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Swallowing, known scientifically as deglutition, is the reflex in the human 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. The mechanism for swallowing 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. Swallowing becomes a great concern for the elderly since strokes and Alzheimer's disease can interfere with the ANS. Speech therapists in the United States are commonly used to correct for this condition since the speech process uses the same neuromuscular structures as swallowing.
Normal swallowing consists of three phases: oral, pharyngeal, and esophageal. Disorders of the oral or pharyngeal phases may lead to oropharyngeal dysphagia and disorders of the esophageal may lead to esophageal dysphagia.
Oral preparatory phase
In this phase, the food is processed by mastication, combined with the movement of the tongue form a bolus to an appropriate size to pass through the pharynx and esophagus.
When the bolus is ready to enter this next stage, it is first moved to the back of the tongue. Next, the anterior tongue lifts to the hard palate and retracts in a posterior direction to force the bolus to the oropharynx. Then, the posterior tongue is lifted by the mylohyoid muscles, which also elevates the soft palate and seals the nasopharynx to prevent nasal aspiration. This phase is voluntary and involves important cranial nerves: V (trigeminal), VII (facial), and (XII) hypoglossal.
In this phase, the bolus is advanced from the pharynx to the esophageal through peristalsis. The soft palate is elevated to the posterior nasopharyngeal wall. 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.
- The respiratory centre of the medulla is directly inhibited by the swallowing centre for the very brief time that it takes to swallow. This is known as deglutition apnoea.
The upper oesophageal 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 buildup of mucous or saliva in the throat and airways, producing a noise known as a death rattle, or agonal respiration.
Swallowing pattern generators
Swallowing involves the coordinated contraction of more than 25 pairs of muscles in the oropharynx, larynx and esophagus, which are active during an oropharyngeal phase, followed by the primary esophageal peristalsis. Swallowing depends on a CPG located in the medulla oblongata, which involves several brain stem motor nuclei and two main groups of interneurons: a dorsal swallowing group (DSG) in the nucleus tractus solitarius and a ventral swallowing group (VSG) located in the ventrolateral medulla above the nucleus ambiguus. Neurons in the DSG are responsible for the generation of the swallowing pattern, while those in the VSG distribute the commands to the various motoneuronal pools. As in other CPGs, the functioning of the central network can be modulated by peripheral and central inputs, so that the swallowing pattern is adapted to the size of the bolus.
Within this network, central inhibitory connections play a major role, producing a rostrocaudal inhibition that parallels the rostrocaudal anatomy of the swallowing tract. Thus, when the neurons controlling the proximal parts of the tract are active, those that command more distal parts are inhibited. Apart from the type of connection between the neurons, intrinsic properties of the neurons, especially those of NTS neurons, probably also contribute to the shaping and timing of the swallowing pattern.
The swallowing CPG is a flexible CPG. This means that at least some of the swallowing neurons may be multifunctional neurons and belong to pools of neurons that are common to several CPGs. One such CPG is the respiratory one, which has been observed interacting with the swallowing CPG.
- Aerophagia - swallowing of air
- Central pattern generators
- Eating disorders
- Levator veli palatini
- Dick TE, Oku Y, Romaniuk JR, Cherniack NS (June 1993). Interaction between central pattern generators for breathing and swallowing in the cat. J. Physiol. (Lond.) 465: 715–30.
- Grélot L, Barillot JC, Bianchi AL (1989). Pharyngeal motoneurones: respiratory-related activity and responses to laryngeal afferents in the decerebrate cat. Exp Brain Res 78 (2): 336–44.
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