![]() The adult human cortex is composed of an estimated 16–30 billion neurons, each making approximately 7000 synaptic connections exchanging information up to a rate of 1000 Hz, with supporting glial cells outnumbering neurons in the cortex at a ratio of less than two ( 1– 4). ■ Animal models of states of health and disease have demonstrated the importance of normal glymphatic system function in homeostasis of the central nervous system interstitium, which is enhanced during slow wave stages of sleep. ■ The astrocytic end feet that make up the glia limitans express a high number of aquaporin 4 water channels to help facilitate rapid transport of water between the interstitium and paravascular space. ■ Paravascular channels are intrinsic and distinctive components of healthy parenchymal blood vessels of the central nervous system. ![]() ■ Paravascular channels, defined here as the spaces between the endothelial basement membrane and glia limitans, connect the central nervous system interstitium to the subarachnoid CSF efflux pathways, allowing removal of fluid and solutes from the central nervous system interstitium through the glia limitans. ■ The glymphatic system is a recently discovered network that enables the exchange of central nervous system interstitial fluid and cerebrospinal fluid (CSF) necessary for homeostasis of the interstitial space. The evolving understanding of glymphatic system characteristics is then used to provide a current interpretation of its physiology that can be helpful for radiologists when interpreting neuroimaging investigations. Glymphatic system behavior in animal models of health and disease, and its enhanced function during sleep, are discussed. The currently understood anatomy and physiology of the glymphatic system is reviewed, with the paravascular space presented as an intrinsic component of healthy pial and parenchymal cerebral blood vessels. Networks of paravascular channels surrounding pial and parenchymal arteries and veins were found that extend into the walls of capillaries to allow fluid transport and exchange between the interstitial and cerebrospinal fluid spaces. ![]() How the central nervous system maintains tight control of extracellular conditions has been a fundamental question in neuroscience until recent discovery of the glial-lymphatic, or glymphatic, system was made this past decade. Lymphatic vessels serve as collectors of this fluid in most organs however, these vessels are absent in the central nervous system. Normal physiologic function of organs requires a circulation of interstitial fluid to deliver nutrients and clear cellular waste products. From the Montreal Neurologic Institute and Hospital, Department of Diagnostic and Interventional Neuroradiology, McGill University Health Centre, 3801 Rue University, Montréal, QC, Canada H3A 2B4 (J.M.K.) Department of Medical Imaging, University of Toronto, Toronto, Canada (J.M.K., D.V., K.D.B., H.G.J.K., T.K., K.P.M., K.G.t.B., D.J.M.) Division of Neuroradiology, Toronto Western Hospital, University Health Network, Toronto, Canada (J.M.K., D.V., K.D.B., H.G.J.K., T.K., K.P.M., K.G.t.B., D.J.M.) Centre Hospitalier de l’Université de Montreal (CHUM), Department of Radiology, Service of Neuroradiology, l’Université de Montreal, Montréal, Canada (J.M.K.) Department of Materials Science & Engineering, Faculty of Applied Science & Engineering, University of Toronto, Toronto, Canada (D.V.) Department of Medical Imaging, Sydney Children’s Hospitals Network, Westmead, Australia (K.D.B.) and Division of Neurosurgery, Department of Surgery, University of Toronto, Toronto, Canada (T.K., K.G.t.B.). ![]()
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