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   The Guardian    
      
      
   Neuroscience Neurophilosophy   
      
   How to optimise your brain's waste disposal system   
      
   New research suggests that body posture during sleep may affect the efficiency   
   of the brain’s self-cleaning process   
      
    Cairo's zabaleen collect the city's waste on donkey carts.   
    Cairo’s zabaleen collect the city’s waste on donkey carts. Photograph:   
   Dave Stamboulis/Alamy   
   Mo Costandi   
   Saturday 22 August 2015 04.00 EDT Last modified on Saturday 22 August 2015   
   14.42 EDT   
      
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   The human brain can be compared to something like a big, bustling city. It has   
   workers, the neurons and glial cells which co-operate with each other to   
   process information; it has offices, the clusters of cells that work together   
   to achieve specific    
   tasks; it has highways, the fibre bundles that transfer information across   
   long distances; and it has centralised hubs, the densely interconnected nodes   
   that integrate information from its distributed networks.   
      
   Like any big city, the brain also produces large amounts of waste products,   
   which have to be cleared away so that they do not clog up its delicate moving   
   parts. Until very recently, though, we knew very little about how this   
   happens. The brain’s waste    
   disposal system has now been identified. We now know that it operates while we   
   sleep at night, just like the waste collectors in most big cities, and the   
   latest research suggests that certain sleeping positions might make it more   
   efficient.   
      
      
   Newly discovered vessels beneath skull could link brain and immune system   
    Read more   
   Waste from the rest of the body is cleared away by the lymphatic system, which   
   makes and transports a fluid called lymph. The lymphatic system is an   
   important component of the immune system. Lymph contains white blood cells   
   that can kill microbes and mop    
   up their remains and other cellular debris. It is carried in branching vessels   
   to every organ and body part, and passes through them, via the spaces between   
   their cells, picking up waste materials. It is then drained, filtered, and   
   recirculated.   
      
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   The brain was thought to lack lymphatic vessels altogether, and so its waste   
   disposal system proved to be far more elusive. Several years ago, however,   
   Maiken Nedergaard of the University of Rochester Medical Center and colleagues   
   identified a system of    
   hydraulic “pipes” running alongside blood vessels in the mouse brain.   
   Using in vivo two-photon imaging to trace the movements of fluorescent   
   markers, they showed that these vessels carry cerebrospinal fluid around the   
   brain, and that the fluid enters    
   inter-cellular spaces in the brain tissue, picking up waste on its way.   
      
   Nedergaard and her colleagues also discovered that proper function of these   
   vessels depends on movements of water around the brain, which are carried out   
   by glial cells called astrocytes, and therefore named them the glymphatic   
   system. They went on to    
   show that inter-cellular spaces expand by up to 60% in the brains of naturally   
   sleeping and anaesthetised mice, and that this expansion drives the clearance   
   of waste from the brain by facilitating the movements of lymph and water.   
      
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   Last month, researchers from the University of Virginia reported the   
   identification of lymphatic vessels in the central nervous system. They   
   demonstrated that the lymphatic system extends into the dura mater, the   
   thickest and outer-most of the three    
   meningeal membranes that envelope the brain and spinal cord. These vessels run   
   parallel to the major veins and arteries, and split to send branches deep into   
   the brain’s crevices. The researchers believe that they could be linked to   
   the glymphatic    
   system, and may be the second stage of the disposal mechanism, which would   
   transport waste out of the brain and spinal cord altogether.   
      
   The latest study from Nedergaard’s group, published in the Journal of   
   Neuroscience earlier this month, shows that body posture affects the   
   efficiency of the glymphatic system’s waste clearance. Using fluorescence   
   microscopy and radioactive tracing    
   once again, they showed that drainage of the cerebrospinal fluid worked best   
   in mice lying on their sides compared to those lying on their back or standing   
   up.   
      
      
   The neuroprotective lifestyle   
    Read more   
   The function of sleep was once deeply mysterious, but there’s plenty of   
   evidence that it is critical for memory consolidation, and it would now seem   
   to be required for the effective removal of waste from the brain, too.   
   Although these studies were    
   performed in mice, preliminary results suggest that lymphatic vessels are also   
   present in the human brain and spinal cord, but further research will be   
   needed to confirm that they actually constitute a working waste disposal   
   system.   
      
   Eventually, the link to sleep could have important implications for the   
   treatment of neurodegenerative diseases such as Alzheimer’s and   
   Parkinson’s, all of which involve the build-up of misfolded proteins within   
   and around nerve cells, because of a    
   defective waste disposal system. Indeed, it is now seems clear that good sleep   
   hygiene has a neuroprotective effect and, in line with this, other research   
   shows that sleep disturbances predict the onset of neurodegeneration.   
      
   Sleeping on the side just happens to be the most popular sleeping posture for   
   both mice and humans, and so this preference may have evolved to optimise the   
   waste disposal system and thus ensure that the metropolis of the brain runs as   
   effectively as    
   possible.   
      
   References   
   Lee, H. et al. (2015). The Effect of Body Posture on Brain Glymphatic   
   Transport. J. Neurosci, 35: 11034-44. DOI: 10.1523/JNEUROSCI.1625-15.2015.   
      
   Louveau, A., et al. (2015). Structural and functional features of central   
   nervous system lymphatic vessels. Nature, 523: 337-41. DOI: 10.1   
   38/nature14432.   
      
   Xu, L., et al. (2014). Sleep Drives Metabolite Clearance from the Adult Brain.   
   Science, 342: 373-7. DOI: 10.1126/science.1241224. [Full text]   
      
   Iliff, J., et al. (2013). A Paravascular Pathway Facilitates CSF Flow Through   
   the Brain Parenchyma and the Clearance of Interstitial Solutes, Including   
   Amyloid β. Sci. Trans. Med., 4: 147ra111. DOI: 10.1126/scitranslmed.3003748.   
   [Full text]   
      
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