A team of the University of Oulu, Finlandhas identified a mechanism in human brain which acts as a “sewer system” during sleep. The finding, described in two studies published in Advanced Science and PNASoffers a new perspective on how the brain eliminates waste while the body rests. During sleep, the brain activates a kind of biological “sewer system” that facilitates the circulation of fluids and the elimination of metabolic waste.

Although this is a metaphorical comparison, it helps to understand a complex physiological process that until now has not been observed with this level of detail. The key to progress lies in a ultrafast magnetic resonance imaging technique developed by the Oulu functional neuroimaging group. This system allows us to follow the movement of water molecules in the cerebrospinal fluid in just five minutes and without the need to use contrast agents. Thanks to this, researchers have been able to measure more precisely how brain flows vary between waking and sleeping states.

The brain eliminates waste during sleep

According to the US National Institutes of Health (NIH), “while you sleep, your brain works. For example, Sleep helps prepare your brain to learn, remember and create. The brain has a drainage system that eliminates toxins during sleep.

The researchers verified that, during sleep, the natural pulsations that drive the movement of blood and cerebrospinal fluid modify their behavior. Specifically, respiratory pulsations and vasomotor waveswhich contribute to the internal transport of fluids in the brain, intensify, while the cardiac ones reduce their rate. This readjustment seems to promote more efficient filtration of water in the brain tissue.

This change is not minor, since it indicates that the sleeping brain reorganizes part of its activity to optimize your own debugging process. As blood vessels dilate and blood pressure decreases, the propagation of certain internal waves increases in speed. According to the Finnish team, this phenomenon coincides with a phase of greater efficiency in the elimination of waste substances accumulated during neuronal activity.

He Professor Vesa Kiviniemi, responsible for the studyexplained that during sleep “vasomotor waves, in particular, slow pulsations below 0.1 hertz, begin to locally influence not only the movement of fluids, but also the electrical activity of the brain.” In these regions, the brain’s “cleaning rate” increases markedly during sleep. The inverse relationship between heart rate and electrical activity not only challenges traditional conceptions of how the brain functions at rest, but also raises new questions about its potential implications for long-term brain health.

One of the applications proposed by researchers is brain aging monitoring. Available data indicate that the efficiency of cerebrospinal fluid circulation decreases with age, promoting waste accumulation and cognitive decline. Kiviniemi noted: ‘New measurement methods open the possibility of monitoring (and in the future, potentially treating) age-related changes in cerebrospinal fluid dynamics2.

In the technological field, the Oulu team has also developed portable devices capable of record brain electrical activity and blood flow during sleepwhich allows evaluating brain clearance without the need to resort to magnetic resonance imaging. The measurements obtained with this technology show high agreement with the results of imaging examinations, suggesting its possible incorporation into clinical settings and routine diagnostics in the near future.

Investigation

Their first study, published in February, evaluated the ability of their technique to magnetic resonance encephalography to record changes in pulse and fluid flow in the brains of 22 volunteers, both during sleep and wakefulness. The second work, published in March in Proceedings of the National Academy of Sciencescombined these resonance methods with monitoring of level-dependent blood oxygenation, cranial fluid flows, as well as infrared monitoring and real-time DC-EEG, analyzing sleep and wake patterns in 24 volunteers.

The entire process can be completed in about five minutes, although for each volunteer approximately 46 minutes of wakefulness and about an hour of sleep in different states. Kiviniemi’s team observed that the intense directional flow to neurons during wakefulness changed when the 24 young, healthy volunteers slept. “During sleep these interactions changed so that the directionality of the network was lost and the interactions became more bidirectional,” they noted in their second study. This increase in bidirectionality was especially evident in brain regions related to sensory function and cognitive processes, such as the posterior insula, thalamus, and superior cerebellum.