Not only are there differences in individual contractions of the intestine, but there also are different patterns of con- tractions. 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).
30 s
Fig. 5.1 Intraluminal pressure changes recorded from the duodenum of a conscious human. Sensors placed 1 cm apart record changes in pressure that are phasic, lasting 4 to 5 seconds. Note that a rather large contrac- tion can take place at one site while nothing is recorded 1 cm away on either side.
39 CHAPTER 5 Motility of the Small Intestine
0 5 10 15 20 25 30 35 40 45
Time (s) 0
1 2 3 4
Occurrence (%)
Fig. 5.2 Frequency distribution of 7572 contractions recorded at one site in the human small intestine. Note that the contractions are most frequent at multiples of 5 seconds, the approximate interval between slow waves in this region. (From Christensen J, Glover JR., Macagno EO, et al: Statistics of contractions at a point in the human duodenum. Am J Physiol 221:1818-1823, 1971.)
A
B
Fig. 5.3 The influence of contractions on contents within a re- gion of intestine. Each panel depicts the region at three consec- utive points in time. (A) A contraction that is neither preceded nor followed by other contractions serves to mix and locally circulate the intestinal contents. (B) Contractions that have an orad-to-caudad (left-to-right) sequence serve to propel contents
This complex actually begins in the stomach (see Chapter 4). (See viedo: http://www.wzw.tum.de/humanbiolo- gy/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 bacte- rial 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.
In nonfasting persons, the MMC disappears, and con- tractions are spread more uniformly over time. In the human upper small bowel, contractions at any one site are present 14% to 34% of the recorded time. The most com- mon pattern consists of one to three sequential contrac- tions separated by periods of 5, 10, 15, or 20 seconds. These contractions are of variable intensity, with none as forceful as the intense contractions that occur during the MMC.
Contractions of the intestine are controlled by activ- ities of the ICCs and smooth muscle cells, as well as by nerves and humoral substances. As in the stomach, smooth muscle cells in the small intestine have a membrane poten- tial that fluctuates rhythmically with cyclic depolarizations and repolarizations of 5 to 15 millivolts (mV) (Fig. 5.6).
This slow wave activity (or basic electrical rhythm) is always present whether contractions are occurring or not.
At any one site in the intestine, slow wave frequency is con-
40 CHAPTER 5 Motility of the Small Intestine
100 mm Hg
100 mm Hg
Distal duodenum Proximal duodenum
100 mm Hg
Jejunum
1 min
Fig. 5.4 Contractions at three loci in the small bowel. Note that at each locus, phases of no or intermittent contractions are followed by a phase of continuous contractions that ends abruptly. Also note that the phase of continuous contractions appears to migrate aborally along the bowel. Such a pattern is called the migrating motor complex. (From Rees WD, Malagelada JR, Miller H: Human interdigestive and postprandial gastroin- testinal motor and gastrointestinal hormone patterns. Dig Dis Sci 27:321-329, 1982.)
50
Minutes 0
Number of contractions/min 12 8 4
0 100 150 200 250 300
0 12 8 4 0 12 8 4
Fig. 5.5 Graphic presentation of the number of contractions at three loci in the small bowel. The number of contractions during each minute of the recording was counted and plotted against the time of recording. The resulting histogram indicates cycles of activity at each locus. Note that migrating motor complexes recur at each locus at intervals of approximately 100 minutes and that the phases of intense contractions appear to migrate aborally. (From Vantrappen G, Janssens J: The interdigestive motor complex of normal subjects and patients with bacterial overgrowth of the small intestine. J Clin Invest 59:1158-1166, 1977.)
41 CHAPTER 5 Motility of the Small Intestine
the bowel. There is a decrease in frequency toward the ile- ocecal junction. In humans the frequency decreases from a mean of approximately 12 cycles/minute (cpm) in the duodenum to a mean of approximately 8 cpm in the termi- nal ileum. The decrease is not linear, because frequency is constant throughout the duodenum and for approximately 10 cm into the jejunum. Beyond that point frequency declines more or less linearly.
Although slow wave frequency is the same over the proximal small intestine, slow waves do not occur simul- taneously at all points. Multiple electrodes detect a proximal-to-distal phase lag that simulates a propagated signal (see Fig. 5.6).
Unlike in the stomach, slow waves themselves do not initiate contractions in the small intestine. Contractions are initiated by a second electrical event, often referred to as spike potential activity. Spike potentials are rapid depo- larizations of the smooth muscle cell membrane that occur only during the depolarization phase of the slow wave.
Because spike potentials are restricted to only one phase of the slow wave cycle, the contractions they initiate are phasic. The muscle relaxes during the repolarization phase of the slow wave cycle.
During periods when every slow wave is accompanied by spike potentials, the intestine at any site contracts at the same frequency as the slow wave frequency at that site.
Thus slow wave frequency sets the maximum frequency of contractions at any one site. In addition, because a gradient exists in the slow wave frequency along the small bowel, there is a gradient in the maximal frequency of contrac- tions. For most of the time, however, spike potentials do not accompany every slow wave. During these periods, contractions at any one site occur at multiples of the slow wave interval. Thus it is no coincidence that in the human proximal bowel, slow waves occur every 5 seconds and contractions occur at multiple intervals of 5 seconds.
Although slow waves at adjacent sites along the bowel are always present and are temporally related, the occur- rence of spike potentials is often localized. Thus sites 1 to 2 cm on either side of an area exhibiting slow waves with spike potentials may exhibit slow waves only. When this is the situation, segmenting contractions occur. Conversely, when the occurrence of spike potentials is not localized, slow waves do influence the spatial relationships of con- tractions at adjacent sites. The phase lag in occurrence of slow waves at adjacent sites imposes a phase lag in the occurrence of contractions at adjacent sites.
As explained previously, not every slow wave is accom- panied by spike potentials and muscle contraction. The occurrence of spike potentials and contractions is regu- lated by nervous activity and by circulating and locally released chemical agents. Certain reflexes depend on the intrinsic neurons, the extrinsic neurons, or both. The per- istaltic reflex (law of the intestines) described previously depends on an intact enteric nervous system. Application of neural blocking agents abolishes or greatly reduces this reflex. Another reflex, the intestino-intestinal reflex, depends on extrinsic neural connections. If an area of the bowel is grossly distended, contractile activity in the rest of the bowel is inhibited. Sectioning of the extrinsic nerves abolishes this reflex. Additionally, it is well known that changes in emotional state can induce alterations in small bowel motility. Thus the small bowel is under the influence of higher centers of the nervous system.
In addition to neural control, many circulating and endogenously released chemicals alter intestinal motility.
Epinephrine released from the adrenal glands tends to inhibit contractions. Serotonin, which is contained in large quantities within the small intestine, stimulates contrac- tions, as do certain of the prostaglandins.
Several hormones also alter intestinal motility. Gastrin, CCK, motilin, and insulin tend to stimulate contractions, whereas secretin and glucagon tend to inhibit them. The exact role of these chemical agents in the regulation of motility is yet to be clarified.
The presence of a characteristic pattern of contractions during fasting—the MMC—indicates complex controlling mechanisms. The hormone motilin may be involved in 1
2
3
4
5
6
Fig. 5.6 Slow waves and spike potentials from multiple sites in the small intestine. Tracings 1 to 6 illustrate activity from progressively distal areas. The solid line connecting the slow waves in tracings 1 to 4 denotes the apparent propagation in the region of a slow wave frequency plateau. Tracings 5 and 6 show decreases in slow wave frequency at more distal areas.
The rapid transients that occur on the peaks of some of the slow waves represent spike potentials.
42 CHAPTER 5 Motility of the Small Intestine regulating MMC cycle length. Plasma levels of motilin
fluctuate with the same periodicity as for the phase of intense duodenal contractions, and exogenously adminis- tered motilin initiates a premature MMC. Enteric nerves also are involved because their disruption alters MMC ini- tiation and migration. Extrinsic nerves do not seem to be required. Segments of intestine that have been extrinsically denervated still exhibit MMCs. However, extrinsic neural activity can modify characteristics of the MMC.
Feeding abolishes MMCs and institutes a pattern of more or less continuous contractions of varying ampli- tude. The change in pattern caused by feeding proba- bly is brought about by hormones such as gastrin and CCK, which are released during feeding, and by neural mechanisms.