There is a paradox in sleep. The apparent tranquility of someone who is resting would suggest total immobility. In reality, behind it lies a frenetic activity that takes place in the sleeper's brain. The night is 'silent' everywhere, but not here. When the person sleeps the neurons 'dance'. Or rather they do the cleaning. During sleep, brain cells produce bursts of electrical impulses that build up into rhythmic waves, a sign of increased brain cell function. But why is the brain active when we are resting? Slow brain waves are associated with restful, restorative sleep. And now scientists at Washington University School of Medicine in St. Louis have discovered that they help eliminate waste from the brain.
How 'washing' happens
Individual nerve cells coordinate to produce rhythmic waves that push fluid through dense brain tissue, washing it in the process. “These neurons – explains the first author of the study published in 'Nature', Li-Feng Jiang-Xie – are miniature pumps. The synchronized neural activity fuels the flow of fluids and the removal of debris from the brain”. The discovery of this process could pave the way towards the “possibility of delaying or even preventing neurological diseases, including Alzheimer's and Parkinson's disease, diseases in which excess waste – such as metabolic waste and junk proteins – they accumulate in the brain and lead to neurodegeneration“. Brain cells orchestrate thoughts, feelings and body movements and form dynamic networks essential for memory formation and problem solving. But to carry out such energy-demanding tasks, they need fuel. Their nutrient consumption from your diet creates metabolic waste in the process.
“It is essential that the brain disposes of this metabolic waste, which can accumulate and contribute to neurodegenerative diseases – explains Jonathan Kipnis, Distinguished Professor of Pathology and Immunology, senior author of the article -. We knew that sleep is a time when the brain starts a cleaning process to eliminate waste and toxins that it accumulates while awake. But we didn't know how this happens. These findings may be able to tell us about strategies and potential therapies to speed up the clearance of harmful waste and remove it before it can lead to disastrous consequences.” But cleaning the dense brain is no simple task.
The cerebrospinal fluid that surrounds the brain enters and winds through intricate cellular networks, picking up toxic waste as it travels. As it exits the brain, the contaminated fluid must pass through a barrier before pouring into the lymphatic vessels of the dura mater, the layer of outer tissue that envelops the brain beneath the skull. But what powers the movement of fluid in, through and out of the brain? By studying the brains of sleeping mice, researchers found that neurons drive the cleaning efforts by sending electrical signals in a coordinated manner to generate rhythmic waves in the brain, Jiang-Xie illustrates. The research team silenced specific regions of the brain so that neurons in those regions did not create rhythmic waves. Result: it was realized that without these waves, fresh cerebrospinal fluid could not flow through the silenced brain regions and trapped waste could not leave the brain tissue.
What happens to those who sleep less
“One of the reasons we sleep is to cleanse the brain,” Kipnis says. “And if we can improve this process, perhaps it is possible to sleep less and stay healthy. Not everyone has the advantage of sleeping 8 hours every night and sleep loss impacts health. Other studies have shown that mice genetically programmed to sleep less have healthy brains. Could it be because they clean waste from their brains more efficiently? Could we help people with insomnia by improving their brain's cleaning capabilities so they can survive on less sleep?” These are some of the questions that remain open.
Brainwave patterns change during sleep cycles. And, the authors add, it should be noted that higher, higher-amplitude brain waves move fluids more forcefully. Researchers are now interested in understanding why neurons emit waves with variable rhythmicity during sleep and which regions of the brain are most vulnerable to waste accumulation. “We think that the process of cleaning the brain is similar to washing dishes,” explains neurobiologist Jiang-Xie. “You start, for example, with a large, slow and rhythmic movement to clean the soluble waste splashed on the plate. Then you decrease the amplitude of the movement and increase the speed of these movements to remove particularly sticky food residues. Although the different amplitude and rhythm of hand movements, the overall goal remains the same: to remove different types of waste from the dishes. Perhaps, therefore, the brain adapts its cleaning method depending on the type and quantity of waste.”
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