QUANTA MAGAZINE (March 26, 2025) by Veronique Greenwood:

Incased in the skull, perched atop the spine, the brain has a carefully managed existence. It receives only certain nutrients, filtered through the blood-brain barrier; an elaborate system of protective membranes surrounds it. That privileged space contains a mystery. For more than a century, scientists have wondered: If it’s so hard for anything to get into the brain, how does waste get out?

What’s more, during NREM sleep norepinephrine levels change rhythmically. This neurotransmitter could help tie together their hypotheses — the physical movement of CSF through brain tissues and the “brainwashing” occurring during sleep.

The brain has one of the highest metabolisms of any organ in the body, and that process must yield by-products that need to be removed. In the rest of the body, blood vessels are shadowed by a system of lymphatic vessels. Molecules that have served their purpose in the blood move into these fluid-filled tubes and are swept away to the lymph nodes for processing. But blood vessels in the brain have no such outlet. Several hundred kilometers of them, all told, seem to thread their way through this dense, busily working tissue without a matching waste system.

However, the brain’s blood vessels are surrounded by open, fluid-filled spaces. In recent decades, the cerebrospinal fluid, or CSF, in those spaces has drawn a great deal of interest. “Maybe the CSF can be a highway, in a way, for the flow or exchange of different things within the brain,” said Steven Proulx, who studies the CSF system at the University of Bern.

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Veronique Greenwood is a science writer and essayist. Her work has appeared in The New York Times Magazine, Smithsonian, Discover, Aeon and other publications.