When we sleep at night, our brain is still active. Not with the same intensity as during the day, because our consciousness rests for a good reason: we should relax. To do this, the brain has to clean, among other things, just like cleaning teams in an office building do after the employees go home. The thinking organ literally flushes waste and toxins that arise from intensive daily activity out of the spaces between the cells and into the body’s wastewater system.
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Now scientists led by Jonathan Kipnis from the Washington University School of Medicine in St. Louis, Missouri, have used animal experiments with mice to find out how the brain drives the flushing process. They published their results in the journal “Nature” at the end of February.
The brain needs a lot of nutrients for its energy-intensive tasks. In complex networks, it controls cognitive processes such as thinking, planning and problem solving. To do this, it plans and initiates movements and processes sensory impressions and feelings. Accordingly, a lot of waste is generated, such as protein residues and messenger substances. The waste must not be left lying around, otherwise the thinking organ would poison itself. “It is critically important that the brain disposes of metabolic waste that… [sonst] accumulate and contribute to neurodegenerative diseases,” says Kipnis.
For a long time it was unclear how waste removal works because the brain did not appear to have a sewage system like the lymphatic system of the rest of the body. In 2013, scientists led by Maiken Nedergaard from the University of Rochester Medical Center in the US state of New York described that the brain has a similar but separate sewage system and called it the “glymphatic system”. The name is the shortened version of “glial-dependent lymphatic transport” because specialized glial cells in it form channels between nerve cells and around brain blood vessels.
Wave pool in the brain
These channels are filled with cerebrospinal fluid, the same fluid that surrounds the outside of the brain. For the flushing process, the glial cells fire coordinated electrical signals and in this way set the brain fluid in rhythmic motion, creating waves. In order to prove that the wave-like pulsation actually causes waste transport, the research team switched off certain brain regions in the test animals so that the neurons in these regions no longer generated rhythmic waves. Without these waves, fresh cerebrospinal fluid could not flow through the deactivated brain regions and trapped waste could not leave the brain tissue.
The wave pattern appears to change during sleep cycles. What is striking, the study authors write, is that higher brain waves with greater amplitude move the fluid with more force. “We think that the brain cleaning process is similar to washing dishes,” explains lead author and neurobiologist Jiang-Xie. “For example, you start with large, slow, rhythmic swipes to remove soluble waste that has splashed onto the plate.” Then scrub small areas with faster movements to remove particularly sticky food residue on the plate. It could be that the brain also adapts the cleaning intensity to the type and amount of waste materials.
Washing, cutting, laying
Sleep researcher Albrecht Vorster from the University Hospital of Bern praises the publication. Since the glymphatic system was first described, several research groups have found evidence of “nocturnal brainwashing”, but the theory was not entirely uncontroversial because important details had not yet been clarified. Research from the University of Washington now closes one of these gaps and describes a possible mechanism for cleaning.
“The important thing is that these rhythmic movements were independent of the actual firing pattern of the nerve cells,” so they have their own task. In addition, they took place almost exclusively in deep sleep. Sleep puts the brain back into a normal state, so to speak, which is important because “a number of diseases are associated with leftover protein residues that can subsequently form clumps,” says Vorster.
These diseases, also known as “dirty brain diseases”, included Alzheimer’s disease and Parkinson’s disease. “The risk of becoming ill increases after years of very short sleep or disturbed sleep – for example due to snoring with pauses in breathing or shift work. Sufficient, regular, restful sleep without snoring protects us from brain diseases.”
Vorster himself likes to describe the meaning of sleep like this: “Sleeping well is like washing, cutting and laying for the brain.” By washing he means flushing out the spaces between the cells. In this way, the brain tries to ward off diseases such as Alzheimer’s and other neurodegenerative diseases.
Cutting means cutting back connections between nerve cells that are no longer needed. Then the next day – similar to a tree that sprouts better after being cut – they can sprout new trees and make new contacts. When laying, the brain organizes, connects and consolidates daily experiences and information.
(jl)
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