Call it a scoop on poop.
Researchers from McMaster University say they have solved a mystery of bowel movement that has puzzled the science community since the gut's nutrient-absorbing function was first discovered in the late 19th century.
The segmentation motion of the gut — the mixing and grinding of food within the intestine that allows nutrients to be absorbed into the bloodstream — is orchestrated by not only the nervous system, but also the intricate interplay between two cell networks, the researchers say.
Named interstitial cells of Cajal, these networks are known as pacemakers that send signals to the muscles and cause them to contract.
The first set of pacemakers, ICC-MP (myenteric plexus), is responsible for propulsion — the motion of pushing the content down the gut in a relatively linear fashion, much like how we squeeze toothpaste out of the tube.
The second set of pacemakers, ICC-DMP (deep myenteric plexus), is triggered only after nutrients are consumed, as the researchers discovered. It then signals the gut to move in “a checkered pattern” to hold and mix food content and promote nutrient absorption.
“So you have two pacemaker activities at the same time. When they interact, you get a very rhythmic motor activity,” said Jan Huizinga, professor in medicine at McMaster University and lead researcher of the study.
“The second pacemaker is absolutely essential, otherwise we wouldn't be able to absorb the nutrients,” he added.
Without ICC-DMP telling the gut muscles to wax and wane, Huizinga said, the omnipresent activity of propulsion would cause food to be flushed out too quickly to allow nutrient absorption.
Dominant theory rejected
The mechanism of propulsion has been well researched, the study says, but segmentation — the most common motor pattern of the small intestine — is still poorly understood, preventing attempts to develop treatment for conditions like constipation, diarrhea, bloating and malabsorption of nutrients.
The next step for Huizinga and his team is to further study the second pacemaker, ICC-DMP, to come up with a treatment for these conditions by modifying segmentation activities.
“The discovery of the second pacemaker gives us the cells we should focus on when we want to change segmentation,” he said.
Previously, the dominant theory suggests that segmentation is controlled by the nervous system only. But that was rejected two years ago when Huizinga's team used a nerve blocker to block all the nervous functions.
“Then we saw beautiful segmentation motor pattern and we said, 'Look at this. This can happen without any interference of the nerve system.' So then we changed all of our hypotheses. We changed all of our protocols, all of our project objectives,” he said.
“I am an expert on the pacemaker cell, but I never thought that segmentation is also caused by the pacemaker system.”
The team filmed the various movements of the rats' guts before and after nutrients were introduced. They then analyzed the footage and studied the electroactivity of the intestine.
Huizinga said collaborating with experts from different fields has been key to the study. For example, to understand how the two pacemaker networks communicate with each other, Huizinga worked with engineers from the University of Toronto. A mathematician from the University of Manitoba and clinicians from Wuhan University in China also took part in the study.
The study was published in science journal Nature Communications on Monday.