Abdominal Contractions May Drive Brain Fluid Flow, Aiding in Neural Waste Clearance

Data from a new study in Nature Neuroscience shows that the brain may be more mechanically connected to the body than previously appreciated. Using mice and computational simulations of fluid motion, the team identified a possible biological mechanism that helps explain why exercise benefits brain health. Specifically, they found that abdominal contractions compress blood vessels that are connected to the spinal cord and brain, which helps the organ move gently within the skull. This movement facilitates the flow of cerebrospinal fluid over the brain, potentially washing away neural waste and preventing the development of neurodegenerative disorders. 

The work, which is described in a paper titled “Brain motion is driven by mechanical coupling with the abdomen,” builds on past studies exploring how sleep and neuron loss influence how and when cerebrospinal fluid flushes the brain, according to Patrick Drew, PhD, a professor of engineering science and mechanics, neurosurgery, biology, and biomedical engineering at Penn State University. Drew is the corresponding author on the study. 

“Our research explains how just moving around might serve as an important physiological mechanism promoting brain health,” said Drew. The contraction of abdominal muscles to push blood from the abdomen into the spinal cord acts “just like in a hydraulic system” that puts pressure on the vertebral venous plexus, a network of veins that connect the abdominal cavity to the spinal cavity which causes the brain to move. Computational simulations show “that this gentle brain movement will drive fluid flow in and around the brain” removing harmful waste. 

To view this mechanism in moving mice, the scientists used two-photon microscopy, which allows for high-definition imaging of living tissue, and microcomputed tomography, which supports high-resolution three-dimensional examination of whole organs. They observed the brains shifting in the moments before the mouse moved and right after their abdominal muscles tightened, anticipating further movement. 

To ensure that the abdominal contractions were the reason for the observed shift rather than other movements, the scientists applied gentle and controlled pressure to the abdomens of anesthetized mice. They observed that the mice’s brains moved in response. “Importantly, the brain began moving back to its baseline position immediately upon relief of the abdominal pressure,” Drew said, suggesting “that abdominal pressure can rapidly and significantly alter the position of the brain within the skull.” 

The next step was digging deeper into the fluid’s movement in the brain as well as assessing if the brain’s movement could induce fluid flow. For this task, members of the team developed various techniques to capture this information including conducting imaging experiments of living mice and generating computational simulations of fluid motion. 

“Modeling fluid flow in and around the brain offers unique challenges because there are simultaneous, independent movements, as well as time-dependent, coupled movements,” explained Francesco Costanzo, PhD, a professor of engineering science and mechanics, biomedical engineering, mechanical engineering, and mathematics, who led the computational modeling aspects of the project. “Accounting for all of them requires accounting for the special physics that happens every time a fluid particle crosses one of the many membranes in the brain. So, we simplified it” using the analogy of a sponge for the brain. By simplifying it in this way, Costanzo explained, the team could model how fluid flows through a structure with varied spaces.  

Sticking with the analogy, “we also thought of it as a dirty sponge—how do you clean a dirty sponge?” Costanzo continued. “You run it under a tap and squeeze it out. In our simulations, we were able to get a sense of how the brain moving from an abdominal contraction can help induce fluid flow over the brain to help clear waste products.”  

Further studies are necessary to understand how this mechanism works in human bodies particularly how it cycle cerebrospinal fluid around the brain, and helps to protect against neurodegenerative disease. “This kind of motion is so small. It’s what’s generated when you walk or just contract your abdominal muscles, which you do when you engage in any physical behavior. It could make such a difference for your brain health,” Drew said.  Overall, “our research shows that a little bit of motion is good, and it could be another reason why exercise is good for our brain health.”  

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