Final answer:
The outer circular layer of pharyngeal muscles, or pharyngeal constrictor muscles, contract to propel a food bolus through the oropharynx and laryngopharynx, allowing the upper esophageal sphincter to relax so the bolus can enter the esophagus and begin the journey toward the stomach through peristalsis.
Step-by-step explanation:
The outer circular layer of pharyngeal muscles, known as the pharyngeal constrictor muscles, play a critical role in deglutition, or swallowing. These muscles contract in a coordinated fashion to move the bolus (a mass of chewed food) through the oropharynx and laryngopharynx regions of the pharynx. As the food moves through the pharynx, the upper esophageal sphincter relaxes, allowing the bolus to enter the esophagus. This action initiates peristalsis, a sequence of wave-like muscle contractions, which propels the food toward the stomach.
The passage of food through the esophagus involves the sphincters and muscularization of the esophagus with the commencement of rhythmic peristalsis waves. This phase is under the control of the medulla oblongata. The muscles work in unison: when the circular muscle layer contracts, it pinches the esophagus, moving the bolus forward while the longitudinal muscles shorten the esophagus and widen it to receive the bolus. This process carries on until the food reaches the stomach, upon which distention of the esophagus signals the next phase.