科学家发现脑部清洁系统

A previously unrecognized system that drains waste from the brain at a rapid clip has been discovered by neuroscientists at the University of Rochester Medical Center. The findings were published online August 15 in Science Translational Medicine. The highly organized system acts like a series of pipes that piggyback（肩扛） on the brain's blood vessels, sort of a shadow plumbing system that seems to serve much the same function in the brain as the lymph system does in the rest of the body -- to drain away waste products.

"Waste clearance is of central importance to every organ, and there have been long-standing questions about how the brain gets rid of its waste," said Maiken Nedergaard, M.D., D.M.Sc., senior author of the paper and co-director of the University's Center for Translational Neuromedicine. "This work shows that the brain is cleansing itself in a more organized way and on a much larger scale than has been realized previously.

"We're hopeful that these findings have implications for many conditions that involve the brain, such as traumatic brain injury, Alzheimer's disease, stroke, and Parkinson's disease," she added.

Nedergaard's team has dubbed the new system "the glymphatic system," since it acts much like the lymphatic system but is managed by brain cells known as glial cells（胶质细胞）. The team made the findings in mice, whose brains are remarkably similar to the human brain.

Scientists have known that cerebrospinal fluid or CSF plays an important role cleansing brain tissue, carrying away waste products and carrying nutrients to brain tissue through a process known as diffusion. The newly discovered system circulates CSF to every corner of the brain much more efficiently, through what scientists call bulk flow（整体流） or convection.

"It's as if the brain has two garbage haulers -- a slow one that we've known about, and a fast one that we've just met," said Nedergaard. "Given the high rate of metabolism in the brain, and its exquisite sensitivity, it's not surprising that its mechanisms to rid itself of waste are more specialized and extensive than previously realized."

While the previously discovered system works more like a trickle, percolating CSF through brain tissue, the new system is under pressure, pushing large volumes of CSF through the brain each day to carry waste away more forcefully.

The glymphatic system is like a layer of piping that surrounds the brain's existing blood vessels. The team found that glial cells called astrocytes use projections known as "end feet" to form a network of conduits around the outsides of arteries and veins inside the brain -- similar to the way a canopy of tree branches along a well-wooded street might create a sort of channel above the roadway.

Those end feet are filled with structures known as water channels or aquaporins, which move CSF through the brain. The team found that CSF is pumped into the brain along the channels that surround arteries, then washes through brain tissue before collecting in channels around veins and draining from the brain.