The Three Phases of Swallowing

As easy as it might seem to you, swallowing is actually one of the most complicated actions carried out by our nervous system. This seemingly simple act requires a series of voluntary and involuntary actions that must occur in a precisely orchestrated three-part sequence, involving the function of multiple areas of the central 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 and chewed with the aid of 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 mouth, and from there into the oropharynx. 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 pharynx, 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 this stage include the trigeminal, facial and hypoglossal nerves.

The Pharyngeal Phase

As the bolus reaches the pharynx, special sensory receptors activate the involuntary part of swallowing. The reflex, which is mediated by the swallowing center in the medulla, causes the food to be further pushed back into the pharynx and esophagus by rhythmic but 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 the lungs.

The closure of the larynx by the epiglottis protects the lungs from injury, as food and other particles 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 rhythmic contractions. The esophagus has two important sphincters, namely the upper and lower esophageal sphincters, which under normal conditions prevent food or saliva from being regurgitated toward the mouth. In doing so, the esophageal sphincters serve as a physical barrier to regurgitated food.

Both esophageal sphincters, first the upper, and then the lower, open reflexively as food 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.

_{Source: Bronwyn Jones; Normal and Abnormal Swallowing, Imaging in Diagnosis and Therapy; 2002 Second Edition - Springer}