Swallowing consists of chewing, a pharyngeal phase, movement of material through the esophagus, and the relaxation of the stomach to receive the ingested material.
Chewing has three major functions: (1) It facilitates swallowing and prevents injury to the lining of the pharynx and esophagus by reducing the size of the ingested particles and mixing them with saliva; (2) starches in food are exposed to salivary amylase, the enzyme that begins their chemical breakdown (digestion); and (3) it increases the surface area of ingested material, speeding up the rate at which it can be acted on by digestive enzymes. Particles are usually reduced to a few cubic millimeters in humans, but other animals such as the dog and cat almost bolt their food, chewing it only into lumps small enough to pass into the pharynx. The act of chewing is both voluntary and involuntary, and most of the time proceeds by reflex actions void of conscious input.
The chewing reflex is initiated by food in the mouth that inhibits muscles of mastication, and the jaw drops. A subsequent stretch reflex of the jaw muscles produces a contraction that automatically raises the jaw, closing the teeth on the food bolus. Compression of the bolus against the mucosal surface of the mouth inhibits the jaw muscles to repeat the process.
Normally, liquids are propelled immediately from the mouth to the oropharynx and swallowed. Swallowing begins with a posterior movement of the tongue, forcing the liquid ahead of it. At the start of a swallow of solid material, the tip of the tongue separates a bolus of material between the tongue and the hard and soft palates (Fig. 3A). The tongue then raises against the hard palate and sweeps backward, forcing the bolus into the oropharynx (Fig. 3B). At this time, the nasopharynx is closed by a downward movement of the soft palate and contraction of the superior constrictor muscles of the pharynx. Simultaneously, respiration is inhibited and the laryngeal muscles contract to close the glottis and elevate the larynx. A peristaltic contraction now begins in the superior constrictor muscles (Fig. 3B) and proceeds through the middle and inferior constrictors, propelling the bolus through the pharynx (Fig. 3C). The upper esophageal sphincter relaxes, allowing these contractions to move the material into the esophagus (Fig. 3D).
Although swallowing can be initiated voluntarily, these efforts fail unless there is something, at least a small amount of saliva, to trigger the swallowing reflex. A voluntary movement of the tongue begins the swallow by forcing material into contact with a large number of receptors lining the pharynx. These receptors initiate impulses via vagal and glossopharyngeal afferents to the swallowing center in the medulla (Fig. 4). Although the oral and pharyngeal phases of swallowing are brief, lasting less than a second, the afferent impulses arriving at the swallowing center evoke a coordinated sequential output of efferent activity lasting as long as 9 sec. This efferent activity occurs via a variety of nuclei including the nucleus ambiguus and those of the trigeminal, facial, and hypoglossal nerves, and sequentially activates the muscles of the pharynx and esophagus.
Receptors present in the pharynx and esophagus send feedback information to the swallowing center via interneurons and vagal afferents, leading to further coordination of the muscular contractions involved. The sequential nature of the firing of efferents from the swallowing center and the overall coordination result in the peristaltic nature of the contractions (Fig. 4).
Because the contractions of the muscles of the pharynx are under control by extrinsic nerves and the swallowing center, a variety of neurologic lesions result in swallowing problems. Cerebrovascular accidents involving the medulla and swallowing center result in a loss of the pharyngeal phase of swallowing. Aspiration is often the result of such lesions, because the movement of material through the pharynx and upper esophageal sphincter is no longer coordinated.
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