Intestinal Contractions and Motility

Two major types of contractions occur in the small intestine: peristalsis and segmentation. Peristalsis is much weaker in the small intestine than in the esophagus and stomach. Intestinal motility—the movement of chyme through the intestine—is relatively slow and is due primarily to the fact that the pressure at the pyloric end of the small intestine is greater than at the distal end.

Interstitial Cell Cajal Slow Waves

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■ Figure 18.15 Slow waves in the intestine. The slow waves are produced by the interstitial cells of Cajal (ICC), not by smooth muscle cells, and are apparently conducted by networks of ICC that are electrically joined together within the intestinal wall. Smooth muscle cells respond to this depolarization by producing action potentials and contracting. Note that the slow waves occur much slower (with a rate measured in seconds) than do the pacemaker potentials in the heart.

The major contractile activity of the small intestine is segmentation. This term refers to muscular constrictions of the lumen, which occur simultaneously at different intestinal segments (fig. 18.14). This action serves to mix the chyme more thoroughly.

Contractions of intestinal smooth muscles occur automatically in response to endogenous pacemaker activity, somewhat analogous to the automatic beating of the heart. In intestinal smooth muscle, however, the rhythm of contractions is paced by graded depolarizations called slow waves (fig. 18.15). Current evidence suggests that the slow waves are produced by unique cells, often associated with autonomic nerve endings. However, these pacemaker cells are neither neurons nor smooth muscle cells; they are the cells identified histologically as the interstitial cells of Cajal. These cells have long processes joined to each other and to smooth muscle cells by gap junctions, which permit the spread of depolarization from one cell to the next (fig. 18.16).

The slow waves are conducted between interconnected interstitial cells of Cajal through electrical synapses between these cells. Current evidence suggests that only the interstitial cells of Cajal can produce and conduct the slow waves. Smooth muscle cells, though electrically joined to each other (through electrical synapses termed nexuses) and to the interstitial cells of Cajal, cannot do this.

The slow waves produced and conducted by the interstitial cells of Cajal serve to depolarize the adjacent smooth muscle cells.

Structures

Functions

Interstitial cells of Cajal

Smooth muscle cells

Autonomic axon

Interstitial cells of Cajal

Smooth muscle cells

Autonomic axon

Cells Cajal

Production of slow waves

Conduction of slow waves to smooth muscle

Depolarization and opening of Ca2+ channels, production of action potentials

Neural input to ICC and smooth muscle

■ Figure 18.16 Cells responsible for the electrical events within the muscularis. The interstitial cells of Cajal (ICC) generate the slow waves, which pace the contractions of the intestine. Slow waves are conducted into the smooth muscle cells, where they can stimulate opening of Ca2+ channels. This produces action potentials and stimulates contraction. Autonomic axons have varicosities that release neurotransmitters, which modify the inherent electrical activity of the interstitial cells of Cajal and smooth muscle cells.

Production of slow waves

Conduction of slow waves to smooth muscle

Depolarization and opening of Ca2+ channels, production of action potentials

Neural input to ICC and smooth muscle

■ Figure 18.16 Cells responsible for the electrical events within the muscularis. The interstitial cells of Cajal (ICC) generate the slow waves, which pace the contractions of the intestine. Slow waves are conducted into the smooth muscle cells, where they can stimulate opening of Ca2+ channels. This produces action potentials and stimulates contraction. Autonomic axons have varicosities that release neurotransmitters, which modify the inherent electrical activity of the interstitial cells of Cajal and smooth muscle cells.

When the slow-wave depolarization exceeds a threshold value, it triggers action potentials in the smooth muscle cells by opening voltage-gated Ca2+ channels. The inward flow of Ca2+ has two effects: (1) it produces the upward depolarization phase of the action potential (repolarization is produced by outward flow of K+); and (2) it stimulates contraction (as described in chapter 12). Contraction may then be aided by additional calcium released from the sar-coplasmic reticulum through calcium-induced calcium release.

Autonomic nerves modify these automatic contractions of the intestine. When acetylcholine (from parasympathetic axons) stimulates its muscarinic ACh receptors in the smooth muscle cells, it increases the amplitude and duration of the slow waves. Thus, it increases the production of action potentials and promotes contractions and motility of the intestine. Inhibitory neu-rotransmitters, by contrast, hyperpolarize the smooth muscle membrane and thereby decrease the activity of the intestine.

Test Yourself Before You Continue

1. Describe the structures that increase the surface area of the small intestine and explain the function of the intestinal crypts.

2. Explain what is meant by the term brush border and give some examples of brush border enzymes. Why is it that many adults cannot tolerate milk?

3. Explain how smooth muscle contraction in the small intestine is regulated. What is the function of segmentation?

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Responses

  • Isengrin Roper
    What are the two types of contractions that occur in the small intestine?
    8 years ago
  • Stella Miller
    How to increase intestinal contractions?
    8 years ago
  • Robur Sackville
    How smooth muscle contraction in small intestines are regulated?
    8 years ago
  • joel
    What are the slow waves in the small intestine smooth muscle cells called?
    8 years ago
  • ulrike
    What are slow waves caused by smooth muscle cells and?
    8 years ago
  • connor muir
    What are intestinal contractions called?
    8 years ago
  • james
    Do peristalsis and segmentation happen simulataneously?
    8 years ago
  • ANNI
    How does the action potential of the axon stimulates the muscle cell?
    8 years ago
  • Klaudia
    How does action potential occur in gastrointestinal smooth muscles in humans?
    8 years ago

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