Swallowing is a series of events where food or liquid is passed from the mouth into esophagus. The process is largely involuntary and is highly coordinated. Approximately 50 pairs of muscles and many nerves are operative in each swallowing event.

When food is ingested, it is prepared for swallowing by chewing (mastication). During mastication, there is an introduction and immersion of the food bolus into saliva. Serving as a predigestive fluid that breaks food down into biochemically useful components, saliva also acts as a lubricant for the swallowing process.

The swallow response begins with a pushing of the food or liquid to the back of the mouth by the tongue. This is known as the oral stage. The next stage, called the pharyngeal stage, starts as the food or liquid moves past the pharynx (a canal that connects the mouth with the esophagus) into the...