Most of us are well-versed on how our digestive system works after we eat. How it mashes food, tosses in some digestive enzymes, and mixes it up so it can be broken down and absorbed.
But did you know that your gut is actually working 24/7, even when you’re not eating? In fact, what goes on during the time you’re not eating is crucial for gut health.
Just like the aftermath of your annual Christmas party, the fun doesn’t end when everyone leaves. Once that last guest leaves through the door the real fun begins: clean up.
Your gut also has this same after party clean up mode called the migrating motor complex(MMC). The migrating motor complex is a wave of electrical activity with the sole purpose to clean your gut.
The migrating motor complex is different from regular motility because it isn’t stimulated by contents in the gut. In fact, the migrating motor complex happens 3 hours after your last meal.
You’ve probably even experienced the migrating motor complex before. Maybe you wake up early and skip breakfast, then at the office you feel your stomach growl. That growling is the migrating motor complex, but only 1 part of it.
Even if you’re aware of the migrating motor complex, you may not be fully aware of the process. Yes, a major part is the transport of bacteria and waste from the stomach and small intestine in to the colon. But that’s not the whole thing.
One of the problems with focusing on the migrating motor complex from a motility viewpoint is that it makes the whole process confusing. What is gastric phase 3 and what is that in relation to duodenal phase 2?
Another big component of the migrating motor complex is the secretion of bile by the gallbladder. If we add this information to the motility picture the whole thing becomes much easier to understand. It also helps us understand why managing stress is so important for gut health. With that in mind, let’s take a look at the whole story behind the migrating motor complex.
In the beginning…
There is no consensus on what initiates the migrating motor complex. One of the reasons for this is that it can start in either the stomach or the duodenum.
The research points to a protein called motilin initiating the process in the stomach but not if it begins in the duodenum(1). Although, an argument can be made that the migrating motor complex always begins in the duodenum.
There are 3 phases of the migrating motor complex when looking at it from a motility perspective.
- Phase 1-No motor activity
- Phase 2-Low motor activity that slowly increases in frequency
- Phase 3-Strong, evenly spaced waves of motor activity that cause peristaltic contractions
A peak in plasma motilin levels does precede phase 3 activity in the stomach, but a couple of other things precede the peak in motilin. Specifically, we see phase 2 activity in the duodenum. This may not seem like such a big deal until you learn that the gallbladder empties bile in to the duodenum during phase 2(2).
Taken from: https://www.nhhepatobiliaryandpancreas.org/portals/583/Images/Content/gallbladder-diagram.jpeg
It’s an absolute necessity to have bile empty in to the duodenum before phase 3 of the stomach. The forceful phase 3 contractions in the stomach force open the pyloric sphincter separating the stomach and duodenum and deposits the acidic stomach contents in to the duodenum.
As phase 3 hits the duodenum, bicarbonate gets secreted to neutralize the acid. But, the pyloric sphincter remains open and some of this bicarbonate backs up in to the stomach. This helps restore it to resting pH(3). At this point most of the bile should be on its way. Reflux of bile back in to the stomach is not good.
From there the peristaltic contractions carry the bile through the gut. Along the way, the antimicrobial effects of bile acids kill unwanted bacteria and waves of muscular contraction sweep them in to the colon.
In the ileum, bile acids bind to receptors that release antimicrobial peptides and send the bile acids back to the liver for recycling. All the while, the gallbladder refills for the next cycle.
What I’ve just described to you is effectively a full cycle of the migrating motor complex and takes 90 minutes from start to finish. But what begins another cycle? Is it motilin?
Remember, the gallbladder dumps bile in to the duodenum during phase 2. Almost immediately, the gallbladder begins to fill again during phase 3. This process occurs through phase 1 in the duodenum until phase 2 causes it to empty again. But what causes phase 2?
One thought was that motilin initiates the whole process by causing gallbladder secretion and phase 3 contractions in the stomach at around the same time. But further investigation found that motilin peaks in the blood hit after the gallbladder empties. This indicates that gallbladder emptying may precede stomach contractions.
This brings the discussion to the natural observation that maybe the migrating motor complex is somehow regulated by the enterohepatic circulation of bile. There is a good amount of evidence to support this:
After a meal, gallbladder filling occurs approximately 3 hours after the meal. This coincides with phase 1 of the migrating motor complex in the duodenum(3)
A study in humans found that motilin spikes come after biliary emptying(4)
Another study in humans found that squeezing the gallbladder to empty it causes a motilin peak but a motilin peak didn’t induce gallbladder contraction(5)
There is also evidence to the contrary showing that motilin causes the gallbladder to empty(6). While this makes the picture a little cloudy on which comes first, one thing that all these studies show is that gallbladder filling and emptying is an integral part of the migrating motor complex.
Bile and the migrating motor complex
Bile doesn’t need to be the trigger of the migrating motor complex to be important. The gallbladder releases more bile for a longer period of time when phase 3 of the migrating motor complex begins in the stomach(7). This may lead to a more efficient housecleaning process.
Bile appears to stimulate motility in the duodenum. In both humans and rats, secretion of bile in to the duodenum precedes phase 3 contractions. Removing bile from the duodenum causes the disappearance of phase 3 contractions(3).
These factors indicate that cycles of the migrating motor complex that begin in the stomach are better. It also implies that bile is important for optimizing the migrating motor complex through 2 mechanisms: an antibacterial effect as well as a motility stimulating effect. For those who believe that they have a defective migrating motor complex, optimizing bile flow will more than likely help.
Stress and the migrating motor complex
Stress is another factor that will affect the migrating motor complex. Phase 3 contractions that begin in the stomach are regulated by the vagus nerve(8) while those in the duodenum are not. In dogs, acoustic stress(loud noises), almost completely eliminates phase 3 in the stomach(body & antrum). It has no effect in the duodenum.
To understand why this happens, it’s important to understand how stress affects the vagus nerve. The autonomic nervous system controls processes that are mostly out of our control such as heart rate, blood pressure, and digestion. There are 2 arms of the autonomic nervous system that work together called the sympathetic and the parasympathetic nervous systems.
You may know the sympathetic system as the fight or flight mode while the parasympathetic system is often referred to as rest and digest. These 2 arms work together like the brake and gas pedal of your car. Think of the sympathetic nervous system as the gas and the parasympathetic nervous system as the brake. When sympathetic nerves are stimulated parasympathetic nerves are inhibited, and vice versa.
When we are stressed, our foot is mashing the gas pedal down as hard as we can. When this happens, the brake doesn’t work well. Since our vagus nerve is made of parasympathetic nerve fibers, high levels of stress inhibit its function.
It only makes sense that stress reduces or eliminates phase 3 contractions that begin in the stomach because stimulating sympathetic nerves via stress inhibits the parasympathetic nerves of the vagus. Since the vagus nerve is how the gut-brain axis communicates, stress can impact any function that relies on this communication hub, especially digestion.
Sure, the gut can function without the brain, but not optimally. In fact, I don’t believe it’s possible to fix a gut disorder without addressing stress. It’s probably the number one reason most people fail.
The migrating motor complex is integral for maintaining gut health. By acting as a housekeeper, the migrating motor complex prevents bacterial overgrowth and optimizes digestion. The most effective cycles of the migrating motor complex begin in the stomach.
While many only focus on the motility component of the migrating motor complex, the secretory component is equally important. The secretion of bile during the migrating motor complex kills bacteria and helps regulate the motility component to sweep them away. Bile gets secreted longer and to a greater extent in cycles that begin in the stomach.
Finally, stress plays a pretty big role in the migrating motor complex. Higher levels of stress inhibit the vagus nerve which controls cycles that begin in the stomach. Therefore, stress can negatively impact the migrating motor complex by reducing motility and decreasing the release of antimicrobial bile acids.
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