What could we do if we didn’t waste so many hours of sleep every day? In the long run, probably not much, because without sleep the brain would gradually poison itself. To prevent this from happening, nerve cells flush the tissue night after night while you sleep, a research team now reports in the scientific journal Nature. The cells therefore generate rhythmic waves that drive fluid through the 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, as the team led by Jonathan Kipnis from Washington University in St. Louis explains. Brain cells have energy-intensive tasks: They control thoughts, feelings and body movements and form dynamic networks that are essential for memory and problem solving. However, the processing of nutrients creates an abundance of metabolic waste.
Brains of higher organisms contain billions of neurons with high metabolic rates in complex networks. “It is critical that the brain dispose of metabolic wastes that can accumulate and contribute to neurodegenerative diseases,” Kipnis explained. It was known that sleep is a time of cleansing of toxins and other waste products that have accumulated while awake. “But we didn’t know how this happened.”
The sewage system in the brain was only discovered a few years ago
In the body, so-called lymph is transported with waste materials via the lymphatic system, which runs through the body in fine branches. The brain, however, is shielded from the rest of the body by the blood-brain barrier, the boundary between the bloodstream and the central nervous system. Thanks to special cells that lie on the outside of the vessel wall, only selected substances can pass into the brain. This protects the brain from harmful substances and pathogens.
But there is also a “wastewater system” in the brain, as was discovered just a few years ago. Similar to the lymphatic system, it is a flowing system. The transport fluid is ultimately released into the lymphatic system. By examining anesthetized and sleeping mice, the researchers found that it is a coordinated activity of neurons that drives the movement of cerebrospinal fluid through the dense brain tissue. The cells generate electrical impulses that condense into rhythmic waves. The waste-laden fluid then drains into the lymphatic vessels in the dura mater – the outer layer of tissue that encases the brain beneath the skull.
The researchers also recorded that brain wave patterns change during sleep cycles. “We think that the cleaning process of the brain is similar to washing dishes,” explained lead author Li-Feng Jiang-Xie from Washington University about the suspected cause. Large, slow pulses may initially remove easily soluble substances, while small, faster pulses may remove more stubborn deposits – similar to how you scrub away particularly sticky food residue on a plate. “Perhaps the brain adjusts its cleaning method depending on the type and amount of waste,” says Jiang-Xie.
If the team switched off certain brain regions in the mice, the neurons in these regions no longer generated rhythmic waves and no fresh cerebrospinal fluid could flow through, and waste products present there were not removed. The scientists hope that the findings gained could provide clues to potential therapies for neurological diseases such as Alzheimer’s and Parkinson’s. It is possible that the removal of certain harmful waste could be accelerated in a targeted manner – with the aim of eliminating it before it leads to dire consequences.