Swallowing

Swallowing occurs in three phases: First, in the oral phase, the tongue, which is under voluntary control, pushes food towards pharynx, which activates the swallowing reflex. The swallowing reflex is mediated by the vagus and glossopharyngeal nerves, which carry information to the medullary swallowing center; the medulla then sends motor signals to the striated muscles of the pharynx and upper esophagus. Then, in the pharyngeal phase, the soft palate pulls upward, blocking off the nasopharynx, and the epiglottis falls over the larynx to larynx to block off the respiratory tract (breathing is inhibited during the pharyngeal phase). The upper esophageal sphincter relaxes, and peristaltic waves propel the food into the esophagus. The esophageal phase is an involuntary phase in which a primary peristaltic wave travels down esophagus, pushing the food; If the primary wave isn’t sufficient to clear the esophagus, a secondary peristaltic wave is initiated by enteric nervous system. During this time, the upper esophageal sphincter closes so food can’t reflux to pharynx. When food arrives at the distal esophagus, the lower esophageal sphincter opens and food passes to the stomach; the lower esophageal sphincter opens in response to stimuli from the vagus nerve. Clinical correlation: In its tonically closed state, the lower esophageal sphincter prevents stomach contents, including acid, from refluxing into the esophagus. However, when intra-abdominal pressure is increased (as in pregnancy or obesity), or when there is an issue with the sphincter itself, stomach contents can be “pushed back” into the esophagus – a condition we call acid reflux, which can cause chest and throat pain known as “heart burn”