Swallowing

Overview
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.

Coordination and control
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 in the pharyngeal and esophgeal phases.

Phases
Normal swallowing consists of four phases: oral preparatory, oral, pharyngeal, and esophageal (not all sources consider oral preparatory a distinct phase).

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.

Oral (or "buccal") phase
When the bolus is ready to enter the oral 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).

Pharyngeal phase
In this phase, the bolus is advanced from the pharynx to the esophagus through peristalsis. 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.

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.

Esophageal phase
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.

Clinicial 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.

Disorders of the oral or pharyngeal phases may lead to oropharyngeal dysphagia and disorders of the esophageal may lead to esophageal dysphagia.