What can you do if you don't waste hours of sleep every day? In the long run, probably not much, because without sleep the brain is gradually poisoned. To prevent this from happening, nerve cells clean the tissue nightly while you sleep. A research team now reports in the journal Science Nature. So the cells create rhythmic waves that push fluid through dense brain tissue and carry away deposits.
Many people associate sleep with rest – but on the contrary, the brain is very busy during these hours, explains a team led by Jonathan Kipnis of Washington University in St. Louis. Brain cells have energy-intensive tasks: they control thoughts, feelings, and body movements and create dynamic networks essential for memory and problem solving. However, the processing of nutrients generates a large amount of metabolic waste.
The brains of higher organisms contain billions of neurons with high metabolic rates in complex networks. “It is critical for the brain to dispose of metabolic waste products that can accumulate and contribute to neurodegenerative diseases,” Kipnis explained. It turns out that sleep is the time to cleanse the body of toxins and other waste products accumulated during waking hours. “But we don't know how this happened.”
The sewage system in the brain was discovered only a few years ago
In the body, what is called lymph is carried along with waste products through the lymphatic system, which travels through the body in fine branches. However, the brain is protected from the rest of the body by the blood-brain barrier, which is the boundary between the bloodstream and the central nervous system. Thanks to special cells on the outside of the vessel wall, only selected substances can enter the brain. It protects the brain from harmful substances and pathogens.
But there is a “wastewater system” in the brain, discovered a few years ago. Like the lymphatic system, it is a flowing system. Transport fluid is eventually released into the lymphatic system. By studying anesthetized and sleeping mice, the researchers found that it was the coordinated activity of neurons that drove cerebrospinal fluid through dense brain tissue. Cells generate electrical impulses that condense into rhythmic waves. The waste-rich fluid then drains into lymphatic vessels in the dura mater – the outer layer of tissue that encloses the brain under the skull.
Researchers have also recorded that brain wave patterns change during sleep cycles. “We think the process of cleaning the brain is similar to washing the dishes,” explained lead author Li-Feng Jiang-Xie of the University of Washington of the suspected cause. Larger, slower pulses can remove material that dissolves more easily at first, while smaller, faster pulses can remove more stubborn deposits – especially like how you wipe sticky food residue off a plate. “Perhaps the brain adjusts its cleaning process depending on the type and amount of waste,” says Jiang-Chee.
If certain brain regions were turned off in a group of mice, the neurons in these regions no longer generated rhythmic waves and new cerebrospinal fluid could not flow, and the waste products there were not removed. Scientists believe the findings could provide clues to potential treatments for neurological diseases such as Alzheimer's and Parkinson's. The disposal of some harmful wastes can be accelerated in a targeted manner – with the aim of removing it before it leads to adverse consequences.