Objectives
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Describe the oral and pharyngeal events taking place during a swallow.
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Describe the pressures within the esophagus and oral stomach at rest and during a swallow.
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Explain the regulation involved during a swallow, including its initiation and peristalsis through the esophagus.
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Understand the process of receptive relaxation of the oral stomach, its function, and regulation.
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Understand gastric esophageal reflux disease (GERD) and its causes.
Swallowing consists of chewing, a pharyngeal phase, movement of material through the esophagus, and the relaxation of the stomach to receive the ingested material. Swallowing is almost purely a motility function. Digestion and absorption are minimal, in part because transport of the bolus into the stomach takes only seconds.
Chewing
Chewing has three major functions: (1) it facilitates swallowing by reducing the size of ingested particles and thus also prevents damage to the lining of the pharynx and esophagus; (2) it mixes food with saliva, which exposes the food to digestive enzymes and lubricates it; and (3) it increases the surface area of ingested material and thereby increases the rate at which it can be digested.
The act of chewing is both voluntary and involuntary, and most of the time it proceeds by reflexes void of conscious input. The chewing reflex is initiated by food in the mouth that inhibits muscles of mastication and causes the jaw to drop. A subsequent stretch reflex of the jaw muscles produces a contraction that automatically raises the jaw and closes the teeth on the bolus of food. Compression of the bolus on the mucosal surface of the mouth inhibits the jaw muscles to repeat the process.
Pharyngeal Phase
Normally liquids are propelled immediately from the mouth to the oropharynx and are swallowed. Swallowing is initiated by propulsion of material into the oropharynx primarily by movements of the tongue . The portion of solid material to be swallowed is separated from other material in the mouth so it lies in a chamber created by placing the tip of the tongue against the hard palate ( Fig. 3.1A ). The material is propelled by elevation and retraction of the tongue against the palate. As the material passes from the oral cavity into the oropharynx , the nasopharynx is closed by movement of the soft palate and contraction of the superior constrictor muscles of the pharynx ( Fig. 3.1B ). Simultaneously, respiration is inhibited, and contraction of the laryngeal muscles closes the glottis and raises the larynx . The bolus is propelled through the pharynx by a peristaltic contraction that begins in the superior constrictor and progresses through the middle and inferior constrictor muscles of the pharynx ( Fig. 3.1C ). These contractions, along with relaxation of the upper esophageal sphincter (UES), propel the bolus into the esophagus ( Fig. 3.1D ).
The oral and pharyngeal phases of swallowing are rapid, taking less than 1 second. Swallowing can be initiated voluntarily, but these efforts fail unless something, at least a small amount of saliva, triggers the swallowing reflex. Once initiated, however, swallowing proceeds as a coordinated involuntary reflex. Coordination is central in origin, and an area within the reticular formation of the brainstem has been identified as the swallowing center . Afferent impulses from the pharynx are directed toward this center, which serves to coordinate the activity of other areas of the brain such as the nuclei of the trigeminal, facial, and hypoglossal nerves, as well as the nucleus ambiguus ( Fig. 3.2 ). Efferent impulses from the center are distributed to the pharynx via nerves from the nucleus ambiguus. The impulses appear to be sequential, so the pharyngeal musculature is activated in a proximal-to-distal manner. This sequencing accounts for the peristaltic nature of the pharyngeal contractions. The center also appears to interact with other areas of the brain involved with respiration and speech. Injury to the swallowing center produces abnormalities in the pharyngeal component of swallowing.
Esophageal Peristalsis
The esophagus propels material from the pharynx to the stomach. This propulsion is accomplished by coordinated contractions of the muscular layers of the body of the esophagus. Because a large segment of the esophagus is located in the thorax, where the pressure is lower than in the pharynx and stomach, the esophagus also must withstand the entry of air and gastric contents. The barrier functions of the esophagus are accomplished by the presence of sphincters at each end of the organ.
Anatomically the esophageal muscle is arranged in two layers: an inner layer with the muscle fibers organized in a circular axis and an outer layer with the fibers organized in a longitudinal axis. The UES consists of a thickening of the circular muscle and can be identified anatomically as the cricopharyngeal muscle. This muscle, like the musculature of the proximal third of the esophageal body, is striated. The distal third of the esophagus is composed of smooth muscle; although the terminal 1 to 2 centimeters (cm) of the musculature acts as a lower esophageal sphincter (LES), no separate sphincter muscle can be identified anatomically. The middle third of the body of the esophagus is composed of a mixture of muscle types with a descending transition from striated to smooth fibers.
The events that occur in the esophagus between and during swallowing are often monitored by placing pressure-sensing devices at various levels in the esophageal lumen. Such devices indicate that between swallows, both the UES and the LES are closed and the body of the esophagus is flaccid ( Fig. 3.3A ). In the region of the UES, pressure is as much as 60 millimeters of mercury (mm Hg) higher than that in the pharynx or body of the esophagus. A zone of elevated pressure also is found at the LES. The length of this zone may range from several millimeters to a few centimeters, and the pressure may be 20 to 40 mm Hg higher than on either side. Pressures in the body of the esophagus are similar to those within the body cavity in which the esophagus lies. In the thorax the pressures are subatmospheric and vary with respiration; they drop with inspiration and rise with expiration. These fluctuations in pressure with respiration reverse below the diaphragm, and intraluminal esophageal pressure reflects intraabdominal pressure, which is slightly higher than atmospheric pressure.
