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We've been doing science for a couple thousand years, so you'd think we'd have most of the nervous system figured out by now. Or at least, that we'd know the parts of it, even if we aren't sure what they do.

Nope. We're still discovering new things all the time. Like, recently, we found a hidden network in the brain called the glymphatic system, whose main job is to clear away waste.

It operates mostly when you're asleep, might prevent people from developing certain diseases, and has impacted the way we understand how our brains works. Basically, finding it was a pretty awesome surprise. Every minute of every day, your cells do all sorts of marvelous things, but they also produce waste -- like carbon dioxide, water, and larger molecules like proteins.

But that doesn't mean you're, y'know... full of garbage. In most of your body, a network of vessels called the lymphatic system acts like a sewer to clear that junk away. It carries waste to your circulatory system, where it can then be taken to your liver and kidneys for disposal.

But up until recently, scientists couldn't find anything like that in the brain. Even though, relative to other parts of your body, it produces a lot of waste. Instead, they thought that waste exited the brain by diffusing into the cerebrospinal fluid -- or the fluid around your brain and spinal cord.

Then it could make its way into the circulatory system. For a while, this made sense, since scientists couldn't see any vessels. And that waste had to be going somewhere.

But in 1971, one scientist named Dr. Helen Cserr realized this method wouldn't actually work. She did the math and realized that some of the large proteins in cellular waste would take too long to diffuse.

Still, it wasn't clear where that gunk was going. And for decades, it was a bit of a head scratcher. Then, in 2012, we finally figured it out.

Scientists at the University of Rochester used a technique called two photon microscopy to look at mouse brains. This technique uses lasers to allow scientists to look deep inside live tissues for extended periods of time. With it, researchers saw that there was an active, directional flow of cerebrospinal fluid into the spaces between brain cells.

Older fluid was draining out, and it was taking the brain's waste with it. They called this discovery the glymphatic system, and we've since found evidence that it's working in human brains as well. Even though we've been studying the brain for a while now, this system was hard to find for a couple of reasons.

One is that, unlike the lymphatic system, the glymphatic system doesn't really have vessels. So there's not a structure you can find in a dissection. Instead, the fluid flows in the space between blood vessels and the cells that surround them.

It's pretty sneaky. Discovering this system was cool because it means we know more about how our bodies work. But it might also have implications for health and medicine in general.

Like, one thing that scientists have learned is that the glymphatic system is almost only turned on when you're sleeping. That's because, when you're awake, your body produces elevated amounts of a chemical called norepinephrine. And at least in mice, which have somewhat similar brains to us, norepinephrine suppresses the glymphatic system.

So, that's one more reason sleep is important for your health -- it keeps your brain clean! Scientists have also learned that, in mice, the glymphatic system gets less active as they get older. This probably happens in humans, too -- right around the time our risk of dementia is also increasing.

This suggests that problems with the glymphatic system could play a role in dementia and neurodegenerative diseases like Alzheimer's and Parkinson's. These diseases are associated with build-ups of large proteins in the brain, which are some of the junk that the glymphatic system is supposed to clear away. So understanding how the system works may help scientists figure out better ways to treat these diseases, along with other protein-related conditions.

But there's definitely a long way to go. Now, finding the glymphatic system seemed like a huge victory -- but the surprises weren't quite over yet. Because in 2017, scientists found lymphatic vessels in the human brain, too.

The ones that weren't supposed to exist. They were hiding in the meninges -- the outer membranes that cover the brain and spinal cord. We didn't know about these vessels before because they're hidden inside a tough membrane.

And they run alongside some pretty prominent blood vessels. When you're looking at a brain -- with an MRI, for example -- these lymphatic vessels are difficult to see because the blood vessels are in the way. Kind of like a photobomb.

Still, now that we've found all these new systems, we kind of have more questions than answers. The next steps are for scientists to figure out how the glymphatic system is involved in human disease -- and how it and those new lymphatic vessels might work together. Then, someday, maybe we can use that knowledge to treat disease.

All this goes to show that, even though we're pretty smart, we still have a lot to learn about our brains. But the next time someone accuses you of having a dirty mind, you can let them know they don't need to worry. Your glymphatic system -- and those lymphatic vessels -- keep your brain sparkling clean!

Thanks for watching this episode of SciShow Psych, a Complexly production. If you'd like to learn even more about your brain's anatomy and the rest of your body, you can check out Crash Course Anatomy & Physiology over at youtube.com/crashcourse. [♪ OUTRO ].