A good night's rest may literally¹ clear the mind. Using mice, researchers showed for the first time that the space between brain cells may increase during sleep, allowing the brain to flush out（冲掉，排出） toxins² that build up during waking hours. These results suggest a new role for sleep in health and disease. The study was funded by the National Institute of Neurological Disorders³ and Stroke (NINDS), part of the NIH. "Sleep changes the cellular⁴ structure of the brain. It appears to be a completely different state," said Maiken Nedergaard, M.D., D.M.Sc., co-director of the Center for Translational Neuromedicine at the University of Rochester Medical Center in New York, and a leader of the study.

 

For centuries, scientists and philosophers have wondered why people sleep and how it affects the brain. Only recently have scientists shown that sleep is important for storing memories. In this study, Dr. Nedergaard and her colleagues unexpectedly found that sleep may be also be the period when the brain cleanses⁵ itself of toxic⁶ molecules⁷.

 

Their results, published in Science, show that during sleep a plumbing⁸ system called the glymphatic system may open, letting fluid flow rapidly through the brain. Dr. Nedergaard's lab recently discovered the glymphatic system helps control the flow of cerebrospinal fluid (CSF脑脊髓液), a clear liquid surrounding the brain and spinal⁹ 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, Ph.D., a program director at NINDS.

 

Initially¹⁰ the researchers studied the system by injecting dye into the CSF of mice and watching it flow through their brains while simultaneously¹¹ monitoring electrical brain activity. The dye flowed rapidly when the mice were unconscious, either asleep or anesthetized. 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," said Dr. Nedergaard. "It suggested that the space between brain cells changed greatly between conscious and unconscious states."

 

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 anesthetized.

 

"These are some dramatic changes in extracellular space," said Charles Nicholson, Ph.D., a professor at New York University's Langone Medical Center and an expert in measuring the dynamics¹² of brain fluid flow and how it influences nerve cell communication.

 

Certain brain cells, called glia, control flow through the glymphatic system by shrinking or swelling¹³. Noradrenaline is an arousing hormone¹⁴ that is also known to control cell volume. Similar to using anesthesia, treating awake mice with drugs that block noradrenaline induced unconsciousness and increased brain fluid flow and the space between cells, further supporting the link between the glymphatic system and consciousness.

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