Swallowing also involves the stomach. In terms of motil- ity functions, the stomach can be divided into two major areas: the orad portion, which consists of the fundus and a
portion of the body; and the caudad portion, which con- sists of the distal body and the antrum (Fig. 3.5). These two regions have markedly different patterns of motility that are responsible, in part, for two major functions: accom- modation of ingested material during swallowing and regulation of gastric emptying. Accommodation is attrib- utable primarily to activities of the oral region, whereas both regions are involved in the regulation of gastric emp- tying (see Chapter 4).
During a swallow, the orad region of the stomach relaxes at about the same time as the LES. Intraluminal pressures in both regions fall before arrival of the swal- lowed bolus because of active relaxation of the smooth muscle in both regions (see Fig. 3.3B). After passage of the bolus, the pressure in the stomach returns to approx- imately what it was before the swallow. This process has been termed receptive relaxation. Because relaxation happens with each swallow, large volumes can be accom- modated with a minimal rise in intragastric pressure. For example, the human stomach can accept 1600 cubic cen- timeters (cc) of air with a rise in pressure of no more than 10 mm Hg.
Receptive relaxation is mediated by a vagovagal reflex, a nervous reflex that has its afferent and efferent path- ways in the vagus nerve. If this nerve is transected, recep- tive relaxation is impaired and the stomach becomes less distensible. Studies indicate that vagal impulses may act through 5-hydroxytryptamine receptors to release NO to cause the muscle to relax.
Fundus
Body
Antrum
Fig. 3.5 Divisions of the stomach. For discussions of secre- tion, the stomach usually is divided into the fundus, body, and antrum. For discussions of motility, it can be divided into an orad area and a caudad area. Shading denotes the approximate extent of the caudad area.
26 CHAPTER 3 Swallowing
S U M M A R Y
• Swallowing is initiated voluntarily, but once initiated, it proceeds as an involuntary reflex.
• Swallowing is accomplished by peristaltic contraction of pharyngeal muscles, during which time the UES relaxes.
This is followed by peristaltic contraction of the esoph- ageal musculature, during which time the LES and the orad region of the stomach relax.
• Peristaltic contraction of the pharyngeal muscles, relax- ation of the UES, and peristaltic contraction of the stri- ated muscle of the upper esophagus are regulated by pathways within the central nervous system. Peristaltic contraction of the smooth muscle of the lower esoph- agus and relaxation of the LES are regulated by path- ways within the central nervous system and by pathways within the intrinsic nerves.
CLINICAL APPLICATIONS
Contractions of pharyngeal muscle are controlled solely by extrinsic nerves. Therefore certain neurologic diseases (e.g., cerebrovascular accident) can have an adverse effect on this phase of swallowing. Aspiration often occurs because contractions in the pharynx and upper esophageal sphincter (UES) are no longer coordinated. A similar clinical picture can be seen in diseases that affect striated muscle or the myoneural junction.
Diseases affecting the smooth muscle portion of the esophagus predictably cause abnormalities in peristalsis and in the tone of the lower esophageal sphincter (LES).
In achalasia, for example, the LES often fails to relax com- pletely with swallowing. This may be coupled with a loss of peristalsis in the esophageal body, a complete absence of contractions, or the appearance of simultaneous rather than sequential contractions, with resulting impaired transit.
Patients with this disorder have considerable difficulty swallowing, often aspirate retained esophageal content, and may become severely malnourished. This disor- der has been attributed to abnormalities in the enteric nerves. In another disorder, diffuse esophageal spasm, simultaneous contractions of a long duration and high amplitude can occur. Affected persons have difficulty swallowing and may complain of chest pain. Although not always symptomatic, abnormalities in the esophageal
component of swallowing can occur as part of a variety of systemic diseases. Examples are diabetes mellitus, chronic alcoholism, and scleroderma.
Motor dysfunction also can play an important contribu- tory role in the pathogenesis of other esophageal diseases.
The most common symptom associated with esopha- geal dysfunction is heartburn. This burning sensation is caused by the reflux of gastric acid into the esophagus and the resulting injury to the esophageal mucosa. This condition may be produced by motor abnormalities that result in abnormally low pressures in the LES or by the failure of secondary peristalsis to empty the esophagus effectively. Reflux may also occur if intragastric pressure increases, as may occur after a large meal, during heavy lifting, or during pregnancy. Persistent reflux and the resulting inflammation lead to gastroesophageal reflux disease (GERD). This condition is usually treated effec- tively with inhibitors of gastric acid secretion. In some cases a region of the proximal stomach moves through the diaphragm into the thorax and produces severe gas- tric reflux. This condition is termed hiatal hernia and is often treated by surgery. Reflux itself is not abnormal, and it occurs several times a day. Under normal conditions the refluxed acid is cleared from the esophagus, and no symptoms develop.
CLINICAL TESTS
Swallowing is assessed clinically by x-ray examination with barium and by esophageal manometry. In the x-ray study, the patient swallows a bolus of liquid barium sulfate. Because this material is radiopaque, it can be observed fluoroscopically and recorded on the x-ray film as it traverses the esophagus, thereby providing a qualita- tive description of motor events in both the pharynx and the esophagus.
If a more detailed description of events is required, or if motor disorders such as those just described are sus- pected, esophageal manometry is often useful. Catheters
are passed through the nose or mouth so that their tips lie in various regions of the esophagus. Pressures detected by the catheters then are recorded between and during swallowing small sips of water. These recordings pro- vide a quantitative description of events occurring in the sphincters and body of the esophagus.
A useful test for episodes of acid reflux is 24-hour moni- toring of intraesophageal pH. A small pH probe is inserted nasally and positioned 5 cm above the lower esophageal sphincter. A small battery-powered computer is used for continuous recordings of pH.
27 CHAPTER 3 Swallowing
• Contraction of the pharynx and esophagus can be initi- ated by swallowing (primary peristalsis). Contraction of the esophagus can be initiated by stimulation of recep- tors within the esophagus (secondary peristalsis).
• Tonic contraction of the LES between swallows is the result of an interplay of excitatory and inhibitory neural and hormonal influences acting on an intrinsic myo- genic contraction. During a swallow, intrinsic nerves
release a transmitter (perhaps NO or VIP) to cause muscle relaxation.
• Contraction of the orad stomach is the result of an inter- play of excitatory and inhibitory neural and hormonal influences acting on an intrinsic myogenic contraction.
During a swallow, the orad stomach relaxes (recep- tive relaxation) in response to activation of inhibitory nerves in the vagus.
K E Y W O R D S A N D C O N C E P T S
Oral ingestion Chewing reflex Oropharynx Tongue Oral cavity Nasopharynx Glottis Larynx
Peristaltic contraction Pharynx
Upper esophageal sphincter Swallowing center
Esophagus
Lower esophageal sphincter Primary peristalsis
Secondary peristalsis Vagotomy
Orad portion Caudad portion Receptive relaxation Vasovagal reflex Achalasia
Diffuse esophageal spasm Heartburn
Gastroesophageal reflux disease Hiatal hernia
S E L F - S T U D Y P R O B L E M S
1. What is the difference between the innervation of the striated and smooth muscle portions of the esophagus and its physiologic consequences?
2. How does the body cope with the fact that much of the esophagus lies within the thorax?
SUGGESTED READINGS
Biancani P, Harnett KM, Behar J. Esophageal motor function.
In: Yamada T, Alpers DH, Laine L, et al., eds. Textbook of Gastroenterology. 3rd ed. Vol. 1. Philadelphia: Lippincott Williams & Wilkins; 1999.
Mittal RK. Motor function of the pharynx, the esophagus, and its sphincters. In: Johnson LR, ed. Physiology of the Gastrointestinal Tract. 5th ed. Vol. 1. San Diego: Elsevier;
2012.
Roman C, Gonella J. Extrinsic control of digestive tract motility.
In: Johnson LR, ed. Physiology of the Gastrointestinal Tract.
2nd ed. Vol. 1. New York: Raven Press; 1987.
Shaker R. Pharyngeal motor function. In: Johnson LR, ed.
Physiology of the Gastrointestinal Tract. 4th ed Vol. 1. San Diego: Elsevier; 2006.
Tack J. Neurophysiologic mechanisms of gastric reservoir func- tion. In: Johnson LR, ed. Physiology of the Gastrointestinal Tract. 5th ed. Vol. 1. San Diego: Elsevier; 2012.
O B J E C T I V E S
• Describe the contractions of the orad and caudad regions of the stomach.
• Explain the regulation of the contractile activity of the stomach, including the role of slow waves.
• List the components of the gastric contents that affect the rate of gastric emptying.
• Understand the role of duodenal receptors in the regu- lation of gastric emptying.
• Describe the changes in motility that regulate gastric emptying.
• Describe the disorders that may result in an impair- ment of gastric emptying.
Gastric Emptying
4
Motility of the stomach and upper small intestine is orga- nized to accomplish the orderly emptying of contents into the duodenum in the presence of ingested material of variable quantity and composition. Accommodation and temporary storage of ingested material result from receptive relaxation of the orad stomach (see Chapter 3).
Emptying, which also requires mixing ingested material with gastric juice and reducing the particle size of any sol- ids that have been swallowed, results from integrated con- tractions of the orad stomach, caudad stomach, pylorus, and duodenum.