<P> After food is chewed into a bolus, it is swallowed and moved through the esophagus . Smooth muscles contract behind the bolus to prevent it from being squeezed back into the mouth . Then rhythmic, unidirectional waves of contractions work to rapidly force the food into the stomach . The migrating motor complex (MMC) helps trigger peristaltic waves . This process works in one direction only and its sole esophageal function is to move food from the mouth into the stomach (the MMC also functions to clear out remaining food in the stomach to the small bowel, and remaining particles in the small bowel into the colon). </P> <P> In the esophagus, two types of peristalsis occur: </P> <Ul> <Li> First, there is a primary peristaltic wave, which occurs when the bolus enters the esophagus during swallowing . The primary peristaltic wave forces the bolus down the esophagus and into the stomach in a wave lasting about 8--9 seconds . The wave travels down to the stomach even if the bolus of food descends at a greater rate than the wave itself, and continues even if for some reason the bolus gets stuck further up the esophagus . </Li> <Li> In the event that the bolus gets stuck or moves slower than the primary peristaltic wave (as can happen when it is poorly lubricated), stretch receptors in the esophageal lining are stimulated and a local reflex response causes a secondary peristaltic wave around the bolus, forcing it further down the esophagus, and these secondary waves continue indefinitely until the bolus enters the stomach . The process of peristalsis is controlled by medulla oblongata . Esophageal peristalsis is typically assessed by performing an esophageal motility study . </Li> </Ul> <Li> First, there is a primary peristaltic wave, which occurs when the bolus enters the esophagus during swallowing . The primary peristaltic wave forces the bolus down the esophagus and into the stomach in a wave lasting about 8--9 seconds . The wave travels down to the stomach even if the bolus of food descends at a greater rate than the wave itself, and continues even if for some reason the bolus gets stuck further up the esophagus . </Li>

What happens when the muscles in the stomach wall relax and contract