Objectives
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Compare the motility patterns of the small intestine during feeding and fasting.
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Describe the functions of slow waves and spike potentials in regulating contractions of the small intestine.
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Describe the functions and control of the migrating motility complex.
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Explain the peristaltic reflex and the intestino-intestinal reflex.
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Describe the motility changes that result in vomiting, and discuss the primary factors controlling it.
Motility of the small intestine is organized to optimize the processes of digestion and absorption of nutrients and the aboral propulsion of undigested material. Thus contractions perform at least three functions: (1) mixing of ingested foodstuffs with digestive secretions and enzymes, (2) circulation of all intestinal contents to facilitate contact with the intestinal mucosa, and (3) net propulsion of the intestinal contents in an aboral direction.
Anatomic Considerations
Contractions of the small intestine are effected by the activities of two layers of smooth muscle cells: an outer layer with the long axis of the cells arranged longitudinally and an inner layer with the long axis of the cells arranged circularly. In general, the circular muscle layer is thicker, and both layers are more abundant in the proximal intestine; they decrease in thickness distally to the level of the ileocecal junction.
The small intestine is richly innervated by elements of the autonomic nervous system. Within the wall of the intestine itself lie neurons, nerve endings, and receptors of the enteric nervous system. These neural elements tend to be concentrated in several plexuses. The most prominent, the myenteric or Auerbach plexus , lies between the circular and longitudinal layers of smooth muscle cells. Plexal neurons receive input from other neurons within the plexus, from receptors located in the mucosa and muscle walls, and from the central nervous system by way of the parasympathetic and sympathetic nerve trunks. Plexal neurons provide integrated output to smooth muscle cells of both muscle layers, to epithelial cells, and perhaps to endocrine and immune cells. Many neurotransmitters are present in the enteric nervous system, including acetylcholine, norepinephrine, vasoactive intestinal peptide, enkephalin, and other peptides. Furthermore, many nerves express nitric oxide synthase activity.
Interstitial cells of Cajal (ICCs) also are prominent in and adjacent to the enteric nerves and muscular layers of the intestine. At least two major classes of ICCs have been identified: those responsible for generating slow waves and those involved in mediating neural input to the smooth muscle cells.
Extrinsic innervation is supplied by the vagus nerve and by nerve fibers from the celiac and superior mesenteric ganglia (see Fig. 2.1 ). Many of the fibers within the vagus are preganglionic, whereas many from the abdominal ganglia are postganglionic. Some of the fibers within the vagus are cholinergic, whereas some from the abdominal plexuses are adrenergic. In addition, nerves that contain somatostatin, substance P, cholecystokinin (CCK), enkephalin, neuropeptide Y, and other transmitters have been identified in afferent and efferent vagal and splanchnic nerves. The exact pathways and physiologic roles of these nerves are being elucidated.
Types of Contractions
Between contractions, pressures within the lumen of the small intestine approximately equal the intraabdominal pressure. When the musculature contracts, the lumen is occluded partially or totally, and the pressure increases. Most contractions are local events and involve only 1 to 4 centimeters (cm) of bowel at a time. The contractions usually produce intraluminal pressure waves that appear as nearly symmetric peaks of uniform shape ( Fig. 5.1 ). In the human upper small bowel, contractions occur at any one site at multiple intervals of 5 seconds ( Fig. 5.2 ).
Occasionally other types of pressure waves can be recorded. One such type consists of an elevated baseline pressure that lasts from 10 seconds to 8 minutes. This wave seldom occurs alone and is usually accompanied by superimposed phasic changes in pressure.
The effect of any contraction on the intestinal contents depends on the state of the musculature above and below the point of the contraction. If a contraction is not coordinated with activity above and below, intestinal contents are displaced both proximally and distally during the contraction and may flow back during the period of relaxation. This serves to mix and locally circulate the contents ( Fig. 5.3A ). Such contractions appear to divide the bowel into segments, and this feature accounts for the name given to this process— segmentation . If, however, the contractions at adjacent sites occur in a proximal-to-distal sequence, aboral propulsion will result. (See http://www.wzw.tum.de/humanbiology/motvid01/movie_65_1mot01.wmv ; accessed March 2018).
The small intestine is also capable of eliciting a highly coordinated contractile response that is propulsive in function. When an area of bowel is stimulated (e.g., by placement of a solid bolus of material in the lumen), the bowel responds with contraction orad and relaxation aborad to the point of stimulation. These events tend to move the material in an aboral direction ( Fig. 5.3B ), and if they occur sequentially, they can propel a bolus the entire length of the gut in a short time. This peristaltic response, first described by Bayliss and Starling, is known as the law of the intestines. Often it is invoked to explain how material is normally propelled through the small bowel. However, peristalsis involving long segments of intestine is seldom seen in normal persons, although peristaltic contractions involving short segments (1 to 4 cm) of intestine have been described.
Patterns of Contractions
Not only are there differences in individual contractions of the intestine, but there also are different patterns of contractions. In fasting humans, contractions do not occur evenly over time. Rather, at each locus cycles consisting of phases of no or few contractions are followed by a phase of intense contractions (phase 3) that ends abruptly ( Fig. 5.4 ). The duration of each cycle is the same at adjacent loci of the bowel; however, the 5- to 10-minute phase of intense contractions does not occur simultaneously at all loci. Instead, this phase appears to migrate aborally, and it takes approximately 1.5 hours to sweep from the duodenum through the ileum. The characteristics of this pattern have earned it the title of migrating motor complex (MMC). This complex actually begins in the stomach (see Chapter 4 ). (See viedo: http://www.wzw.tum.de/humanbiology/motvid02/movie_06_1mot02.wmv; http://www.wzw.tum.de/humanbiology/motvid02/movie_08_1mot02.wmv accessed March 2018). Its functions appear to be to sweep undigested contents from the stomach, through the small intestine, and into the colon and to maintain low bacterial counts in the upper intestine. MMCs cycle at intervals of approximately every 1.5 hours ( Fig. 5.5 ) as long as the person is fasting.
