Recent research published in natural neuroscience This suggests that the brain is more mechanically connected to the body than previously realized. Scientists have discovered that contractions of the abdominal muscles compress blood vessels connected to the spine and brain, pushing out fluid within the skull that gently moves the brain. This physical shake provides evidence of how exercise benefits brain health by flushing out cellular waste.
Scientists set out to understand the specific mechanical origins of brain movements in awake animals. The central nervous system is encased in thick bone and appears insulated from the physical forces of the rest of the body. But Patrick Drew, a professor of engineering science and mechanics, neurosurgery, biology and biomedical engineering at Penn State, said the study builds on previous research detailing how sleep and neuron loss affect when and how cerebrospinal fluid flows through the brain.
“Our study explains how just moving around acts as an important physiological mechanism that promotes brain health,” said Drew, corresponding author of the paper. “In this study, we found that when your abdominal muscles contract, they push blood from your abdomen into your spinal cord, putting pressure on your brain and causing it to move, just like a hydraulic system.”
“Simulations show that this gentle brain movement facilitates fluid flow in and around the brain,” Drew said. “Fluid movement within the brain is thought to be important for removing waste products and preventing neurodegenerative diseases. Our study shows that a little movement is good, and that may be another reason why exercise is good for brain health.”
To observe these minute movements, the authors used two-photon microscopy, which allows high-resolution imaging of biological tissue. They examined 24 awake mice whose heads were temporarily held in place on a spherical treadmill. A special transparent window was surgically placed in the mouse’s thinned skull, allowing scientists to see the outer surface of the brain.
The scientists quickly switched the focus of the microscope between tiny glowing beads placed on the skull and specific cells in brain tissue. They precisely measured these shifts, capturing video frames approximately 40 times per second. The authors found that the brain shifts slightly forward and sideways during running.
Interestingly, the researchers observed that the brain changed in the moments before the mice moved, but immediately after they had to tighten their abdominal muscles to move their bodies further. Because the brain moved just before the legs moved, scientists suspected that the core muscles were to blame. Sensors implanted in the mice recorded spikes in abdominal muscle activity that perfectly predicted the timing of brain movements.
Drew, who also holds the title of deputy director of Penn State’s Huck Institute for Life Sciences, explained how pumps create pressure that drives fluid flow in hydraulic systems. In this case, the pump is an abdominal contraction that can be as light as the tension before sitting or taking a step.
To understand the physical connection, scientists mapped the blood vessels that connect the abdomen to the spine. They injected two mice with a special dye and scanned the animals using microcomputed tomography. This makes it possible to perform high-resolution 3D examinations of the entire organ. The scan revealed a special network of veins called the spinal venous plexus.
The contractions put pressure on the spinal venous plexus, a network of veins that connect the abdominal and spinal cavities, causing the brain to move. When your abdominal muscles contract, they squeeze blood from your abdomen and send it to your spinal column. The increased volume inside the spine forces fluid upwards, creating pressure waves that physically move the brain.
To confirm that it was abdominal contractions, and not other movements, that acted as a pump, the researchers applied gentle, controlled pressure to the abdomens of lightly anesthetized mice. They created a custom pneumatic belt that resembles a miniature blood pressure cuff. When the mice were subjected to no movement other than local mechanical pressure, which was lower than that experienced by humans with a blood pressure cuff, the mice’s brains changed.
“Importantly, the brain began to return to its baseline position as soon as the abdominal pressure was released,” Drew said. “This suggests that abdominal pressure can rapidly and significantly change the position of the brain within the skull.”
Now that we have confirmed the link between abdominal contractions and brain movement, the next step is to understand how fluid moves in the brain and whether brain movement can trigger fluid flow, Drew said. However, until now, there were no existing imaging techniques to visualize such rapid and subtle dynamics of fluid flow.
“Fortunately, an interdisciplinary team at Penn State was able to develop these techniques, including conducting live mouse imaging experiments and creating computer simulations of fluid motion,” Drew said. “This combination of expertise is critical to understanding these types of complex systems and how they impact health.”
Francesco Costanzo, professor of engineering science and mechanics, biomedical engineering, mechanical engineering and mathematics at Penn State, led the computational modeling. This model represented a simplified geometry of the mouse central nervous system.
“Modeling fluid flow in and around the brain presents unique challenges because there are not only simultaneous, independent movements, but also coupled, time-dependent movements,” Costanzo said. “To explain all of that, you have to account for the special physics that happens every time a fluid particle passes through one of the many membranes in the brain.”
“So we simplified it,” Costanzo said. “The brain is similar to a sponge in that it has a soft skeleton and body fluids can move through it.”
Costanzo explained that by simplifying the shape of the brain to the shape of a sponge, researchers can model how fluids flow within structures with various spaces, such as the brain’s wrinkles or the pores of a sponge.
“Continuing with the idea of the brain as a sponge, we also thought of the brain as a dirty sponge. How do you clean a dirty sponge?” Costanzo asked. “You turn on the faucet and squeeze it out. In our simulations, we were able to understand how the movement of the brain through abdominal contractions directs the flow of fluid throughout the brain and helps remove waste products.”
The simulations showed that sudden movement of the brain forces interstitial fluid (the fluid found in the microscopic spaces around cells) out of brain tissue. This outward flow is the opposite of what happens during sleep, when fluids are flushed deep into the brain to remove waste products.
Although these findings provide insight, there are some limitations that should be considered. The experiment required the mouse’s head to be kept completely still. In freely moving animals, the physical forces produced by moving the head up and down also act on the brain, adding further complexity.
Furthermore, the scientists only imaged the topmost part of the brain cortex. Deep regions of the brain tend to experience different types of stretching and movement during abdominal contractions. Computer simulations also rely on simplified shapes of the brain and spinal cord, which may not capture how fluids move through the complex anatomy of the real nervous system.
Professor Drew stressed that although further research is needed to fully understand the effects in humans, this study suggests that physical movement may help circulate cerebrospinal fluid around and within the brain, removing waste products and protecting against neurodegenerative diseases associated with waste accumulation.
“This kind of movement is very small,” Drew said. “It’s produced when you walk, contract your abdominal muscles, and do any physical activity. It can make a big difference to your brain health.”
The study, “Brain movement is driven by mechanical connections with the abdomen,” was authored by C. Spencer Garborg, Beatrice Ghitti, Qingguang Zhang, Joseph M. Ricotta, Noah Frank, Sara J. Mueller, Denver I. Greenawalt, Kevin L. Turner, Ravi T. Kedarasetti, Marceline Mostafa, Hyunseok Lee, Francesco Costanzo, and Patrick J. Drew.
