Deglutition is the scientific term for the word swallowing. It came from the Latin word deglutire, which means "to swallow down." Muscles in the tongue, pharynx, and esophagus are involved in the deglutition process, which is divided into three phases: the oral phase, pharyngeal phase, and esophageal phase. Cranial nerves also play many important roles in these phases.
The oral phase is usually a voluntary phase, which means that it is often done consciously by the individual. It starts when food enters the mouth. Through the process of mastication, or chewing, food is broken into smaller pieces. Then the tongue pushes the bits of food towards the pharynx, or back of the throat. The cranial nerves that are involved during the oral phase are the trigeminal nerve, the facial nerve, and the hypoglossal nerve.
As soon as the food reaches the back of the throat, the pharyngeal phase, which is mostly involuntary, occurs. The uvula and soft palate usually prevent the passage of food towards the nose by covering the nasopharynx, which opens to the nasal cavities. At the same time, the deglutition reflex starts by propelling the foods towards the esophagus and away from the lungs with the help of the epiglottis, which is a flap of tissue which covers the larynx. During this process, deglutition apnea occurs, which means breathing stops for a very short time. Cranial nerves involved during this phase include the vagus, accessory, and hypoglossal nerves.
The esophageal phase, which also occurs involuntarily, starts with the opening of the esophageal sphincter to let the food enter. When food reaches the end of the esophagus, another muscle, called the cardiac sphincter, opens to let food pass to the stomach for digestion. A sphincter is a muscular ring that contracts and relaxes, and either prevents or allows the passage of materials from one area to the next, as between in the esophagus and the stomach.
Deglutition disorders sometimes occur in the elderly, especially after a stroke. Since the cranial nerves are often affected in stroke patients, food particles can sometimes enter their lungs and cause infection. Any birth defect, infection, obstruction, injury, or cancer growth in the pharynx or the esophagus can affect the deglutition process. The most common symptom of a deglutition disorder is dysphagia, or difficulty in swallowing. Another symptom is odynophagia, or pain during swallowing.