Swallowing, sometimes called deglutition in scientific contexts, is the process in the human or animal body that allows for a substance to pass from the mouth, to the

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

, and into the

esophagus The esophagus ( American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to ...

, while shutting the

epiglottis 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 in ...

. Swallowing is an important part of

eating Eating (also known as consuming) is the ingestion of food, typically to provide a heterotrophic organism with energy and to allow for growth. Animals and other heterotrophs must eat in order to survive — carnivores eat other animals, he ...

and

drinking Drinking is the act of ingesting water or other liquids into the body through the mouth, proboscis, or elsewhere. Humans drink by swallowing, completed by peristalsis in the esophagus. The physiological processes of drinking vary widely am ...

. If the process fails and the material (such as food, drink, or medicine) goes through the

trachea The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...

, then

choking Choking, also known as foreign body airway obstruction (FBAO), is a phenomenon that occurs when breathing is impeded by a blockage inside of the respiratory tract. An obstruction that prevents oxygen from entering the lungs results in oxygen dep ...

or pulmonary aspiration can occur. In the human body the automatic temporary closing of the epiglottis is controlled by the swallowing reflex. The portion of food, drink, or other material that will move through the neck in one swallow is called a bolus. In

colloquial English Colloquialism (), also called colloquial language, everyday language or general parlance, is the linguistic style used for casual (informal) communication. It is the most common functional style of speech, the idiom normally employed in conversa ...

, the term "swallowing" is also used to describe the action of taking in a large mouthful of food without any biting, where the word gulping is more adequate.

In humans

Swallowing comes so easily to most people that the process rarely prompts much thought. However, from the viewpoints of

physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

, of

speech–language pathology Speech-language pathology (or speech and language pathology) is a healthcare field of expertise practiced globally. Speech-language pathology (SLP) specializes in the evaluation, diagnosis, treatment, and prevention of communication disorders ( ...

, and of

health care Health care or healthcare is the improvement of health via the prevention, diagnosis, treatment, amelioration or cure of disease, illness, injury, and other physical and mental impairments in people. Health care is delivered by health profe ...

for people with difficulty in swallowing (dysphagia), it is an interesting topic with extensive

scientific literature : ''For a broader class of literature, see Academic publishing.'' Scientific literature comprises scholarly publications that report original empirical and theoretical work in the natural and social sciences. Within an academic field, scie ...

Coordination and control

Eating and swallowing are complex neuromuscular activities consisting essentially of three phases, an 'oral, pharyngeal and esophageal phase. Each phase is controlled by a different neurological mechanism. The oral phase, which is entirely voluntary, is mainly controlled by the medial

temporal lobe The temporal lobe is one of the four Lobes of the brain, major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe ...

s and

limbic system The limbic system, also known as the paleomammalian cortex, is a set of brain structures located on both sides of the thalamus, immediately beneath the medial temporal lobe of the cerebrum primarily in the forebrain.Schacter, Daniel L. 2012. ''Ps ...

of the

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consistin ...

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 the medulla oblongata and

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...

. The reflex is initiated by touch receptors in the

as a bolus 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 The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...

) and smooth muscles of the pharynx and

. The autonomic nervous system (ANS) coordinates this process in the pharyngeal and esophageal phases.

Phases

Oral phase

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 the

salivary glands The salivary glands in mammals are exocrine glands that produce saliva through a system of ducts. Humans have three paired major salivary glands ( parotid, submandibular, and sublingual), as well as hundreds of minor salivary glands. Salivary gl ...

( parasympathetic). 2) Mastication Food is mechanically broken down by the action of the teeth controlled by the muscles of mastication (V₃) acting on the

temporomandibular joint In anatomy, the temporomandibular joints (TMJ) are the two joints connecting the jawbone to the skull. It is a bilateral synovial articulation between the temporal bone of the skull above and the mandible below; it is from these bones that it ...

. This results in a bolus which is moved from one side of the oral cavity to the other by the tongue.

Buccinator The buccinator () is a thin quadrilateral muscle occupying the interval between the maxilla and the mandible at the side of the face. It forms the anterior part of the cheek or the lateral wall of the oral cavity.Illustrated Anatomy of the Head ...

(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 the

lingual nerve The lingual nerve carries sensory innervation from the anterior two-thirds of the tongue. It contains fibres from both the mandibular division of the trigeminal nerve (CN V3 ) and from the facial nerve (CN VII). The fibres from the trigeminal nerv ...

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 the mylohyoid (mylohyoid nerve—V₃),

genioglossus The genioglossus is one of the paired extrinsic muscles of the tongue. The genioglossus is the major muscle responsible for protruding (or sticking out) the tongue. Structure Genioglossus is the fan-shaped extrinsic tongue muscle that forms the ma ...

styloglossus The styloglossus, the shortest and smallest of the three styloid muscles, arises from the anterior and lateral surfaces of the styloid process near its apex, and from the stylomandibular ligament. Passing inferiorly and anteriorly between the int ...

and

hyoglossus The hyoglossus, thin and quadrilateral, arises from the side of the body and from the whole length of the greater cornu of the hyoid bone, and passes almost vertically upward to enter the side of the tongue, between the styloglossus and the inf ...

(the rest XII)) such that the tongue slopes downwards posteriorly. The contraction of the

and

(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 the

palatoglossal arch The palatoglossal arch (glossopalatine arch, anterior pillar of fauces) on either side runs downward, lateral (to the side), and forward to the side of the base of the tongue, and is formed by the projection of the glossopalatine muscle with its ...

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 the

superior laryngeal nerve The superior laryngeal nerve is a branch of the vagus nerve. It arises from the middle of the inferior ganglion of vagus nerve and in its course receives a branch from the superior cervical ganglion of the sympathetic nervous system. The superi ...

. This phase is ''voluntary'' and involves important

cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

: V (trigeminal), VII (facial) and XII (hypoglossal).

Pharyngeal phase

For the pharyngeal phase to work properly all other egress from the pharynx must be occluded—this includes the

nasopharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struct ...

and the

larynx 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 ...

. 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 by tensor palatini (Vc), and then elevated by

levator palati The levator veli palatini () is the elevator muscle of the soft palate in the human body. It is supplied via the pharyngeal plexus. During swallowing, it contracts, elevating the soft palate to help prevent food from entering the nasopharynx. ...

ni (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 the

suprahyoid The suprahyoid muscles are four muscles located above the hyoid bone in the neck. They are the digastric, stylohyoid, geniohyoid, and mylohyoid muscles. They are all pharyngeal muscles, with the exception of the geniohyoid muscle. The digastric ...

and longitudinal pharyngeal muscles –

stylopharyngeus The stylopharyngeus is a muscle in the head that stretches between the temporal styloid process and the pharynx. Structure The stylopharyngeus is a long, slender muscle, cylindrical above, flattened below. It arises from the medial side of the ...

(IX),

salpingopharyngeus The salpingopharyngeus muscle is a muscle of the pharynx. It arises from cartilage around the Eustachian tube, and inserts into the palatopharyngeus muscle by blending with its posterior fasciculus. It raises the pharynx and larynx during deglu ...

(pharyngeal plexus—IX, X) and

palatopharyngeus The palatopharyngeus (palatopharyngeal or pharyngopalatinus) muscle is a small muscle in the roof of the mouth. It is a long, fleshy fasciculus, narrower in the middle than at either end, forming, with the mucous membrane covering its surface, the ...

(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 the

auditory tube Auditory means of or relating to the process of hearing: * Auditory system, the neurological structures and pathways of sound perception ** Auditory bulla, part of auditory system found in mammals other than primates ** Auditory nerve, also known ...

The actions of the

ni (pharyngeal plexus—IX, X), tensor palatini (Vc) and

(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 the

oropharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...

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 the

lateral cricoarytenoids The lateral cricoarytenoid (also anterior cricoarytenoid) muscles extend from the lateral cricoid cartilage to the muscular process of the arytenoid cartilage. By rotating the arytenoid cartilages medially, these muscles adduct the vocal cords and ...

and the oblique and transverse

arytenoids The arytenoid cartilages () are a pair of small three-sided pyramids which form part of the larynx. They are the site of attachment of the vocal cords. Each is pyramidal or ladle-shaped and has three surfaces, a base, and an apex. The arytenoid ...

(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 the laryngeal 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 the piriform 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 is ''passively'' controlled reflexively and involves cranial nerves V, X (vagus), XI (accessory) and XII (hypoglossal). The respiratory 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 as ''deglutition apnea''. 10)

Hyoid The hyoid bone (lingual bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies between the base of the mandible and the third cervical vertebra. ...

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 pharyngeal

peristalsis Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, whic ...

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 The inferior pharyngeal constrictor muscle is a skeletal muscle of the neck. It is the thickest of the three outer pharyngeal muscles. It arises from the sides of the cricoid cartilage and the thyroid cartilage. It is supplied by the vagus nerve ...

) 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.

Esophageal phase

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 the

lower esophageal sphincter The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...

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.

Clinical significance

Swallowing becomes a great concern for the elderly since strokes and

Alzheimer's disease Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...

can interfere with the autonomic 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 a

death rattle Terminal respiratory secretions (or simply terminal secretions),, known colloquially as a death rattle, are sounds often produced by someone who is near death as a result of fluids such as saliva and bronchial secretions accumulating in the throa ...

(not to be confused with

agonal respiration Agonal respiration, gasping respiration or agonal breathing is a distinct abnormal pattern of breathing and brainstem reflex characterized by gasping, labored breathing, accompanied by strange vocalizations and myoclonus. Possible causes include ...

, which is an abnormal pattern of breathing due to cerebral ischemia or hypoxia). Abnormalities of the pharynx and/or oral cavity may lead to

oropharyngeal dysphagia Oropharyngeal dysphagia arises from abnormalities of muscles, nerves or structures of the oral cavity, pharynx, and upper esophageal sphincter. Signs and symptoms Some signs and symptoms of swallowing difficulties include difficulty controlling f ...

. Abnormalities of the esophagus may lead to

esophageal dysphagia Esophageal dysphagia is a form of dysphagia where the underlying cause arises from the body of the esophagus, lower esophageal sphincter, or cardia of the stomach, usually due to mechanical causes or motility problems. Signs and symptoms Patients ...

. The failure of the lower esophagus sphincter to respond properly to swallowing is called

achalasia Esophageal achalasia, often referred to simply as achalasia, is a failure of smooth muscle fibers to relax, which can cause the lower esophageal sphincter to remain closed. Without a modifier, "achalasia" usually refers to achalasia of the esoph ...

. 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 forges breathing and priorities the swallowed matter. This intermediate level of muscle manipulation is similar to the techniques used by sword swallowers.

In non-mammal animals

In many birds, the esophagus is largely a mere gravity chute, and in such events as a

seagull Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. They are most closely related to the terns and skimmers and only distantly related to auks, and even more distantly to waders. Until the 21st century, m ...

swallowing a fish or a

stork Storks are large, long-legged, long-necked wading birds with long, stout bills. They belong to the family called Ciconiidae, and make up the order Ciconiiformes . Ciconiiformes previously included a number of other families, such as herons an ...

swallowing a

frog A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-f ...

, 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. In

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

, 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 its

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

s. In

snake Snakes are elongated, Limbless vertebrate, limbless, carnivore, carnivorous reptiles of the suborder Serpentes . Like all other Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales. Ma ...

s, 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.

References

External links

*
Overview at nature.com

Anatomy and physiology of swallowing at dysphagia.com

Swallowing animation (flash) at hopkins-gi.org
* rticle on French WikipediaSee : " déglutition atypique" = unfunctional or pathological swallowing.
Normal Swallowing and Dysphagia: Pediatric Population
{{Authority control Reflexes Physiology Articles containing video clips