In humans and most other mammals, food is chewed in the mouth and mixed with saliva. Periodically the tongue pushes a bolus (mass) of the chewed food toward the throat. By making contact with the soft palate at the back of the mouth cavity, the food bolus initiates swallowing, which is a complex series of reflexes. Swallowing propels the food through the pharynx (where the mouth cavity and nasal passages join) and into the esophagus (food tube). To prevent food from entering the trachea (windpipe), the larynx (voice box) closes, and a flap of tissue called the epiglottis covers the entrance to the larynx (Figure 50.11A).

Once a bolus of food enters the esophagus, it is moved toward the stomach both by gravity and by waves of muscle contraction called peristalsis (Figure 50.11B). The muscle of the upper region of the esophagus is striated (i.e., skeletal muscle) and is controlled by the CNS reflexes of swallowing. The muscles of the rest of the esophagus are smooth muscles controlled by the autonomic and enteric nervous systems.

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Smooth muscles of the gut contract in response to being stretched (see Figure 47.8). When a bolus of food reaches the smooth muscle region of the esophagus and stretches it, the muscle responds by contracting, thus pushing food toward the stomach. Why doesn’t the contraction of the esophageal smooth muscle push food back toward the mouth? The nerve net between the two smooth muscle layers coordinates the muscles so that when a region of the gut smooth muscle contracts, the circular smooth muscle just beyond it relaxes. This action, controlled by the enteric nervous system, is called an anticipatory wave of relaxation. The contractions of smooth muscles behind the relaxing area push food into it—in the direction of the anus. The resulting stretch of that region stimulates its circular smooth muscle to contract as the anticipatory wave of relaxation causes the smooth muscle of the next region to relax. The peristaltic waves of contraction and relaxation of the gut smooth muscle move food along the gut from the mouth to the anus.

At the junction of the esophagus and stomach is an esophageal sphincter, a ring of circular smooth muscle that is normally constricted. Waves of peristalsis cause the sphincter to relax enough to let food pass from the esophagus into the stomach. Sphincter muscles are found throughout the digestive tract: the pyloric sphincter governs the passage of stomach contents into the small intestine; the ileocaecal sphincter controls the flow of food between the small and large intestines; and the anal sphincter relaxes to allow defecation.

Some movements of the stomach are not as coordinated as the peristaltic movements of the esophagus and serve to move the contents of the stomach around and mix them with the digestive juices. However, peristaltic waves of contraction reorganize in the lower region of the stomach and push the semidigested material up against the pyloric sphincter which controls the opening to the small intestine. In the small intestine, peristalitic activity continues to move the gut contents toward the large intestine.