Describe the sequence of events in the pharyngeal-esophageal phase of swallowing, including the role of the swallowing center and the protective mechanisms that prevent food from entering the respiratory passages.

The pharyngeal-esophageal phase begins when a bolus stimulates tactile receptors in the posterior pharynx, triggering an involuntary swallowing reflex controlled by the swallowing center in the medulla and lower pons. Motor impulses, mainly via the vagus nerves, coordinate the pharyngeal and esophageal muscles. Protective mechanisms include momentary inhibition of respiration, blocking of all routes except the digestive tract, and closure of the epiglottis over the larynx to route food posteriorly into the esophagus.

In the pharyngeal-esophageal phase of deglutition, a bolus that has left the mouth stimulates tactile receptors in the posterior pharynx, which initiates the involuntary part of swallowing. The swallowing center in the medulla and lower pons controls this phase and sends motor impulses to the muscles of the pharynx and esophagus, primarily through the vagus nerves. Once food enters the pharynx, respiration is momentarily inhibited and all routes except the desired one into the digestive tract are blocked. The epiglottis closes off the larynx so that food is routed posteriorly into the esophagus rather than into the respiratory passageways. Muscular contractions, including those of the pharyngeal constrictor muscles, then propel the bolus into the esophagus. Solid foods pass from the oropharynx to the stomach in about 8 seconds, while fluids pass in 1 to 2 seconds and are aided by gravity.