Factors that control motility of the large intestine are com- plex and poorly understood. As in the stomach and small intestine, motility in the large intestine is influenced by at least four factors: interstitial cells of Cajal–smooth muscle properties, enteric nerves, extrinsic nerves, and circulating or locally released chemicals.
The tone of the ileocecal sphincter is basically myo- genic. It is modified, however, by nervous and humoral factors. Distention of the colon causes an increase in sphincteric tension, a reflex mediated by enteric nerves (see Fig. 6.2B). Distention of the ileum causes relaxation, also mediated by enteric nerves (see Fig. 6.2A). Relaxation of the sphincter and an increase in the contractile activity of the ileum occur with or shortly after eating. This has been termed the gastroileal reflex. One view is that the reflex is mediated by the gastrointestinal (GI) hormones, primarily gastrin and cholecystokinin (CCK). Both these hormones cause an increase in the contractile activity of the ileum, as well as relaxation of the ileocecal sphinc- ter. Some investigators, however, think that this reflex is mediated via the extrinsic autonomic nerves to the intestine.
Smooth muscle cells of the ascending, transverse, descending, and sigmoid colon and of the rectum exhibit fluctuations in their membrane potential. Cyclic depo- larizations and repolarizations that possess some of the characteristics of small intestinal slow waves can be recorded. As in the small intestine, these slow waves are thought to depend on interactions between smooth mus- cle cells and interstitial cells of Cajal. Potential changes that resemble spike potentials also are recorded. These changes probably initiate contractions, but the exact rela- tionships between changes in potential and contractile activity have not been clarified. In addition, investigators have recorded various oscillations in membrane potential that fit descriptions of neither slow wave nor spike poten- tial activities. The origin and function of these phenom- ena are less clear.
30 20
Rectal distention
Change in pressure (mm Hg)
Internal sphincter 10
0
External sphincter 20
15 10 5 0
Fig. 6.4 Intraluminal pressure recorded at the level of the internal and external anal sphincters. Rectal distention caus- es relaxation of the internal sphincter and contraction of the external sphincter. Note, however, that the changes in sphinc- teric pressures are transient even though rectal distention is maintained. This finding is related to the accommodation of the stretch receptors within the wall of the rectum. (Modified from Schuster MM, Hookman P, Hendrix TR: Simultaneous manometric recording of internal and external anal sphincteric reflexes. Johns Hopkins Med J 116: 70-88, 1965.)
49 CHAPTER 6 Motility of the Large Intestine
Enteric neurons are involved in the control of colonic contractions; aperistaltic reflex can be initiated in the colon, and this reflex is mediated by nerves within the myenteric plexus. These plexal nerves seem to be pre- dominantly inhibitory because in their absence the colon is contracted tonically. Several colonic reflexes have their pathways in the extrinsic nerves. Distention of remote areas of the bowel induces an inhibition of contractions.
The pathway for this reflex includes the inferior mesenteric ganglion and also may include the spinal cord. In addition, several investigations have demonstrated that emotional state has a marked influence on colonic motility. These influences of the central nervous system are mediated by the extrinsic nerves.
The GI hormones, as well as epinephrine and the prosta- glandins, affect colonic motility. Gastrin and CCK increase
colonic activity and have been implicated in the mass movement that sometimes occurs after eating. Epinephrine inhibits all contractile activity, whereas the prostaglandins (primarily E type) decrease segmenting contractions and increase propulsive activity. The importance of these agents in regulating colonic motility is not known.
The rectosphincteric reflex and the act of defecation are under neural control. Part of the control lies in the enteric nervous system. The reflex, however, is reinforced by activ- ity of neurons within the spinal cord. Destruction of the nerves to the anorectal area can result in fecal retention.
The sensation of rectal distention, as well as voluntary control of the external anal sphincter, is mediated by path- ways within the spinal cord that lead to the cerebral cortex.
Destruction of these pathways causes a loss of voluntary control of defecation.
CLINICAL SIGNIFICANCE
Abnormal transit of material through the colon is com- mon. Delayed transit leads to constipation; in most situations, however, this is dietary in origin. There is a direct correlation among increased dietary fiber, increased colonic intraluminal bulk, and enhanced transit through the colon. How motility of the colon contributes to these changes in transit is not known.
Laxatives work either by osmotic effects (polyethylene glycol, magnesium citrate, lactulose, and sorbitol) or by increasing colonic propulsion (bisacodyl, sodium pico- sulfate, and glycerol). Alterations in motility and tran- sit are frequently caused by emotional factors and are indicative of the strong influence of the higher centers of the central nervous system on motility. The final effects of stress on colonic motility vary greatly from individual to individual. Most students are familiar with diarrhea previous to an important examination. The severity of the problem is usually related inversely to how well the student is prepared for the test.
A particularly interesting and dramatic clinical disorder in which severe constipation is seen is congenital mega- colon (Hirschsprung’s disease), which is characterized by an absence of the enteric nervous system in the dis- tal colon. The internal anal sphincter is always involved, and often the disease extends proximally into the rec- tum. The involved segment exhibits increased tone, has a very narrow lumen, and is devoid of propulsive activity.
As a result, the colon proximal to the diseased segment
becomes dilated, thus producing a megacolon. This con- dition is treated through surgical removal of the diseased segment.
In adults, the most common gastrointestinal disorder for which medical advice is sought is irritable bowel syndrome. This disorder gives rise most often to abdom- inal pain and altered bowel habit (constipation and/or diar- rhea). In limited observations, exaggerated segmental contractions in the sigmoid colon have been seen, partic- ularly in response to stimulants such as morphine. During stress, patients with irritable bowel syndrome and con- stipation exhibit increased segmentation in the sigmoid colon, whereas those with diarrhea exhibit decreased segmentation. In addition to motility disorders, sensory hypersensitivity to visceral stimulation may play a role.
The cause of this disorder remains unknown. One theory suggests that altered motility may reflect the condition- ing of autonomic responses from repeated exposure to stressful situations. Another suggests that gastrointesti- nal infections and alterations in normal colonic flora play a role in the origin of irritable bowel syndrome.
In older age groups, diverticula (outpouchings of mucosa that extend through the muscular wall) frequently develop in the colon. Evidence suggests that abnormal colonic motil- ity leads to diverticulum formation because of the genera- tion of increased intraluminal pressure. However, a direct correlation among abnormal motility, symptoms, and the presence of diverticula cannot always be demonstrated.
50 CHAPTER 6 Motility of the Large Intestine
S U M M A R Y
• The muscular anatomy of the colon is characterized by concentration of the longitudinal muscle into bands called taeniae coli. Contraction of the taeniae coli and the circular muscle results in haustrations.
• Most colonic contractions are of the segmenting type, which aid in the absorption of water and electrolytes.
The frequency of segmenting contractions is higher in the descending and sigmoid colon than in areas located more orad. This retards aboral progression.
• Aboral movement of contents is slow, usually taking days to pass material through the colon. Most aboral movement takes place during infrequent peristaltic contractions called mass movements.
• Tonic contraction of the internal anal sphincter maintains closure of the anal canal. Distention of the rectum elicits
relaxation of the internal anal sphincter and causes the urge to defecate. Defecation can be prevented by volun- tary contraction of the external anal sphincter while the rectum accommodates to the distention and the internal anal sphincter regains its tone. Relaxation of the external anal sphincter during this time leads to defecation.
• Colonic slow waves are cyclic depolarizations and repo- larizations of muscle cell membranes that appear to set the timing of segmental contractions. Neural activity and hormone levels influence the intensity of segmental contractions.
• Mass movements are regulated by activities of the intrin- sic and extrinsic nerves and possibly by the hormones gastrin and CCK. The rectosphincteric reflex is regulated by intrinsic, extrinsic autonomic, and somatic nerves.
K E Y W O R D S A N D C O N C E P T S
Cecum
Ascending/transverse/descending/sigmoid colon Rectum
Anal canal Taeniae coli
Internal anal sphincter External anal sphincter Haustra/haustrations Shunt fascicles
Ileocecal junction Mass movement Rectosphincteric reflex Defecation
Gastroileal reflex Constipation
Hirschsprung’s disease Irritable bowel syndrome Diverticula
S E L F - S T U D Y P R O B L E M S
1. What is the function of the ileocecal reflex, and how is it regulated?
2. How does the motility pattern of the colon lead to the storage of fecal material?
CLINICAL TESTS
Despite the large numbers of patients in whom disor- dered colonic motility is suspected, techniques for mon- itoring contractions are not in general clinical use. Most often, radiologic procedures are used to provide limited information. In one test, radiopaque markers are ingested daily for 3 days. On the fourth day, a radiograph is taken, and the number of markers in each region of the colon is noted and compared with normal values. Measure- ments of intraluminal pressures and myoelectric activity are feasible, especially in the sigmoid colon and rectum,
because these areas are readily accessible. To date, such techniques are mainly used in investigative studies.
The behavior of both the internal and the external anal sphincters, and the response to rectal distention, can be measured by the careful placement of two small intralu- minal balloons in the anal canal. A third balloon is placed in the rectum and is distended to monitor the compo- nents of the defecation reflex. This technique is useful in patients with suspected neurologic disorders that result in impaired defecation.
51 CHAPTER 6 Motility of the Large Intestine
SUGGESTED READINGS
Bharucha AE, Brookes SJH. Neurophysiologic mechanisms of human large intestinal motility. In: Johnson LR, ed.
Physiology of the Gastrointestinal Tract. 5th ed. Vol 1. San Diego: Elsevier; 2012.
Christensen J. The motility of the colon. In: Johnson LR, ed.
Physiology of the Gastrointestinal Tract. 3rd ed. Vol 1. New York: Raven Press; 1994.
Phillips SF. Motility disorders of the colon. In: Yamada T, Alpers DH, Laine L, et al., eds. Textbook of Gastroenterology. 3rd ed.
Vol 1. Philadelphia: Lippincott Williams & Wilkins; 1999.
Sarna S. Large intestinal motility. In: Johnson LR, ed. Physiology of the Gastrointestinal Tract. 4th ed. Vol 1. San Diego:
Elsevier; 2006.
O B J E C T I V E S
• Discuss the constituents and various functions of saliva.
• Understand the mechanisms leading to the formation of saliva.
• Explain the processes resulting in the tonicity of saliva and concentrations of its various ions.
• Describe the regulation of salivary secretion.
Salivary Secretion
7
Although the salivary glands are not essential to life, their secretions are important for the hygiene and comfort of the mouth and teeth. The functions of saliva may be divided into those concerned with lubrication, protection, and digestion. An active process produces saliva in large quan- tities relative to the weight of the salivary glands. Saliva is hyposmotic at all rates of secretion, and in contrast to the other gastrointestinal (GI) secretions, the rate of secretion is almost totally under the control of the nervous sys- tem. Another characteristic of this regulation is that both branches of the autonomic nervous system (ANS) stimu- late secretion. However, the parasympathetic system pro- vides a much greater stimulus than does the sympathetic system.