Porth's Essentials of Pathophysiology, 4e

685

Structure and Function of the Gastrointestinal System

C h a p t e r 2 8

FIGURE 28-6. Two types of small intestine movements: (A) Mixing segmentation waves in which slow contractions of the circular muscle layer occlude the lumen and drive the contents forward and backward; and (B) propulsive peristaltic movements in which segmental contractions followed by sequential relaxation moves the contents forward.

A

B

occur because of inflammation or are secondary to the effects of toxins or unabsorbed materials. Delayed pas- sage of chyme in the small intestine also can be a prob- lem. Transient interruption of intestinal motility often occurs after GI surgery. Intubation with suction often is required to remove the accumulating intestinal contents and gases until activity is resumed. Colonic Motility and Defecation The storage function of the colon dictates that move- ments in this section of the gut are different from those in the small intestine. Movements in the colon are of two types. First are the segmental mixing movements, called haustrations , so named because they occur within sacculations called haustra. These movements produce a local digging-type action, which ensures that all portions of the fecal mass are exposed to the intestinal surface. Second are the propulsive mass movements, in which a large segment of the colon ( ≥ 20 cm) contracts as a unit, moving the fecal contents forward as a unit. Mass move- ments last approximately 30 seconds, followed by a 2- to 3-minute period of relaxation, after which another con- traction occurs. A series of mass movements lasts only for 10 to 30 minutes and may occur only several times a day. Defecation normally is initiated by the mass movements. Defecation (discharge of feces from the rectum) is con- trolled by the action of two sphincters, the internal and external anal sphincters. The internal sphincter is a several- centimeters-long, circular thickening of smooth muscle that lies inside the anus. The external sphincter, which is com- posed of striated voluntary muscle, surrounds the internal sphincter. The external sphincter is controlledbynervefibers in the pudendal nerve, which is part of the somatic nervous system and therefore under voluntary control. Defecation is controlled by defecation reflexes. One of these reflexes is the intrinsic myenteric reflex mediated by the local enteric nervous system. It is initiated by distention of the rectal wall, with initiation of reflex peristaltic waves that spread through the descending colon, sigmoid colon, and rectum.

A second defecation reflex, the parasympathetic reflex, is integrated at the level of the sacral cord. When the nerve endings in the rectum are stimulated, signals are transmit- ted first to the sacral cord and then reflexively back to the descending colon, sigmoid colon, rectum, and anus by the pelvic nerves. These impulses greatly increase peristaltic movements as well as relax the internal sphincter. To prevent involuntary defecation from occurring, the external anal sphincter is under the conscious con- trol of the cortex. As afferent impulses arrive at the sacral cord, signaling the presence of a distended rectum, messages are transmitted to the cortex. If defecation is inappropriate, the cortex initiates impulses that con- strict the external sphincter and inhibit efferent para- sympathetic activity. Normally, the afferent impulses in this reflex loop fatigue easily, and the urge to defecate soon ceases. At a more convenient time, contraction of the abdominal muscles compresses the contents in the large bowel, reinitiating afferent impulses to the cord. ■■ Smooth muscle of the digestive tract propels food and fluids along its length with each part of the GI tract undergoing movements consistent with its digestive, absorptive, and storage functions. ■■ The smooth muscle in the wall of the GI tract undergoes cyclic self-propagating changes in membrane potentials, called slow waves, that maintain motility. Although the activity of GI smooth muscle is self-propagating, its rate and strength of contractions are regulated by input from the enteric and autonomic nervous systems. ■■ The enteric nervous system, which lies entirely within the gut, controls motility, intestinal SUMMARY CONCEPTS

(continued)