We all need to clear our heads, sometimes literally – and now scientists have learned how our neurological plumbing system works.

Every organ produces waste, and the brain is no exception. But unlike the rest of our body, it doesn't have a lymphatic system, a network of vessels that filter out junk. Now, a new study of mouse brains suggests how ours handle waste: by rapidly pumping fluid along the outside of blood vessels, literally flushing waste away. The finding, reported Aug. 15 in Science Translational Medicine, could hint at how diseases like Alzheimer's develop and might be treated.

"If you look at a body-wide map of the lymphatic system, you see a great big void in the brain," said neuroscientist Jeffrey Iliff of the University of Rochester Medical Center. He and his colleagues found that puzzling, given how active the brain is and how sensitive it is to waste buildup.

Scientists long suspected that the brain's refuse ended up in the cerebrospinal fluid, which cushions the brain inside the skull. In the 1980s, some researchers proposed that the fluid might be pumped into the brain to wash it, then pumped out again. Other researchers weren't convinced.

Thanks to new imaging techniques that made it possible to peer inside the brain of a living mouse, Iliff's team saw the process in action. Cerebrospinal fluid flowed along the outside of blood vessels, carried through a network of pipe-like protein structures. The fluid picked up waste that accumulated between cells, then drained out through major veins.

: J. Iliff and M. Nedergaard

"These experiments validate a powerful 'prevailing current' of cerebrospinal fluid in brain extracellular space that effectively clears metabolic garbage," said neurologist Bruce Ransom of the University of Washington, who was not affiliated with the study.

Iliff's group went on to show that mice brains without these channels did poorly at clearing waste, including amyloid protein, buildup of which is linked to Alzheimer's disease. They cleared waste 70 percent more slowly than mice possessing the channels.

Healthy brains produce amyloid normally, but this system clears it out frequently, the researchers suspect. In an Alzheimer's brain, "it builds up and builds up and builds up, until eventually it forms the plaques that can clog up the brain," said Iliff.

In theory, you could prevent or slow that buildup by improving the brain's flushing system. "The key is, we have to find a way to 'turn up' the system," Iliff said. This could also apply to other brain conditions, such as Parkinson's disease or stroke.

Iliff and colleagues used a combination of old and new imaging methods to visualize fluid circulation. The standard method involves injecting a tracer chemical into a slice of brain tissue, shining light on it, and watching how it fluoresces (above right). But this method only provides snapshots of dead brains. With a new technique called two-photon imaging, which can detect fluorescent tracers hit with just two low-energy photons, the researchers could see deep into the brain of a living mouse.

More research is needed to fully understand how the waste system works in human brains, which can't be studied as easily as mice. Still, the findings are important, said Ransom. "After decades of uncertainty about cerebrospinal fluid movement in brain, we finally know which way the wind is blowing," he said.

Citation: "A Paravascular Pathway Facilitates CSF Flow Through the Brain Parenchyma and the Clearance of Interstitial Solutes, Including Amyloid β." By Jeffrey J. Iliff, Minghuan Wang, Yonghong Liao, Benjamin A. Plogg, Weiguo Peng, Georg A. Gundersen, Helene Benveniste, G. Edward Vates, Rashid Deane, Steven A. Goldman, Erlend A. Nagelhus, Maiken Nedergaard. Science Translational Medicine, Vol. 4 Issue 147, August 15, 2012.