The speech may help Students understand how to save time for sleep and stay away from the video games.
Sleep. Its somethingwe spend about a third of our lives doing, but do any of us really understandwhat its all about?
Two thousand yearsago, Galen, one of the most prominent medical researchers of the ancient world,proposed that while were awake, our brains motive force, its juice, wouldflow out to all the other parts of the body, animating them but leaving thebrain all dried up, and he thought that when we sleep, all this moisture thatfilled the rest of the body would come rushing back, rehydrating the brain andrefreshing the mind. Now, that sounds completely ridiculous to us now, butGalen was simply trying to explain something about sleep that we all deal withevery day. See, we all know based on our own experience that when you sleep, itclears your mind, and when you dont sleep, it leaves your mind murky. Butwhile we know a great deal more about sleep now than when Galen was around, westill havent understood why it is that sleep, of all of our activities, hasthis incredible restorative function for the mind.
So today I want totell you about some recent research that may shed new light on this question.Weve found that sleep may actually be a kind of elegant design solution tosome of the brains most basic needs, a unique way that the brain meets thehigh demands and the narrow margins that set it apart from all the other organsof the body.
So almost all thebiology that we observe can be thought of as a series of problems and theircorresponding solutions, and the first problem that every organ must solve is acontinuous supply of nutrients to fuel all those cells of the body. In thebrain, that is especially critical; its intense electrical activity uses up aquarter of the bodys entire energy supply, even though the brain accounts foronly about two percent of the bodys mass. So the circulatory system solves thenutrient delivery problem by sending blood vessels to supply nutrients andoxygen to every corner of our body.
You can actually seeit in this video here. Here, were imaging blood vessels in the brain of aliving mouse. The blood vessels form a complex network that fills the entirebrain volume. They start at the surface of the brain, and then they dive downinto the tissue itself, and as they spread out, they supply nutrients andoxygen to each and every cell in the brain.
Now, just as everycell requires nutrients to fuel it, every cell also produces waste as abyproduct, and the clearance of that waste is the second basic problem thateach organ has to solve. This diagram shows the bodys lymphatic system, whichhas evolved to meet this need. Its a second parallel network of vessels thatextends throughout the body. It takes up proteins and other waste from thespaces between the cells, it collects them, and then dumps them into the bloodso they can be disposed of.
But if you lookreally closely at this diagram, youll see something that doesnt make a lot ofsense. So if we were to zoom into this guys head, one of the things that youwould see there is that there are no lymphatic vessels in the brain. But thatdoesnt make a lot of sense, does it? I mean, the brain is this intenselyactive organ that produces a correspondingly large amount of waste that must beefficiently cleared. And yet, it lacks lymphatic vessels, which means that theapproach that the rest of the body takes to clearing away its waste wont workin the brain.
So how, then, doesthe brain solve its waste clearance problem? Well, that seemingly mundanequestion is where our group first jumped into this story, and what we found aswe dove down into the brain, down among the neurons and the blood vessels, wasthat the brains solution to the problem of waste clearance, it was reallyunexpected. It was ingenious, but it was also beautiful. Let me tell you aboutwhat we found.
So the brain has thislarge pool of clean, clear fluid called cerebrospinal fluid. We call it theCSF. The CSF fills the space that surrounds the brain, and wastes from insidethe brain make their way out to the CSF, which gets dumped, along with thewaste, into the blood. So in that way, it sounds a lot like the lymphaticsystem, doesnt it? But whats interesting is that the fluid and the waste frominside the brain, they dont just percolate their way randomly out to thesepools of CSF. Instead, there is a specialized network of plumbing thatorganizes and facilitates this process. You can see that in these videos. Here,were again imaging into the brain of living mice. The frame on your left showswhats happening at the brains surface, and the frame on your right showswhats happening down below the surface of the brain within the tissue itself.Weve labeled the blood vessels in red, and the CSF thats surrounding thebrain will be in green. Now, what was surprising to us was that the fluid onthe outside of the brain, it didnt stay on the outside. Instead, the CSF waspumped back into and through the brain along the outsides of the blood vessels,and as it flushed down into the brain along the outsides of these vessels, itwas actually helping to clear away, to clean the waste from the spaces betweenthe brains cells. If you think about it, using the outsides of these bloodvessels like this is a really clever design solution, because the brain isenclosed in a rigid skull and its packed full of cells, so there is no extraspace inside it for a whole second set of vessels like the lymphatic system.Yet the blood vessels, they extend from the surface of the brain down to reachevery single cell in the brain, which means that fluid thats traveling alongthe outsides of these vessels can gain easy access to the entire brainsvolume, so its actually this really clever way to repurpose one set ofvessels, the blood vessels, to take over and replace the function of a secondset of vessels, the lymphatic vessels, to make it so you dont need them. Andwhats amazing is that no other organ takes quite this approach to clearingaway the waste from between its cells. This is a solution that is entirelyunique to the brain.
我们的大脑有一个区域，里面装满了干净透明的液体，那种液体叫做脑脊液我们简称CSF（脑脊液） CSF（脑脊液）填充了大脑周围的空间。来自大脑中的废物从大脑中出来，到达CSF（脑脊液）中 CSF和废物一起卸下，进入血液。照这样，听起来这就像一个淋巴系统，不是吗？但有趣的是，从大脑中出来的液体和废物它们不是随意地渗透到 CSF的区域中相反，有一个专门的管道网络，来组织和促进这个过程。你可以从这些视频中看到。看，我们又一次看到了活老鼠的大脑在你左边的组织展示了大脑表层正在发生的事，而你右边的组织显示了大脑表层底下和它的组织所发生的事。我们把血管标记成红色把大脑周围的CSF标记成绿色那么，让我们吃惊的是大脑外面的液体（脑脊液）它们不在外面呆着相反，CSF（脑脊液）沿着血管外壁，流回大脑中，并且在它沿着这些血管流向大脑的同时，它正在忙于将废物从这些脑细胞的间隙中带走。如果你仔细想想就会发现利用这些血管外壁真的是一个巧妙设计的解决方法。因为大脑在一个坚硬的头骨中是封闭的，并且它里面装满了细胞。所以在大脑中已经没有空余的位置留给第二套像淋巴系统那样的管路然而这些血管却从大脑的表层延伸出来到达大脑中的每一个独立的细胞，这也就意味着，脑脊液只要沿着这些血管的外壁流动就可以轻易到达大脑中。所以这就是，安排一套血管去代替第二套如淋巴管一样功能的一种巧妙办法。这样就不需要那另外的一套淋巴管了。更神奇的是，没有其他任何一种器官具有这种清理细胞之间废物的功能这是大脑所独有的解决办法。
But our mostsurprising finding was that all of this, everything I just told you about, withall this fluid rushing through the brain, its only happening in the sleepingbrain. Here, the video on the left shows how much of the CSF is moving throughthe brain of a living mouse while its awake. Its almost nothing. Yet in thesame animal, if we wait just a little while until its gone to sleep, what wesee is that the CSF is rushing through the brain, and we discovered that at thesame time when the brain goes to sleep, the brain cells themselves seem toshrink, opening up spaces in between them, allowing fluid to rush through andallowing waste to be cleared out.
So it seems thatGalen may actually have been sort of on the right track when he wrote aboutfluid rushing through the brain when sleep came on. Our own research, now its2,000 years later, suggests that whats happening is that when the brain isawake and is at its most busy, it puts off clearing away the waste from thespaces between its cells until later, and then, when it goes to sleep anddoesnt have to be as busy, it shifts into a kind of cleaning mode to clearaway the waste from the spaces between its cells, the waste thats accumulatedthroughout the day. So its actually a little bit like how you or I, we put offour household chores during the work week when we dont have time to get to it,and then we play catch up on all the cleaning that we have to do when theweekend rolls around.
Now, Ive just talkeda lot about waste clearance, but I havent been very specific about the kindsof waste that the brain needs to be clearing during sleep in order to stayhealthy. The waste product that these recent studies focused most on isamyloid-beta, which is a protein thats made in the brain all the time. Mybrains making amyloid-beta right now, and so is yours. But in patients withAlzheimers disease, amyloid-beta builds up and aggregates in the spacesbetween the brains cells, instead of being cleared away like its supposed tobe, and its this buildup of amyloid-beta thats thought to be one of the keysteps in the development of that terrible disease. So we measured how fastamyloid-beta is cleared from the brain when its awake versus when its asleep,and we found that indeed, the clearance of amyloid-beta is much more rapid fromthe sleeping brain.
So if sleep, then, ispart of the brains solution to the problem of waste clearance, then this maydramatically change how we think about the relationship between sleep,amyloid-beta, and Alzheimers disease. A series of recent clinical studiessuggest that among patients who havent yet developed Alzheimers disease,worsening sleep quality and sleep duration are associated with a greater amountof amyloid-beta building up in the brain, and while its important to point outthat these studies dont prove that lack of sleep or poor sleep cause Alzheimersdisease, they do suggest that the failure of the brain to keep its house cleanby clearing away waste like amyloid-beta may contribute to the development ofconditions like Alzheimers.
So what this newresearch tells us, then, is that the one thing that all of you already knewabout sleep, that even Galen understood about sleep, that it refreshes andclears the mind, may actually be a big part of what sleep is all about. See,you and I, we go to sleep every single night, but our brains, they never rest.While our body is still and our mind is off walking in dreams somewhere, theelegant machinery of the brain is quietly hard at work cleaning and maintainingthis unimaginably complex machine. Like our housework, its a dirty and athankless job, but its also important. In your house, if you stop cleaningyour kitchen for a month, your home will become completely unlivable veryquickly. But in the brain, the consequences of falling behind may be muchgreater than the embarrassment of dirty countertops, because when it comes tocleaning the brain, it is the very health and function of the mind and the bodythats at stake, which is why understanding these very basic housekeepingfunctions of the brain today may be critical for preventing and treatingdiseases of the mind tomorrow.