The Three 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 actions that must occur in a precisely orchestrated three-part sequence, involving multiple areas of the nervous system. 

There are voluntary, or deliberate actions involved in swallowing, as well as involuntary, or reflexive actions involved in swallowing.

The three phases of swallowing are described below:

The Oral Phase

Swallowing starts with the oral phase. This phase begins when food is placed in the mouth and moistened with saliva. The moistened food is called a food bolus.

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 smaller size that is well lubricated so that it can be easily passed from the front to the back of the mouth. The food bolus is then voluntarily moved further into the oropharynx (upper part of the throat).

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). This step also requires the voluntary elevation of the soft palate in order to prevent food from entering the nose.

The muscles that control the oral phase of swallowing are stimulated by nerves located in the brainstem, called cranial nerves.

The cranial nerves involved in coordinating this stage include the trigeminal nerve, the facial nerve, and the hypoglossal nerve.

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 the esophagus (food pipe) by rhythmic and involuntary contractions of several muscles in the back of the mouth, pharynx, and esophagus.

Because the mouth and throat serve as an entryway for both food and air, the mouth provides a route for air to get into the windpipe and into the lungs, and it also provides a route for food to get into the esophagus and into the stomach.

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. These actions prevent food from going “down the wrong pipe” into the trachea (windpipe).

The closure of the larynx by the epiglottis protects the lungs from injury, as food and other particles that enter 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 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 esophagus has two important muscles that open and close reflexively as the food bolus is brought down during swallowing. These muscles, called sphincters, allow the food bolus to flow in a forward direction while preventing it from going in the wrong direction (regurgitation).

Both esophageal sphincters, first the upper, and then the lower, open in response to the pressure of the food bolus and close after the food bolus passes.

The upper esophageal sphincter prevents food or saliva from being regurgitated back into the mouth, while the lower esophageal sphincter ensures that food remains in the stomach, preventing regurgitation back into the esophagus.

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.

A Word From Verywell

If you have experienced a stroke or another neurological illness, you may undergo a swallowing evaluation to determine whether you have dysphagia. If you have signs of dysphagia, you will need to have speech and swallow therapy so that your swallowing muscles can have the chance to improve as much as possible.


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

 

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