Brain harbors a unique ability to, figuratively speaking, shift its gears. During wakefulness, the brain is geared fully towards processing information and behaving, while homeostatic functions predominate during sleep. The blood-brain barrier establishes a stable environment that is optimal for neuronal function, yet the barrier imposes a physiological problem; transcapillary filtration that forms extracellular fluid in other organs is reduced to a minimum in brain. Consequently, the brain depends on a special fluid (the cerebrospinal fluid; CSF) that is flushed into brain along the unique perivascular spaces created by astrocytic vascular endfeet. We describe this pathway, coined the term glymphatic system, based on its dependency on astrocytic vascular endfeet and their adluminal expression of AQP4 water channels facing towards CSF-filled perivascular spaces. Glymphatic clearance of potentially harmful metabolic or protein waste products, such as amyloid-β is primarily active during sleep, when its physiological drivers, the cardiac cycle, respiration, and slow vasomotion, together efficiently propel CSF inflow along periarterial spaces. The brain's extracellular space contains an abundance of proteoglycans and hyaluronan, which provide a low-resistance hydraulic conduit that rapidly can expand and shrink during the sleep-wake cycle. We describe this unique fluid system of the brain, which meets the brain's requisites to maintain homeostasis similar to peripheral organs, considering the blood-brain-barrier and the paths for formation and egress of the CSF

中文翻译：

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大脑中的液体运输

打个比方来说，大脑拥有一种独特的转换能力。在清醒期间，大脑完全致力于处理信息和行为，而在睡眠期间，体内平衡功能占主导地位。血脑屏障建立了一个最适合神经元功能的稳定环境，但该屏障也带来了生理问题；在其他器官中形成细胞外液的经毛细血管过滤在大脑中减少到最低限度。因此，大脑依赖于一种特殊的液体（脑脊液；CSF），这种液体沿着星形细胞血管端足形成的独特的血管周围空间冲入大脑。我们描述了这条通路，创造了术语“类淋巴系统”，基于其对星形胶质细胞血管末足的依赖性及其面向充满脑脊液的血管周围空间的 AQP4 水通道的腔内表达。潜在有害的代谢或蛋白质废物（例如β淀粉样蛋白）的类淋巴系统清除主要在睡眠期间活跃，此时其生理驱动因素、心动周期、呼吸和缓慢的血管舒缩共同有效地推动脑脊液沿动脉周围空间流入。大脑的细胞外空间含有丰富的蛋白聚糖和透明质酸，它们提供了一个低阻力的液压管道，可以在睡眠-觉醒周期中快速膨胀和收缩。我们描述了这种独特的大脑液体系统，考虑到血脑屏障以及脑脊液的形成和流出路径，它满足大脑维持类似于周围器官的稳态的要求