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Swallowing, also calleddeglutition oringlutition[1] in scientific and medical contexts, is a physical process of ananimal'sdigestive tract (e.g. that of ahuman body) that allows for aningested substance (typicallyfood) to pass from themouth to thepharynx and then into theesophagus. Incolloquial English, the term "swallowing" is also used to describe the action ofgulping, i.e. taking in a large mouthful of food without anybiting.
Swallowing is performed by an initial push from back part of thetongue (with the tongue tip contacting thehard palate for mechanical anchorage) and subsequent coordinatedcontractions of thepharyngeal muscles. The portion of food,drink and/or other material (e.g.mucus,secretions andmedications) that moves into the gullet in one swallow is called abolus, which is then propelled through to thestomach for furtherdigestion by autonomicperistalsis of the esophagus.
Swallowing is an important part ofeating anddrinking. If the process fails and the bolus to be swallowed mistakenly goes into thetrachea, thenchoking orpulmonary aspiration can occur. In the human body, such incidents are prevented by an automatictrapdoor-like inversion of theepiglottis to temporarily cover thelarynx and close off theupper airway, controlled by a complexreflex that facilitates the elevation of thehyoid bone andthyroid cartilage at the same time. The body will also initiate acough reflex to expel any unwanted material that have accidentally entered the airway. A separategag reflex, which involves the elevation of theuvula and tightening of thesoft palate, prevents food from wrongly entering thenasal cavity above during swallowing.
Swallowing comes so easily to most people that the process rarely prompts much thought. However, from the viewpoints ofphysiology, ofspeech–language pathology, and ofhealth care for people withdifficulty in swallowing (dysphagia), it is an interesting topic with extensivescientific literature.
Eating and swallowing are complex neuromuscular activities consisting essentially of three phases, anoral,pharyngeal andesophageal phase. Each phase is controlled by a different neurological mechanism. The oral phase, which is entirely voluntary, is mainly controlled by themedialtemporal lobes andlimbic system of thecerebral cortex with contributions from the motor cortex and other cortical areas. The pharyngeal swallow is started by the oral phase and subsequently is coordinated by the swallowing center on themedulla oblongata andpons. The reflex is initiated by touch receptors in thepharynx as abolus of food is pushed to the back of the mouth by the tongue, or by stimulation of the palate (palatal reflex).
Swallowing is a complex mechanism using both skeletal muscle (tongue) and smooth muscles of the pharynx andesophagus. Theautonomic nervous system (ANS) coordinates this process in the pharyngeal and esophageal phases.
Prior to the following stages of the oral phase, the mandible depresses and the lips abduct to allow food or liquid to enter the oral cavity. Upon entering the oral cavity, the mandible elevates and the lips adduct to assist in oral containment of the food and liquid. The following stages describe the normal and necessary actions to form the bolus, which is defined as the state of the food in which it is ready to be swallowed.
1)Moistening
Food is moistened by saliva from thesalivary glands (parasympathetic).
2)Mastication
Food is mechanically broken down by the action of the teeth controlled by the muscles of mastication (V3) acting on thetemporomandibular joint. This results in a bolus which is moved from one side of the oral cavity to the other by the tongue.Buccinator (VII) helps to contain the food against the occlusal surfaces of the teeth. The bolus is ready for swallowing when it is held together by saliva (largely mucus), sensed by thelingual nerve of the tongue (VII—chorda tympani and IX—lesser petrosal) (V3). Any food that is too dry to form a bolus will not be swallowed.
3)Trough formation
A trough is then formed at the back of the tongue by the intrinsic muscles (XII). The trough obliterates against the hard palate from front to back, forcing the bolus to the back of the tongue.The intrinsic muscles of the tongue (XII) contract to make a trough (a longitudinal concave fold) at the back of the tongue. The tongue is then elevated to the roof of the mouth (by themylohyoid (mylohyoid nerve—V3),genioglossus,styloglossus andhyoglossus (the rest XII)) such that the tongue slopes downwards posteriorly. The contraction of thegenioglossus andstyloglossus (both XII) also contributes to the formation of the central trough.
4)Movement of the bolus posteriorly
At the end of the oral preparatory phase, the food bolus has been formed and is ready to be propelled posteriorly into the pharynx. In order for anterior to posterior transit of the bolus to occur, orbicularis oris contracts and adducts the lips to form a tight seal of the oral cavity. Next, the superior longitudinal muscle elevates the apex of the tongue to make contact with the hard palate and the bolus is propelled to the posterior portion of the oral cavity. Once the bolus reaches thepalatoglossal arch of the oropharynx, the pharyngeal phase, which is reflex and involuntary, then begins. Receptors initiating this reflex are proprioceptive (afferent limb of reflex is IX and efferent limb is the pharyngeal plexus- IX and X). They are scattered over the base of the tongue, the palatoglossal and palatopharyngeal arches, the tonsillar fossa, uvula and posterior pharyngeal wall. Stimuli from the receptors of this phase then provoke the pharyngeal phase. In fact, it has been shown that the swallowing reflex can be initiated entirely by peripheral stimulation of the internal branch of thesuperior laryngeal nerve. This phase isvoluntary and involves importantcranial nerves:V (trigeminal),VII (facial) andXII (hypoglossal).
For the pharyngeal phase to work properly all other egress from the pharynx must be occluded—this includes thenasopharynx and thelarynx. When the pharyngeal phase begins, other activities such as chewing, breathing, coughing and vomiting are concomitantly inhibited.
5)Closure of the nasopharynx
The soft palate is tensed bytensor palatini (Vc), and then elevated bylevator palatini (pharyngeal plexus—IX, X) to close the nasopharynx. There is also the simultaneous approximation of the walls of the pharynx to the posterior free border of the soft palate, which is carried out by the palatopharyngeus (pharyngeal plexus—IX, X) and the upper part of the superior constrictor (pharyngeal plexus—IX, X).
6)The pharynx prepares to receive the bolus
The pharynx is pulled upwards and forwards by thesuprahyoid and longitudinal pharyngeal muscles –stylopharyngeus (IX),salpingopharyngeus (pharyngeal plexus—IX, X) andpalatopharyngeus (pharyngeal plexus—IX, X) to receive the bolus. The palatopharyngeal folds on each side of the pharynx are brought close together through the superior constrictor muscles, so that only a small bolus can pass.
7)Opening of theauditory tube
The actions of thelevator palatini (pharyngeal plexus—IX, X),tensor palatini (Vc) andsalpingopharyngeus (pharyngeal plexus—IX, X) in the closure of the nasopharynx and elevation of the pharynx opens the auditory tube, which equalises the pressure between the nasopharynx and the middle ear. This does not contribute to swallowing, but happens as a consequence of it.
8)Closure of theoropharynx
The oropharynx is kept closed by palatoglossus (pharyngeal plexus—IX, X), the intrinsic muscles of tongue (XII) and styloglossus (XII).
9)Laryngeal closure
The primary laryngopharyngeal protective mechanism to prevent aspiration during swallowing is via the closure of the true vocal folds. The adduction of the vocal cords is affected by the contraction of thelateral cricoarytenoids and the oblique and transversearytenoids (all recurrent laryngeal nerve of vagus). Since the true vocal folds adduct during the swallow, a finite period of apnea (swallowing apnea) must necessarily take place with each swallow. When relating swallowing to respiration, it has been demonstrated that swallowing occurs most often during expiration, even at full expiration a fine air jet is expired probably to clear the upper larynx from food remnants or liquid. The clinical significance of this finding is that patients with a baseline of compromised lung function will, over a period of time, develop respiratory distress as a meal progresses.Subsequently, false vocal fold adduction, adduction of the aryepiglottic folds and retroversion of the epiglottis take place. The aryepiglotticus (recurrent laryngeal nerve of vagus) contracts, causing the arytenoids to appose each other (closes thelaryngeal aditus by bringing the aryepiglottic folds together), and draws the epiglottis down to bring its lower half into contact with arytenoids, thus closing the aditus. Retroversion of the epiglottis, while not the primary mechanism of protecting the airway from laryngeal penetration and aspiration, acts to anatomically direct the food bolus laterally towards thepiriform fossa.Additionally, the larynx is pulled up with the pharynx under the tongue by stylopharyngeus (IX), salpingopharyngeus (pharyngeal plexus—IX, X), palatopharyngeus (pharyngeal plexus—IX, X) and inferior constrictor (pharyngeal plexus—IX, X). This phase ispassively controlled reflexively and involves cranial nerves V,X (vagus),XI (accessory) andXII (hypoglossal). Therespiratory center of the medulla is directly inhibited by the swallowing center for the very brief time that it takes to swallow. This means that it is briefly impossible to breathe during this phase of swallowing and the moment where breathing is prevented is known asdeglutitionapnea.
10)Hyoid elevation
The hyoid is elevated by digastric (V & VII) and stylohyoid (VII), lifting the pharynx and larynx up even further.
11)Bolus transits pharynx
The bolus moves down towards the esophagus by pharyngealperistalsis which takes place by sequential contraction of the superior, middle and inferior pharyngeal constrictor muscles (pharyngeal plexus—IX, X). The lower part of the inferior constrictor (cricopharyngeus) is normally closed and only opens for the advancing bolus. Gravity plays only a small part in the upright position—in fact, it is possible to swallow solid food even when standing on one's head. The velocity through the pharynx depends on a number of factors such as viscosity and volume of the bolus. In one study, bolus velocity in healthy adults was measured to be approximately 30–40 cm/s.[2]
12)Esophageal peristalsis
Like the pharyngeal phase of swallowing, the esophageal phase of swallowing is under involuntary neuromuscular control. However, propagation of the food bolus is significantly slower than in the pharynx. The bolus enters the esophagus and is propelled downwards first by striated muscle (recurrent laryngeal, X) then by the smooth muscle (X) at a rate of 3–5 cm/s. The upper esophageal sphincter relaxes to let food pass, after which various striated constrictor muscles of the pharynx as well as peristalsis and relaxation of thelower esophageal sphincter sequentially push the bolus of food through the esophagus into the stomach.
13)Relaxation phase
Finally the larynx and pharynx move down with the hyoid mostly by elastic recoil. Then the larynx and pharynx move down from the hyoid to their relaxed positions by elastic recoil.Swallowing therefore depends on coordinated interplay between many various muscles, and although the initial part of swallowing is under voluntary control, once the deglutition process is started, it is quite hard to stop it.
Swallowing becomes a great concern for the elderly sincestrokes andAlzheimer's disease can interfere with theautonomic nervous system.Speech pathologists commonly diagnose and treat this condition since the speech process uses the same neuromuscular structures as swallowing. Diagnostic procedures commonly performed by a speech pathologist to evaluate dysphagia include Fiberoptic Endoscopic Evaluation of Swallowing and Modified Barium Swallow Study. Occupational Therapists may also offer swallowing rehabilitation services as well as prescribing modified feeding techniques and utensils. Consultation with a dietician is essential, in order to ensure that the individual with dysphagia is able to consume sufficient calories and nutrients to maintain health. In terminally ill patients, a failure of the reflex to swallow leads to a build-up of mucus or saliva in the throat and airways, producing a noise known as adeath rattle (not to be confused withagonal respiration, which is an abnormal pattern of breathing due to cerebral ischemia or hypoxia).
Abnormalities of the pharynx and/or oral cavity may lead tooropharyngeal dysphagia. Abnormalities of the esophagus may lead toesophageal dysphagia.The failure of the lower esophagus sphincter to respond properly to swallowing is calledachalasia.
M-Type Swallowing
With practice, people can learn to swallow fluidly without closing the mouth by merely manipulating the tongue and jaw to drive fluids or foods down the esophagus. With a continuous motion, an individual forgoes breathing and prioritizes the swallowed matter. This intermediate level of muscle manipulation is similar to the techniques used by sword swallowers.
In many birds, the esophagus is largely a meregravity chute, and in such events as aseagull swallowing a fish or astork swallowing afrog, swallowing consists largely of the bird lifting its head with its beak pointing up and guiding the prey with tongue and jaws so that the prey slides inside and down.
Infish, the tongue is largely bony and much less mobile and getting the food to the back of the pharynx is helped by pumping water in its mouth and out of itsgills.
Insnakes, the work of swallowing is done by raking with the lower jaw until the prey is far enough back to be helped down by body undulations.
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