Scientists have demonstrated for the first time that the space between brain cells may increase during sleep, allowing the brain to flush out toxins that build up during waking hours.
"Sleep changes the cellular structure of the brain. It appears to be a completely different state," said Maiken Nedergaard, co-director of the Center for Translational Neuromedicine at the University of Rochester Medical Center in New York, and leader of the study.
In the study, Nedergaard and her colleagues unexpectedly found that sleep may be also be the period when the brain cleanses itself of toxic molecules.
During sleep a plumbing system called the glymphatic system may open, letting fluid flow rapidly through the brain.
Nedergaard's lab recently discovered the glymphatic system helps control the flow of cerebrospinal fluid (CSF), a clear liquid surrounding the brain and spinal cord.     

"It's as if Dr Nedergaard and her colleagues have uncovered a network of hidden caves and these exciting results highlight the potential importance of the network in normal brain function," said Roderick Corriveau, a programme director at National Institute of Neurological Disorders and Stroke (NINDS) in US.
Initially the researchers studied the system by injecting dye into the CSF of mice and watching it flow through their brains while simultaneously monitoring electrical brain activity.
The dye flowed rapidly when the mice were unconscious, either asleep or anaesthetized. In contrast, the dye barely flowed when the same mice were awake.
"We were surprised by how little flow there was into the brain when the mice were awake. It suggested that the space between brain cells changed greatly between conscious and unconscious states," Nedergaard said.
To test this idea, the researchers used electrodes inserted into the brain to directly measure the space between brain cells. They found that the space inside the brains increased by 60 percent when the mice were asleep or anaesthetized.
"These results may have broad implications for multiple neurological disorders. This means the cells regulating the glymphatic system may be new targets for treating a range of disorders," said Jim Koenig, a programme director at NINDS.
The study was published in journal Science.


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