Difficulty in swallowing is known as dysphagia, and pain on swallowing is defined as odynophagia. They may occur for both organic and nonorganic reasons. Because normal swallowing involves the integrated and coordinated function of the oral, pharyngeal, and esophageal muscles, disorders in any of these regions may cause dysphagia. To understand dysphagia, one must understand the swallowing mechanism. Swallowing involves coordination of both voluntary and involuntary muscular contraction, which can be divided into three phases—oral, pharyngeal, and esophageal.

Food is mixed with saliva during chewing to form the food bolus. The oral phase of swallowing is the only voluntary phase of the sequence. It occurs when the tongue moves the food bolus along its dorsum and propels it into the pharynx. The pharyngeal phase is triggered primarily by contact between the food bolus and the pharynx. Once triggered, the sequence of muscular contractions that make up the swallow continues involuntarily. First, the soft palate, posterior pharyngeal wall, and fossae contract to prevent nasopharyngeal reflux. The airway is protected as the laryngeal muscles contract. This draws the larynx upward beneath the hood of the base of the tongue as the epiglottis is depressed over the glottic inlet. With progressive pharyngeal muscle contraction, the bolus is moved into the hypopharynx and approaches the pharyngoesophageal sphincter (cricopharyngeus muscle), which relaxes and allows the bolus to be propelled into the upper esophagus to begin the esophageal phase. Peristaltic action of the upper, middle, and lower esophagus further propels the bolus toward the stomach. Appropriate relaxation of the lower esophageal sphincter is essential. Once the bolus has entered the stomach, the lower esophageal sphincter contracts and prevents reflux of gastric contents into the esophagus.