Swallowing
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Swallowing, known scientifically as deglutition, is the process in the human or animal body that makes something pass from the mouth, to the pharynx, into the esophagus, with the shutting of the epiglottis. If this fails and the object goes through the trachea, then choking or pulmonary aspiration can occur. In the human body it is controlled by the swallowing reflex.
[edit] In humans
[edit] 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 lobes and limbic system of the cerebral cortex with contributions from the motor cortex and other cortical areas ( ). The pharyngeal swallow is started by the oral phase and subsequently is co-ordinated by the swallowing centre in the medulla oblongata and pons. The reflex is initiated by touch receptors in the pharynx as a bolus of food is pushed to the back of the mouth by the tongue.
Swallowing is a complex mechanism using both skeletal muscle (tongue) and smooth muscles of the pharynx and esophagus. The autonomic nervous system (ANS) coordinates this process in the pharyngeal and esophageal phases.
[edit] Phases
Normal swallowing consists of three phases: oral preparatory and transport, pharyngeal, and esophageal.
[edit] Oral preparatory phase
In humans this consists of opening and closing the mouth, moistening food, mastication, preparing an appropriate size bolus with the movement of the tongue, and cheek muscles. Some animals do not chew but swallow the bolus whole.
[edit] Oral transport (or "buccal") phase
Buccal phase begins with the compression of the food bolus against the hard palate. Next, the tongue retracts in a posterior direction to force the bolus to the oropharynx. Then, the posterior tongue is lifted by the styloglossus and palatoglossus muscles, which also elevates the uvula and seals the nasopharynx to prevent nasal aspiration. This phase is voluntary and involves important cranial nerves: V (trigeminal), VII (facial), and XII (hypoglossal).
[edit] Pharyngeal phase
In this phase, the bolus is advanced from the pharynx to the esophagus through sequential contraction of the constrictor muscles. The soft palate is elevated to the posterior nasopharyngeal wall, through the action of the levator veli palatini. 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. Then the larynx and hyoid are elevated and pulled forward to the epiglottis to relax the cricopharyngeus muscle.
This passively shuts off its entrance and the vocal cords are pulled close together, narrowing the passageway between them. This phase is passively controlled reflexively and involves cranial nerves V, X (vagus), XI (accessory), and XII (hypoglossal). The respiratory centre of the medulla is directly inhibited by the swallowing centre 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.
The bolus moves through the pharynx at a speed of 25 feet per second (8 m/s)[1].
[edit] Esophageal phase
The upper esophageal sphincter relaxes to let food past, after which various striated constrictor muscles of the pharynx as well as peristalsis and relaxation of the lower esophageal sphincter sequentially push the bolus of food through the esophagus into the stomach.
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, or agonal respiration.
[edit] Clinical significance
Swallowing becomes a great concern for the elderly since strokes and Alzheimer's disease can interfere with the ANS. Speech therapy is commonly used to correct this condition since the speech process uses the same neuromuscular structures as swallowing.
Abnormalities of the pharynx and/or oral cavity may lead to oropharyngeal dysphagia. Abnormalities of the esophagus may lead to esophageal dysphagia. The failure of the lower esophagous sphincter to respond properly to swallowing is called achalasia.
[edit] In animals
In many birds, the oesophagus is largely merely a gravity chute, and in such events as a seagull swallowing a fish or a stork swallowing a frog, 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, 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 gills.
In snakes, 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.
[edit] See also
[edit] References
- ^ Adams, Cecil (1976-09-17). "How does peristalsis let you swallow upside down?". The Straight Dope. Chicago, IL, United States: Creative Loafing Media, Inc.. http://www.straightdope.com/columns/read/70/how-does-peristalsis-let-you-swallow-upside-down. Retrieved on 2009-03-18. "food passing through the pharynx is rammed down the line at a speed of about 25 feet per second."
[edit] External links
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Look up gulp or swallow in Wiktionary, the free dictionary. |
- Physiology at MCG 6/6ch3/s6ch3_15
- Overview at nature.com
- Anatomy and physiology of swallowing at dysphagia.com
- swallowing animation (flash) at hopkins-gi.org
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