The Three Phases of Swallowing

Understand These Three Important Phases of Swallowing

Woman Touching Throat
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As easy as it might seem, swallowing is actually one of the most complicated actions carried out by our bodies. This seemingly simple and automatic action involves a series of voluntary and involuntary actions that must occur in a precisely orchestrated three-part sequence, involving multiple areas of the nervous system. 

The three phases of swallowing are described below:

The Oral Phase

Swallowing starts with the oral phase, in which food is placed in the mouth and moistened with saliva.

The food bolus is voluntarily chewed with the teeth that are controlled by the muscles of mastication (chewing). During this phase food is “prepared” into a pellet of an appropriate size so that it can be easily passed from the front to the back of the mouth, and from there into the oropharynx (upper part of the throat). This pellet of food is typically referred to as the food bolus.

From the oropharynx, the food bolus is further channeled by the back of the tongue and other muscles into the lower part of the pharynx (throat), a step that also requires the voluntary elevation of the soft palate in order to prevent food from entering the nose. The cranial nerves involved in coordinating this stage include the trigeminal, facial and hypoglossal nerves.

The Pharyngeal Phase

As the food bolus reaches the pharynx, special sensory nerves activate the involuntary phase of swallowing. The swallowing reflex, which is mediated by the swallowing center in the medulla (the lower part of the brainstem), causes the food to be further pushed back into the pharynx and esophagus (food pipe) by rhythmic and involuntary contractions of several muscles in the back of the mouth, pharynx, and esophagus.

A critical part of the pharyngeal phase is the involuntary closure of the larynx by the epiglottis and vocal cords, and the temporary inhibition of breathing, both of which prevent food from going “down the wrong pipe” into the trachea and traveling to the lungs.

The closure of the larynx by the epiglottis protects the lungs from injury, as food and other particles that get into the lungs can lead to severe infections and irritation of the lung tissue.

Lung infections caused by problems with the pharyngeal phase of the swallowing reflex are commonly known as aspiration pneumonia.

The Esophageal Phase

As food leaves the pharynx, it enters the esophagus, a tube-like muscular structure which leads food into the stomach due to its powerful coordinated muscular contractions. The esophagus has two important muscles that open and close to allow the food bolus to flow in a forward direction, while preventing it from going in the wrong direction (regurgitation). These muscles are called sphincters.

Both esophageal sphincters, first the upper, and then the lower, open reflexively as the food bolus is brought down during swallowing. The passage of food through the esophagus during this phase requires the coordinated action of the vagus nerve, the glossopharyngeal nerve, and from nerve fibers from the sympathetic nervous system.

The upper and lower esophageal sphincters prevent food or saliva from being regurgitated toward the mouth. In doing so, the esophageal sphincters serve as a physical barrier to regurgitated food.

Dysphagia

In general, healthy people can swallow with very little deliberate thought and effort. If the nervous system is disrupted, due to a stroke or another disease, then problems with swallowing can occur.

Swallowing difficulties are referred to as dysphagia. Dysphagia can lead to problems such as choking, lack of appetite and weight loss, and aspiration pneumonia. Find out how you can optimize your swallowing function if you have dysphagia.
Sources:

Relationship between Dysphagia, National Institutes of Health Stroke Scale Score, and Predictors of Pneumonia after Ischemic Stroke, Ribeiro PW, Cola PC, Gatto AR, da Silva RG, Luvizutto GJ, Braga GP, Schelp AO, de Arruda Henry MA, Bazan R, J Stroke Cerebrovasc Dis. 2015 Sep;24(9):2088-94

Edited by Heidi Moawad MD

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