As you learned in lab, the alimentary canal wall contains a muscular layer, composed of both circular and longitudinal smooth muscle layers. (The esophagus also has some skeletal muscle in it). In addition, the intestines tend to also have a smooth muscle layer that runs oblique to the canal axes. All of these sets of smooth muscle layers are used by the alimentary canal to move food and chyme along.
First, consider just the circular and longitudinal muscle layers. I have included the animation seen to the left to help you think about the muscular contractions. If only the longitudinal smooth muscle layer contracted, the canal would shorten. The longitudinal muscle would run from left to right in the animation, so its contraction would shorten the distance of the tube from left to right. At the same time that it is shortening, the canal would be forced into having a wider opening.
Meanwhile, if only the circular smooth muscle layer contracted, then the canal would get longer, but also narrower. Circular muscle runs in a ring around the tube, and its contraction is like the tightening of a girdle around the canal.
Oblique smooth muscle layers cause movements that look less coordinated, more like bending movements.
The alimentary canal uses these smooth muscle layers to make two general types of movement:
I will go through each of these on this webpage, and then I will also provide some location-specific details on how these movements are used.
When the contractions of the muscular layers are not coordinated, whatever is in the lumen of the alimentary canal ends up being mixed, mechanically crushed a bit, and generally thrown around. This is useful for breaking down food material by causing it to break into smaller pieces and by exposing all of it to digestive enzymes. When done more slowly, mixing can also be used to push material together, as seen in the large intestines.
For mixing, the muscular layers contract rhythmically, typically every 20 seconds or so. Contraction can occur rhythmically (meaning, repeated over intervals in time) without occurring in a coordinated fashion (meaning over intervals in space, or distance, along the canal).
This is a coordinated movement, caused by coordinated contractions of the muscular layers. It is shown nicely in Figure 17.4b in your book. Circular muscle contracting behind a bolus of food or behind the chyme, pushes it along. This is most elegantly done in the esophagus. After one location of circular smooth muscle contracts, the adjacent location contracts, then the next adjacent location contracts, etc., propelling the food along. Meanwhile, the circular smooth muscle relaxes right after it contracts.
When studying general biology, instead of A&P, one learns about many examples of peristaltic movement. Besides our alimentary canals moving food this way, other animals also use peristalsis to move food in their digestive systems. Some animals, like earthworms, move their entire bodies this way, using rhythmic, coordinated contractions.
How are these movements used by each digestive region?
No mixing, only peristalsis, occurs in the esophagus. See Figure 17.4b for this.
Figure 17.22 in your book shows stomach movements nicely. The mixing movements are the more random squeezing of the stomach wall, and are facilitated by a closed pyloric sphincter. The peristaltic movements are more coordinated and tend to push food into the pyloric region and even into the duodenum when the pyloric sphincter opens.
Because the small intestines have to do a lot of absorption, they do not want the chyme zooming through them quickly. Peristalsis occurs, but weakly, and it doesn't push the food far. Mixing occurs quite a bit, but a particular kind of mixing called segmentation. Segmentation would look almost like random squeezing movements and allow for the chyme to get continually broken up... it also can push the chyme backward a bit, enabling more time for absorption.
Here, the goal of the movement is to gather the chyme materials into a mass, forming the feces, and send this mass toward the anus. The movements that enable the large intestines to reach their goal are still just mixing and peristalsis. But now, these movements are slower, and more coordinated along longer stretches of the large intestines. The peristaltic waves are referred to as "mass movements," since they push the mass within them toward the anus. These larger movements force accumulation of materials, rather than break up of the mass. It is also peristalsis that enables the feces to leave the rectum when the anal sphincters are relaxed.
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