After studying this chapter, you should be able to:
List the major forms of motility in the gastrointestinal tract and their roles.
Distinguish between peristalsis and segmentation.
Explain the electrical basis of gastrointestinal contractions and the role of basic electrical activity in governing motility patterns.
Describe how gastrointestinal motility changes during fasting.
Understand how food is swallowed.
Define the factors that govern gastric emptying and the abnormal response of vomiting.
Define how the motility patterns of the colon help to desiccate and evacuate the stool.
Digestion and absorption depend on a variety of mechanisms that soften the food, propel it through the length of the gastrointestinal tract, and mix it with bile and digestive enzymes. Some of these mechanisms depend on intrinsic properties of the intestinal smooth muscle. Others involve the operation of reflexes involving the neurons intrinsic to the gut, reflexes involving the central nervous system (CNS), paracrine effects of chemical messengers, and gastrointestinal hormones.
GENERAL PATTERNS OF MOTILITY
Peristalsis is a reflex response that is initiated when the gut wall is stretched, and occurs in all parts of the gastrointestinal tract. The stretch initiates a circular contraction behind the stimulus and an area of relaxation in front of it (Figure 27–1). The wave of contraction then moves in an oral-to-caudal direction, propelling the contents forward. Peristalsis can be increased or decreased by autonomic input, but its occurrence is independent of extrinsic innervation. It is an excellent example of the integrated activity of the enteric nervous system. Local stretch releases serotonin, which activates sensory neurons that activate the myenteric plexus. Cholinergic neurons in this plexus passing in a retrograde direction activate neurons that release substance P and acetylcholine, causing smooth muscle contraction behind the bolus. At the same time, cholinergic neurons passing in an anterograde direction activate neurons that secrete NO and vasoactive intestinal polypeptide (VIP), producing the relaxation ahead of the stimulus.
Patterns of gastrointestinal motility and propulsion. An isolated contraction moves contents orally and aborally. Segmentation mixes contents over a short stretch of intestine, as indicated by the time sequence from left to right. In the diagram on the left, the vertical arrows indicate the sites of subsequent contraction. Peristalsis involves both contraction and relaxation, and moves contents aborally.
When the meal is present, a motility pattern known as segmentation occurs that is designed to retard the movement of the intestinal contents to provide time for digestion and absorption (Figure 27–1). A segment of bowel contracts at both ends, and then a second contraction occurs in the center of the segment to force the contents (chyme) both backward and forward. Unlike peristalsis, retrograde ...