Question

What is/are the function(s) of the Muscularis externa portion of the GI wall? Compare & contrast the two types of motility performed.

Answer 1

The function(s) of the Muscularis externa portion of the GI wall are segmental contractions and peristalsis.

Segmentation Contractions
Segmentation contractions are the common type of movements of small intestine, which occur regularly or irregularly, but in a rhythmic fashion. So, these movements are also called rhythmic segmentation contractions.
The contractions occur at regularly spaced intervals along a section of intestine. The segment of the intestine involved in each contraction is about 1 to 5 cm long. The segments of intestine in between the contracted segments are relaxed. The length of the relaxed segments is same as that of the contracted segments. These alternate segments of contraction and relaxation give appearance of rings, resembling the chain of sausages.
After sometime, the contracted segments are relaxed and the relaxed segments are contracted (Fig. 43.2). Therefore, the segmentation contractions chop the chyme many times. This helps in mixing of chyme with digestive juices.

Peristalsis is defined as the wave of contraction followed by wave of relaxation of muscle fibers. In GI tract, it always travels in aboral direction. Stimulation of smooth muscles of intestine initiates the peristalsis. It travels from point of stimulation in both directions. But under normal conditions, the progress of contraction in an oral direction is inhibited quickly and the contractions disappear. Only the contraction that travels in an aboral direction persists.