The esophagus and stomach have specific motor functions that propel ingested material through the upper gastrointestinal tract, while the stomach also helps to grind the food into a more digestible form. The proximal, striated muscle portion of the esophagus quickly moves the bolus into the distal esophagus where smooth muscle contractions propel it through the lower esophageal sphincter into the stomach. In addition to allowing the bolus to pass, the lower esophageal sphincter is tonically contracted in its resting state, which prevents gastroesophageal reflux. The proximal stomach receptively relaxes to accommodate the swallowed bolus, while the distal stomach has functions to grind the food into smaller sizes to facilitate digestion. The antrum and pylorus have an additional function as a “sieve” to prevent emptying of particles until they have been reduced to an appropriate size. The stomach has a specific region that coordinates the motor activity of the stomach and to a degree the entire upper gastrointestinal tract (pacemaker region). This region initiates the periodic contraction profile that pushes both digested and undigested material through the gastrointestinal tract (phase III of the migrating motor complex). This complicated physiology is affected by both hormones and extrinsic innervation, but the pacemaker resides in the specialized nervous system of the gastrointestinal tract, most likely in the interstitial Cajal cells.
Practical Gastroenterology and Hepatology: Esophagus and Stomach p. 9-15