Peristalsis – the Dance of the Bowel
When we eat food, the content enters the lumen of the bowel. This initiates a cascade we call peristalsis.
To put it plain and simple, it is a universal phenomenon that occurs from the esophagus to the rectum as a response to food.
It is a combination of simultaneous contraction and relaxation of the bowel wall.
The stretch stimulus provided by the food leads to the contraction of circular muscles behind the stimulus while simultaneously causing an area of relaxation in front of it!
We can compare this to a swell (or a ripple) in the ocean, which, you know, has a trough followed by a crest.
This wave of contraction moves from oral to caudal direction propelling the chyme (partially digested food) forward at a rate of 2 - 25 cm/s.
The Mechanism Behind the Dance
The local stretch reflex stimulates the release of serotonin which stimulates the myenteric plexus of Auerbach
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These cholinergic neurons conduct both in anterograde as well as a retrograde fashion.
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Neurons that perform anterograde conduction secrete NO (nitric oxide) and VIP (vasoactive intestinal peptide), leading to relaxation in front of the stimulus.
Neurons that conduct in a retrograde fashion release substance P and acetylcholine, leading to smooth muscle contraction behind the chyme.
Segmentation
Now, the question arises, if the food keeps moving, how does it get digested and absorbed, right?
So in order to keep the food moving forward, but at the same time, get digested and absorbed, the intestine retards the movement of chyme down the GIT by what we call segmental motility.
Author: DrV
So what happens if you DON’T eat?
In periods of fasting, we know that the pacemaker cells of GIT don’t sit still. The pacemaker cells are the interstitial cells of Cajal, which lie within the wall of the GIT. They produce what is known as the basal electrical rhythm (BER) or the gastric slow waves.
“Now, it would be prudent for us not to confuse basal electrical rhythm (BER) and migrating motor complexes (MMC). They are two different entities.”
But do keep in mind that both are closely related;
When there are no MMCs ⟶ BER shows rhythmic oscillations.
In the presence of MMCs ⟶ The spikes are superimposed over the electrical oscillations.
The illustration below will help differentiate the two.
Author: DrV
Now let us see what happens between meals.
In such periods, the pattern of electrical and motor activity that occurs is called the migrating motor complex (MMC). This electrical activity spans across the GIT from the stomach to the distal ileum.
This electrical activity is initiated by motilin and cycles every 90-100 mins.
It is a cyclical electrical activity that occurs in three phases:
- Phase I: It is a quiescent phase with no electrical or mechanical activity
- Phase II: This phase consists of irregular electrical activity causing contractions that do not propagate.
- Phase III: This is the phase that consists of regular electrical activity lasting for about 5 minutes, causing contractions that propagate along the length of the intestine, from the stomach to the distal ileum.
Now, since motilin secretion is suppressed temporarily after a meal, the MMCs are abolished temporarily, and the segmental peristaltic activity occurs instead.
MMCs resume after 90-120 mins when motilin is secreted after digestion and absorption are complete!
This phenomenon is represented by the image given below.
Author: DrV
Wisdom from Grand Wizard Zeo
“MMCs occur in what we call an “inter-digestive period”. They aid in preparing the bowel for the next meal, and thus, they are dubbed as the inter-digestive housekeepers! Also, your stomach growling during your daily rounds is due to hunger contractions, which are caused by the MMCs!”
References:
- Ganong’s Review of Medical Physiology, 26th edition - Page no: 485 - 487
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3174087/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4885840/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1420140/
Author’s footnote
It so happens that peristalsis and MMCs are frequently tested in various competitive exams and are considered high yielding topics. Feel free to click on the references for a more in-depth reading if you so desire.
Feel free to click on the references for a more in-depth reading if you so desire.
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