a leaf-shaped flap in the throat that prevents food and water from entering the trachea and the lungs. It stays open during breathing, allowing air into the larynx. During swallowing, it closes to prevent aspiration of food into the lungs, forcing the swallowed liquids or food to go along the esophagus toward the stomach instead. It is thus the valve that diverts passage to either the trachea or the esophagus.

<span class="mw-page-title-main">Solitary nucleus</span> Sensory nuclei in medulla oblongata

The solitary nucleus is a series of sensory nuclei forming a vertical column of grey matter in the medulla oblongata of the brainstem. It receives general visceral and/or special visceral inputs from the facial nerve, glossopharyngeal nerve and vagus nerve ; it receives and relays stimuli related to taste and visceral sensation. It sends outputs to various parts of the brain, such as the hypothalamus, thalamus, and reticular formation. Neuron cell bodies of the SN are roughly somatotopically arranged along its length according to function.

<span class="mw-page-title-main">Nucleus ambiguus</span>

The nucleus ambiguus is a group of large motor neurons, situated deep in the medullary reticular formation named by Jacob Clarke. The nucleus ambiguus contains the cell bodies of neurons that innervate the muscles of the soft palate, pharynx, and larynx which are associated with speech and swallowing. As well as motor neurons, the nucleus ambiguus contains preganglionic parasympathetic neurons which innervate postganglionic parasympathetic neurons in the heart.

<span class="mw-page-title-main">Stylopharyngeus muscle</span> Muscle

The stylopharyngeus muscle is a muscle in the head. It originates from the temporal styloid process. Some of its fibres insert onto the thyroid cartilage, while others end by intermingling with proximal structures. It is innervated by the glossopharyngeal nerve. It acts to elevate the larynx and pharynx, and dilate the pharynx, thus facilitating swallowing.

<span class="mw-page-title-main">Tensor veli palatini muscle</span> Muscle of the soft palate

The tensor veli palatini muscle is a thin, triangular muscle of the head that tenses the soft palate and opens the Eustachian tube to equalise pressure in the middle ear.

The cough reflex occurs when stimulation of cough receptors in the respiratory tract by dust or other foreign particles produces a cough, which causes rapidly moving air which usually remove the foreign material before it reaches the lungs. This typically clears particles from the bronchi and trachea, the tubes that feed air to lung tissue from the nose and mouth. The larynx and carina are especially sensitive. Cough receptors in the surface cells (epithelium) of the respiratory tract are also sensitive to chemicals. Terminal bronchioles and even the alveoli are sensitive to chemicals such as sulfur dioxide gas or chlorine gas.

<span class="mw-page-title-main">Pharyngeal plexus of vagus nerve</span> Nerve fibers innervating most of the palate and pharynx

The pharyngeal plexus is a nerve plexus located upon the outer surface of the pharynx. It contains a motor component, a sensory component, and sympathetic component.

Gastrointestinal physiology is the branch of human physiology that addresses the physical function of the gastrointestinal (GI) tract. The function of the GI tract is to process ingested food by mechanical and chemical means, extract nutrients and excrete waste products. The GI tract is composed of the alimentary canal, that runs from the mouth to the anus, as well as the associated glands, chemicals, hormones, and enzymes that assist in digestion. The major processes that occur in the GI tract are: motility, secretion, regulation, digestion and circulation. The proper function and coordination of these processes are vital for maintaining good health by providing for the effective digestion and uptake of nutrients.

Bulbar palsy refers to a range of different signs and symptoms linked to impairment of function of the glossopharyngeal nerve, the vagus nerve, the accessory nerve, and the hypoglossal nerve. It is caused by a lower motor neuron lesion in the medulla oblongata, or from lesions to these nerves outside the brainstem, and also botulism. This may be caused by any of a number of genetic, vascular, degenerative, inflammatory, and other underlying conditions. It can be differentiated from pseudobulbar palsy. When there is airway obstruction, intubation is used.

<span class="mw-page-title-main">Pharynx</span> Part of the throat that is behind the mouth and nasal cavity

The pharynx is the part of the throat behind the mouth and nasal cavity, and above the esophagus and trachea. It is found in vertebrates and invertebrates, though its structure varies across species. The pharynx carries food to the esophagus and air to the larynx. The flap of cartilage called the epiglottis stops food from entering the larynx.

<span class="mw-page-title-main">Human digestive system</span> Digestive system in humans

The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion. Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the cephalic phase, the gastric phase, and the intestinal phase.

Flexible Endoscopic Evaluation of Swallowing with Sensory Testing (FEESST), or laryngopharyngeal sensory testing, is a technique used to directly examine motor and sensory functions of swallowing so that proper treatment can be given to patients with swallowing difficulties to decrease their risk of aspiration and choking. FEESST was invented by Dr. Jonathan E. Aviv MD, FACS in 1993, and has been used by otolaryngologists, pulmonologists, gastroenterologists, intensivists and speech-language pathologists for the past 20 years.

References

  1. 1 2 Wilson, Tracy V. (29 June 2007). "How Sword Swallowing Works". HowStuffWorks .
  2. 1 2 Davies AE, Kidd D, Stone SP, MacMahon J (February 1995). "Pharyngeal sensation and gag reflex in healthy subjects". Lancet. 345 (8948): 487–8. doi:10.1016/s0140-6736(95)90584-7. PMID   7861875. S2CID   38343691.
  3. ↑ Eachempati P, Kumbargere Nagraj S, Kiran Kumar Krishanappa S, George RP, Soe HH, Karanth L (November 2019). "Management of gag reflex for patients undergoing dental treatment". The Cochrane Database of Systematic Reviews. 2019 (11). doi: 10.1002/14651858.cd011116.pub3 . PMC   6953338 . PMID   31721146.
  4. ↑ Dua K, Surapaneni SN, Kuribayashi S, Hafeezullah M, Shaker R (August 2011). "Pharyngeal airway protective reflexes are triggered before the maximum volume of fluid that the hypopharynx can safely hold is exceeded". American Journal of Physiology. Gastrointestinal and Liver Physiology. 301 (2): G197-202. doi: 10.1152/ajpgi.00046.2011 . PMC   3154610 . PMID   21566013.
  5. ↑ Dua K, Bardan E, Ren J, Sui Z, Shaker R (October 1998). "Effect of chronic and acute cigarette smoking on the pharyngo-upper oesophageal sphincter contractile reflex and reflexive pharyngeal swallow". Gut. 43 (4): 537–41. doi: 10.1136/gut.43.4.537 . PMC   1727281 . PMID   9824582.
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