Swallowing begins with a voluntary (oral) phase that includes preparation during which food is masticated and mixed with saliva. This is followed by a transfer phase during which the bolus is pushed into the phar- ynx by the tongue. Bolus entry into the hypophar- ynx initiates the pharyngeal swallow response, which is centrally mediated and involves a complex series of actions, the net result of which is to propel food through the pharynx into the esophagus while preventing its entry into the airway. To accomplish this, the larynx is elevated and pulled forward, actions that also facilitate upper esophageal sphincter (UES) opening. Tongue pul- sion then propels the bolus through the UES, followed by a peristaltic contraction that clears residue from the 

pharynx and through the esophagus. The lower esoph- ageal sphincter (LES) relaxes as the food enters the esophagus and remains relaxed until the peristaltic con- traction has delivered the bolus into the stomach. Peri- staltic contractions elicited in response to a swallow are called primary peristalsis and involve sequenced inhibi- tion followed by contraction of the musculature along the entire length of the esophagus. The inhibition that precedes the peristaltic contraction is called deglutitive inhibition. Local distention of the esophagus anywhere along its length, as may occur with gastroesopha- geal reflux, activates secondary peristalsis that begins at the point of distention and proceeds distally. Tertiary esophageal contractions are nonperistaltic, disordered esophageal contractions that may be observed to occur spontaneously during fluoroscopic observation. 

The musculature of the oral cavity, pharynx, UES, and cervical esophagus is striated and directly inner- vated by lower motor neurons carried in cranial nerves (Fig. 4-1). Oral cavity muscles are innervated by the fifth (trigeminal) and seventh (facial) cranial nerves; the tongue, by the twelfth (hypoglossal) cranial nerve. Pha- ryngeal muscles are innervated by the ninth (glossopha- ryngeal) and tenth (vagus) cranial nerves. 

Physiologically, the UES consists of the cricopharyngeus muscle, the adjacent inferior pharyngeal constrictor, and the proximal portion of the cervical esophagus. UES innervation is derived from the vagus nerve, whereas the innervation to the musculature acting on the UES to facilitate its opening during swallowing comes from the fifth, seventh, and twelfth cranial nerves. The UES remains closed at rest owing to both its inherent elastic properties and neurogenically mediated contraction of the cricopharyngeus muscle.