Mastication, or chewing, mixes food with salivary mucus. This action subdivides food and exposes ingested starch to salivary amylase to begin the digestive process. Mastication is not essential for normal GI function but facilitates the process.
Swallowing propels food from the mouth into the esophagus. The initiation of swallowing is voluntary, but once started, the process continues involuntarily. During the voluntary oral phase, the tongue positions food against the hard palate. Entrance of the bolus into the pharynx initiates a swallowing reflex. The pharyngeal phase is involuntary. The reflex is initiated by tactile receptors at the entrance to the pharynx. There is integration at the medulla and pons in the “swallowing center.” Motor output is via cranial nerves, including the vagus.
During swallowing, the soft palate retracts to close the nasopharynx, and the vocal cords contract. The epiglottis moves to close to the trachea to prevent aspiration. Simultaneously, respiration is inhibited. The upper esophageal sphincter relaxes, and the pharyngeal muscles contract to move the bolus through the pharynx. The peristaltic wave forces food past the relaxed upper esophageal sphincter and into the esophagus.
The esophagus is a combination of striated and smooth muscle. The upper third of the esophagus is skeletal muscle. Both types of muscle are present in the middle third of the esophagus, and the lower third is only smooth muscle. In the esophagus, the vagus innervates both smooth and skeletal muscle. Sphincters isolate the lumen of the esophagus from the remainder of the GI tract. Contraction of the upper esophageal sphincter prevents entry of tracheal air. Contraction of the lower esophageal sphincter prevents reflux of gastric contents.
The entry of a bolus of food into the esophagus initiates the esophageal phase of swallowing. While the oral phase of swallowing is voluntary, the esophageal phase is involuntary. Contraction of the upper esophageal sphincter isolates the pharynx from the esophagus. A primary peristaltic wave moves food through the esophagus in 10 seconds. Secondary peristalsis is initiated by esophageal distention via the enteric nervous system, and it propels food still remaining in the esophagus toward the stomach. Relaxation of the lower esophageal sphincter is mediated by the vagus and enteric nerves, using the neurotransmitters nitric oxide (NO) and vasoactive intestinal peptide (VIP) to inhibit smooth muscle contraction.
Abnormalities of the swallowing reflex include disorders of both the esophageal smooth muscle and the muscle tone of the lower esophageal sphincter. A diffuse esophageal spasm results from an inappropriately strong peristaltic contraction. Failure of the lower esophageal sphincter to remain closed allows gastric reflux, causing esophagitis (heartburn). Chronic gastric reflux causes ulceration of the esophagus and can lead to a series of histologic changes known as Barrett’s esophagus. Achalasia occurs if the lower esophageal sphincter does not relax sufficiently to allow food to pass.