Fluid transport in the brain

Abstract

The brain harbors a unique ability to, figuratively speaking, shift its gears. During wakefulness, the brain is geared fully toward 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 aquaporin-4 water channels facing toward 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.

Keywords: brain clearance; brain extracellular matrix; brain fluid transport; cerebrospinal fluid; glymphatic system.

Graphical abstract

FIGURE 1.

Comparison of fluid and solute flow in peripheral tissues and brain. The source of fluid in peripheral tissue is the porous capillaries, which are permeable to plasma and globular proteins like albumin (gray circles). In addition, endothelial cells express aquaporin-1 (AQP1) water channels. A hydrostatic pressure gradient drives an ultrafiltrate of plasma and globular proteins into the tissue at the arterial end of the capillary bed, while most of the fluid and extracellular proteins, including waste products (black circles) are transported out of peripheral tissues by lymphatic vessels. In the brain, the blood brain barrier (BBB) formed by capillary endothelial cell tight junctions largely prevents influx of vascular fluid and proteins. Instead, the brain produces its own fluid, cerebrospinal fluid (CSF). CSF is transported into deep parts of the brain along the periarterial spaces. The unique perivascular spaces of the central nervous system are annular CSF-filled tunnels with low resistance to fluid flow, which are created by a convoluted surface of loosely interconnected astrocytic endfeet plastering the entire cerebral vasculature, including arteries, capillaries and veins. The arterial wall pulsatility drives CSF influx into the neuropil, facilitated by the aquaporin-4 (AQP4) water channels on the astrocytic endfeet. AQP4 channels form square arrays that occupy up to 60% of the surface of astrocytic endfeet facing the perivascular space (John Rash, personal communication). In contrast, brain endothelial cells express few or no water channels. The extracellular fluid and protein waste produced during neural activity are transported to and leave the brain along perivenous spaces for ultimate export by meningeal lymphatic vessels and along cranial and spinal nerve sheaths.

FIGURE 2.

Timeline of significant events and discoveries pertaining to brain fluid transport. The black arrow superimposed over the cerebral artery illustrates the passage of time. The timeline spans over millenniums and includes the first clinical description of cerebral spinal fluid (CSF) in ancient Egypt found in the Edwin Smith papyrus scroll from 1500 BC, the first morphologically correct drawing of the ventricular system by Leonardo da Vinci in 15th century, and the description of CSF flow as the third circulation by Harvey Cushing in the 20th century. Many other substantial discoveries extending beyond those illustrated in the figure established the foundation to our modern understanding of brain fluid transport but due to space limitations cannot be included in the figure (see FIGURE 3). Images are courtesy of The New York Academy of Medicine Library (Edwin Smith Papyrus scroll) and from Ref. (Da Vinci drawings; with permission from Springer International), Ref. (schematic by Harvey Cushing; with permission from The Archives of Surgery), and Ref. (Arachnoid granulations drawing by Key and Retzius).

FIGURE 3.

Schematic of brain fluid transport. Cerebrospinal fluid (CSF) is produced primarily in the choroid plexus which sits in the lateral, 3rd, and 4th ventricles, but it has been suggested that a substantial contribution from extrachoroidal sources contributes to the overall production of CSF. CSF produced within the ventricles flows into the subarachnoid space through the lateral and median apertures and into the basal cisterns (see sect. 5) (33, 52). The subarachnoid space contains around 80% of the total CSF volume, compared with 20% within the ventricles (53). Subarachnoid CSF extends all over the brain and spinal cord and keeps the nervous system buoyant and thus allows it to maintain its shape, despite the lack of a fibrous extracellular matrix. From the subarachnoid space, CSF can either flow into the glymphatic system or egress from the intracranial or intraspinal space directly into the peripheral circulation (see sect. 6 for intra-axial fluid dynamics and sect. 12 for CSF egress). The CSF that enters the periarterial spaces is pumped forward in an anterograde manner along the major cerebral arteries and from there it enters the brain through the perivascular spaces of the thousands of penetrating arteries that perforate the brain tissue (1) (see sect. 9 for physiological drivers) (4, 21, 54). From the periarterial and periarteriolar spaces, CSF is driven into the brain extracellular space over the glia limitans perivascularis in a process that is supported by the polarized expression of aquaporin 4 (AQP4) water channels on the abluminal membrane of the astrocytic endfeet (2) (see sect. 7 for details on fluid flow in brain extracellular space and sect. 8 for roles of AQP4 in brain fluid transport) (4, 22, 55). The flow of fluid through the tortuous brain extracellular space is probably driven by a combination of diffusion and convection. Extracellular fluid with metabolites and waste products is drained into perivenous spaces and finds efflux from the brain following the large cerebral veins (3) (4, 56, 57). CSF from the subarachnoid space and extracellular fluid finds egress from the intracranial compartment to peripheral circulation through several different egress pathways, including dural lymphatics, perineuronal pathways, parasagittal spaces, arachnoid villi, and granulations and adventitia of large cerebral vessels (58). PVS, perivascular space; SAS, subarachnoid space.

FIGURE 4.

Development of the glymphatic system. The glymphatic system develops postnatally. The earliest signs of perivascular cerebral spinal fluid (CSF) flow appear proximal to Circle of Willis at embryonic day 17.5 (E17.5) in mice. Actual parenchymal CSF inflow is first evident in the hippocampus at postnatal day 1 (P1). From P7 to P14 the neocortex starts to show widespread CSF tracer inflow, proceeding from the basal to the dorsal aspect of the brain. By P14, the development of the murine glymphatic system is functionally and morphologically complete. The onset of glymphatic flow requires the development of astrocytic endfeet with polarized expression of aquaporin-4 (AQP4). BBB, blood-brain barrier; ECS, extracellular space.

FIGURE 5.

Development of ventricles, cerebral spinal fluid (CSF) spaces, and egress sites. The initial development of the cerebral ventricles occurs during neurulation, when the neural plate infolds to form the neural tube, which is initially in open contact with the amniotic fluid. Around day 28 in human gestation the anterior and posterior neuropores close, and the first CSF is formed from the captive remnant of amniotic fluid trapped in the neural tube lumen. During the coming weeks and months, infolding of the neural tube results in the formation of the primitive ventricular system consisting of the three brain vesicles. With further growth of the central nervous system (CNS), these vesicles eventually inflate under CSF pressure to form the ventricles of the adult CNS. Strap junctions between ependymal cells create a tight barrier between the CSF compartment and the brain extracellular space, which likely allows for this inflation. In humans, the ventricular compartment and subarachnoid space become communicated between embryonic months 4 and 6, with the opening of the foramina of Luschka and Magendie. Vascularization and formation of the choroid plexus occurs very early in brain development, with onset at embryonic day 10 (E10) in mice and around embryonic week 6 in human brain. CSF egress systems develop only after birth, although there must be some mechanism supporting egress in utero. PNVP, perineural vascular plexus; W, gestational week; M, gestational month.

FIGURE 6.

Phylogenetic development of the glymphatic system. When the nervous system evolved from diffuse neural networks to form centralized ganglia, the first supporting neuroglial cells also appeared. Thus, all bilateria have neuroglia. Ventricles developed at a phylogenetic stage when the nervous system progressed from being a plate-like structure to form a neural tube with a lumen containing cerebral spinal fluid (CSF), which is a feature of all chordates. In vertebrate evolution, brains might have grown so large that the tissue could not be nourished by diffusion from a vascular surface plexus, thus requiring the development of intrinsic brain vascularization. The subarachnoid space covering the external surface of the brain with CSF is seemingly present in all terrestrial vertebrates, such that extensions of the subarachnoid space into the brain parenchyma (i.e., the perivascular spaces) are likely also to be present in amphibians, reptiles, birds, and mammals. The evolutionary innovation of a perivascular endfeet layer of protoplasmic astrocytes with expression of aquaporin 4 (AQP4) polarized toward the blood vessel seems to be a feature of mammalian and avian brains. Thus, since the glymphatic pathway appears fully developed in mammals and birds, we surmise that it must also have been present in their common ancestor, before the divergence of sauropsids (ancestors of reptiles and avians) and synapsids (ancestors of mammals). CNS, central nervous system; SAS, subarachnoid space.

FIGURE 7.

Capillary conformations in evolution. Intrinsic brain vasculature takes two forms in vertebrates. A capillary loop structure (left) consists of a single artery and single vein that feed into a capillary with no or very few branch points and without anastomoses with other capillaries. A capillary meshwork (right) consists of arteries and veins that penetrate singly into the brain and feed into a rich anastomosing network of capillaries. These 2 conformations of vasculature also result in 2 different types of perivascular spaces. In capillary loops, a single perivascular space contains artery, vein, and capillary. In capillary meshworks, the artery and vein run in their own perivascular spaces. It is not yet established whether the hypothetical pericapillary space actually exists, but certain data supports that proposition. CSF, cerebral spinal fluid.

FIGURE 8.

Barriers separate the central nervous system (CNS) from peripheral tissues, but fluid can exchange with relative ease between all the CNS compartments due to the lack of tight barriers within the CNS. Middle: the tight barriers of the adult human brain in red and porous membranes in green. CNS is separated from peripheral tissues by several tight barriers (red lines), which include the blood brain barrier (BBB), the blood-choroid barrier, and the arachnoid membrane. Each of these barriers is created by expression of tight junctions containing claudins and other junctional proteins. The brain surfaces, i.e., the pial membrane and the ependymal cells layers covering the ventricles, are permeable membranes (green lines). Both the cells covering the ventricular surface (ependymal cells) and the cortical surfaces (pial cells) are connected by gap junctions, but gaps allow exchange of cerebral spinal fluid (CSF) with the extracellular fluid (green lines). Of note, astrocytes rather than endothelial cells form a vascular barrier in the external and intermediate zones of the median eminence and the adjacent ventral part of the arcuate nucleus, but not in other regions of the hypothalamus (187, 188). Also shown is the localization of the choroid plexus in the lateral, 3rd, and 4th ventricles, the meningeal layer, and the subarachnoid space. Top left inset: the BBB is a tight barrier that restricts fluid and solute exchange with the vascular compartment while the perivascular endfeet of astrocytes are loosely connected by gap junctions. Bottom left inset: The meningeal membranes consist of dura mater, arachnoid mater, and pia mater. The arachnoid membrane is a tight barrier expressing claudin-11 that separates the peripheral tissue and dura mater from the subarachnoid space. Pia mater covers all surfaces of the brain and spinal cord, but does not represent a barrier for fluid exchange. Together, these 2 membranes are known as the leptomeninges. Top right inset: choroid plexus is a simple structure consisting of a single layer of tight junction-coupled choroidal epithelial cells resting on a basement membrane containing a complex network of fenestrated capillaries. Bottom right inset: The brain surface is covered by loosely connected pial cells, whereas the ependymal lining of the ventricular surfaces also lacks tight junctions. Astrocytic processes contact both the pial and the ependymal layers, thereby creating a second semipermeable layer that possibly acts as a filter. Thus CSF can exchange with brain extracellular fluid at all of the brain surface.

FIGURE 9.

The classical model of cerebral spinal fluid (CSF) production. 1: In this model of CSF secretion, the Na⁺-K⁺-ATPase is the primary driver of CSF production, and secretes Na⁺ directly into CSF. 2: The activity of the Na⁺-K^+-ATPase lowers intracellular Na⁺, which in turn promotes Na⁺ influx across the basolateral membrane, which depends on Na⁺/${\text{HCO}}_3^{-}$/Cl⁻ cotransporters to produce an increase in cytosolic anions (Cl^- and ${\text{HCO}}_3^{-}$) concentrations. 3: In turn, the high density of anion channels at the apical membrane facilitates efflux of Cl^- and ${\text{HCO}}_3^{-}$. 4: Water is drawn into CSF by the increase in osmolarity supported by aquaporin 1 (AQP1) water channels. AQP1 at the basolateral membrane likely also contributes to water influx to the cell. NCBE, Na⁺-driven Cl⁻/${\text{HCO}}_3^{-}$ exchangers.

FIGURE 10.

Influx and efflux of intra-axial fluid follows the arterial and venous vascular territories in the human and rodent brain. Cerebrospinal fluid (CSF) enters the brain along the arterial vascular system of the brain that consists primarily of the 3 large cerebral arteries: the anterior cerebral artery (ACA), the middle cerebral artery (MCA), and the posterior cerebral artery (PCA). Intra-axial fluid exits along the perivascular spaces surrounding the draining venous system, which consists of a superficial and deep system.

FIGURE 11.

The extracellular matrix components and their specialization in the basal lamina. A: the extracellular matrix of peripheral tissue contains the fibrous proteins collagen, laminin, and fibronectin, which are embedded in proteoglycans and hyaluronan. Proteoglycans and hyaluronan both contain glycosaminoglycans (GAGs), which have a large capacity for binding water and Na⁺. Insert: proteoglycans contain a core protein, which is the binding target of numerous repeating disaccharide chains. Hyaluronan is also composed of a repeating disaccharide chain but differs with respect to its lack of core proteins. Hyaluronan interacts with proteoglycans but not with collagen, laminin, and fibronectin. Most of the extracellular matrix proteins interact with multiple classes of membrane receptors, including integrins and CD44. These receptors are coupled via linker proteins on the cytosolic site with the actin cytoskeleton. B: the basal lamina consists of a repetitive, highly organized interweaving of collagen IV, laminin, fibronectin, and proteoglycans. The basal lamina is permeable to soluble proteins such as albumin, and the presence of gaps between the endothelial cells allows free passage of an ultrafiltrate of plasma into the tissue, driven by hydrostatic pressure (blue arrows). The basal lamina induces polarized expression of channels, transporters, and ion pumps in the basolateral membrane (e.g., exocrine glands).

FIGURE 12.

Schematic overview of the lymphatic system in peripheral tissues. The lymphatic vascular tree is composed of several distinct segments. I: blind lymphatic capillaries, for which the primary function is the uptake of excess extracellular fluid. The capillaries have no smooth muscle cells, and their fluid uptake is believed to rely on the unique valve-like microstructural organization of endothelial cells’ flaps and button. A magnified illustration of flaps extending from the borders of the lymphatic endothelial cell. The flaps lack cell adhesions at their tips and are only anchored on the sides by buttons (red), which are cell-cell junctions created by cadherin and tight junction proteins. Uptake of fluid and solutes is unidirectional, and efflux cannot occur because any increase in the intraluminal back-pressure will close the flaps. Also, the basal lamina surrounding lymphatic capillaries is incomplete, facilitating uptake of larger structures, such as immune cells. II: lymphatic collectors are surrounded by smooth muscle cells that are in part oriented along the lymphatic vessel wall rather than encircling the vessel as in blood vessels. They are also endowed with valves that open in response to an increase in pressure created by contraction of the smooth muscle cells. The segment between two valves is called a lymphangion. Intrinsic properties of the muscle cells, the fluid load, and the autonomic nervous system regulate lymph vessel contractility. III: lymph nodes receive lymph from large prenodal collecting lymph vessels. Lymph within the node is transported from the afferent to the efferent lymphatics via a peripheral path (blue arrows pointing to flow from the subcapsular to the medullary sinus) or a central path (flow from the subcapsular sinus to the centrally placed B-cell follicles and T-cell cortex, not shown). IV: postnodal collecting lymphatic channel is shown. Approximately 50% of lymph fluid is transferred to the venous compartment within the node, whereas little exchange of proteins take place at this site. Thus most proteins entering the node, and ∼50% of the total lymph production, exit via the postnodal collecting lymphatic channels and are returned to the venous circulation by the thoracic or right lymphatic ducts (not shown) (49).

FIGURE 13.

Starling’s principle of hydrostatic pressure driving convective fluid flow in peripheral tissue. Capillaries in peripheral tissues are fenestrated and covered by a basal lamina. This arrangement allows an ultrafiltrate of plasma containing relatively low molecular weight solutes such as glucose and globular proteins to flow into the tissue, driven by hydrostatic pressure (P_c). P_c oscillates with the cardiac cycle, exhibiting higher pressures during systole and lower pressures during diastole, which drive temporally changing filtration volumes rather than continuous inflow. The directional flow of fluid and its solutes away from the arterial end of the capillary bed is supported by the colloid osmotic (oncotic) pressure (π_c) absorbing fluid in the venous end of the capillary bed. The remaining fluid and solutes (ΔP − Δπ) are transported out of the tissue by lymphatic capillaries (green). The ultrafiltrate of plasma do not experience unrestricted flow within the tissue, because of interference from cells and the fibrous elements of the extracellular matrix proteins (mostly collagen). Fluid flows primarily within the gelatinous pockets of the proteoglycans/hyaluronan containing glycosaminoglycan (GAGs.) When the hydration of GAGs is high, the hydraulic resistance is low, resulting in faster and more substantial fluid transport. On the other hand, dehydration of proteoglycans/hyaluronan will increase the hydraulic resistance and thereby reduce the inflow of the plasma ultrafiltrate. The relative contribution of the endothelial cell glycocalyx and hydration of GAG in regulating the rate of plasma ultrafiltration has not been determined. The tension within the tissue is created by the fibrillary network of collagen connected to the cytoskeleton by integrin receptors and serves to limit the extent of GAG hydration.

FIGURE 14.

Glycosaminoglycans (GAGs) swell in the setting of injury and increases tissue fluid flow. During normal physiological conditions, solid elements in the extracellular space, including collagen, as well as the cellular tension, together limit GAG hydration in peripheral tissue (355). After a burn injury, the degradation and conformational changes of collagen and cellular elements lead to rapid influx of vascular fluid resulting in massive swelling and an enormous increase in GAG hydration (204, 344, 358, 359). Tissue swelling is slower in onset and less severe in the setting of inflammation or tissue injury. In these conditions, matrix metalloproteinases (MMPs) are activated, resulting in a breakdown of the fibrillar extracellular matrix proteins and interruption of cellular adhesion complexes, mainly integrins. The resultant decrease in tissue tension leads to an increase in GAG hydration and tissue swelling. Ca²⁺ signaling in the setting of tissue injury can also be associated with a breakdown of the actin cytoskeleton and thus result in a more modest increase in GAG hydration (360). In fibrotic scar tissue, the relative amounts of the fibrillar extracellular matrix structures and the actin cytoskeleton increase, thus placing GAGs under additional pressure and reducing their hydration. Indeed, hydraulic resistance has an inverse relationship with GAG hydration. The more water that GAGs bind, the lower their resistance to flow, resulting in an increase in convective flow. On the other hand, fibrosis reduces the tissue fluid flow rate.

FIGURE 15.

Brain extracellular matrix. A: the extracellular matrix (ECM) in the brain is distinct from that in the periphery with respect to its near absence of protein components, being practically devoid of collagen, except collagen IV in the basement membranes, fibronectin, elastin, and laminins. The brain extracellular matrix is primarily composed of glycosaminoglycans (GAGs), mostly hyaluronan and chondroitin sulfate proteoglycans (CSPGs). CSPGs have a wide array of isoforms termed lecticans, including aggrecan, neurocan, brevican, and versican, which are linked together in a matrix via tenascin R. The actin cytoskeleton of cells in the neuropil bind to the CSPGs in the extracellular matrix via CD44 and other non-CD44 adhesion receptors. B: the vascular basement membrane is one of the few locations in the CNS where the extracellular matrix is fibrous, being composed of structural proteins similar to those found in peripheral basement membranes. In particular, the vascular basement membrane is composed of a 3-dimensional matrix of collagen IV and laminins. The isoform of laminin varies between different segments of the vascular network. The protein-rich vascular basement membrane matrix is stabilized by cross-linked nidogen and perlecan. Brain endothelial cells are interconnected via cadherin and the zonula occludens tight junctions, which together form the blood-brain barrier (BBB). The BBB inhibits the free entry of plasma water and many of its solutes from the vasculature. Endothelial cells attach onto the basement membrane via CD44 and integrin receptors in an interaction resulting in the upregulation of tight junction proteins, thus conferring to them their barrier properties. Astrocyte endfeet connected by gap junctions line the opposite side of the basement membrane and attach to the basement membrane via dystroglycan and integrin receptors. The dystroglycan receptor is composed of α- and β-subunits. The attachment of the α-dystroglycan to agrin in the basement membrane, together with the dystrophin-associated protein complex (i.e., dystrophin, dystrobrevin, and α-syntrophin), stabilize the localization of orthogonal arrays of intramembranous proteins composed primarily of aquaporin-4 (AQP4) water channels.

FIGURE 16.

Schematic representation of the organization of the glymphatic system. Cerebrospinal fluid (CSF) flow into the glymphatic system initiates at the basal cisterns of the subarachnoid space, flowing toward perivascular spaces of large cerebral arteries (see text for details). 1: From here, CSF flows into the perivascular spaces of penetrating arterioles that enter the brain parenchyma (4). In a process facilitated by aquaporin 4 (AQP4) channels expressed on the adluminal cell membrane of perivascular astrocytic endfeet, inward flow occurs over the perivascular glia limitans and into the brain extracellular space (22). 2: A directional flow of extracellular fluid is established through the narrow extracellular space toward the perivenous spaces (56). This flow rids the extracellular space of metabolic products and accumulated fluid, and provides a source of fresh extracellular fluid (1). 3: From the perivenous spaces, extracellular fluid exits the brain parenchyma. The glymphatic system has been shown to form a closed connection with dural lymphatics, so it seems likely that perivenous extracellular fluid finds egress through these channels draining into the venous system (70). Other egress pathways are perineuronal paths, perivenous spaces along dural sinuses and major cerebral vessels exiting the skull, and arachnoid granulations. See sect. 12 for details on egress of fluid from the intracranial compartment.

FIGURE 17.

Plausible fluid transport routes and their dependence on the polarized expression of aquaporin 4 (AQP4) at perivascular astrocytic endfeet. Directional fluid transport from arterial to venous vasculature would require differential conditions at the site of influx (left) and efflux (right ) of fluid. Possible flow pathways can be largely divided into transcellular or paracellular transport routes. Transcellular routes (top row) would consist of direct entry of fluid through AQP4 channels, and could be subdivided into the following: 1: trans-astrocytic fluid transport, where fluid enters the astrocyte at the endfoot and continues through the soma and the greater glial syncytium following a largely intracellular pathway, or 2: trans-endfoot fluid transport, where fluid enters into the endfoot and rapidly exits into the extracellular space (ECS) without entering the soma, thus following a largely extracellular pathway interspersed with short segments of intracellular transport. Alternatively, fluid may also follow paracellular routes (bottom row), which would consist of fluid transport through the clefts formed by neighboring endfeet. This primarily extracellular transport route would rely indirectly on AQP4 water channel expression. 3: Hydrostatic pressure gradients, between the arterial and venous sides of the extracellular space could drive directional fluid flow toward the region of lower pressure (P_artery > P_ECS > P_vein). 4: osmotic pressure gradients could be generated by astrocytic ionic fluxes (e.g., K⁺ spatial buffering) that in part depend on AQP4 polarization. Directional transport of extracellular osmoles could potentially drive fluid entry between endfeet toward efflux sites (σπ_vein > σπ_ECS > σπ_artery). PVS, perivascular space.

FIGURE 18.

Intracranial oscillations as drivers for brain fluid transport. A: oscillations can be measured by different techniques in separate compartments, such as neural oscillations measured using EEG or local field potential (493). They display important state-dependent changes such as the predominance of delta waves, K complexes, and sleep spindles during nonrapid eye movement sleep (494). Vascular oscillations are divided into extracranial and intracranial oscillations. Extracranial oscillations are the cardiac (very high frequency) and respiratory (high frequency) cycles, while intracranial oscillations are divided into myogenic (low), neurogenic (very low), and endothelial components [Kiviniemi et al. (495) and Stefanovska et al (496)]. Endothelial oscillations consist of nitric oxide (NO)-dependent (ultralow) and NO-independent (superlow) mechanisms (497). Several neural and vascular oscillations have been measured in CSF. Arterial pulsations from the very high frequency cardiac cycle drive perivascular flow [Mestre et al. (21), Iliff et al. (54), and Harrison et al. (407)]. Flow changes are also driven by the respiratory cycle (498) (Strik et al. (499). Myogenic oscillations, also known as vasomotion, have been measured in cerebral spinal fluid (CSF) [van Veluw et al. (439)]. These myogenic oscillations are sometimes described as ultraslow, Mayer (M) waves, or Lundberg C waves (499) [Drew et al. (500), Lundberg (769), Lang et al. (770)]. B: diagram of coupled oscillators that drive CSF flow. Extracranial oscillators such as the cardiac and respiratory (Resp.) cycles transmit pressure waves into the intracranial compartment by modulating the amount of blood entering the vascular compartment, which is measured as cerebral blood flow (CBF) and cerebral blood volume (CBV). Neural processes like functional hyperemia in turn also modulate CBF and CBV [Buszaki et al. (493)]. Changes in CBF and CBV can be evaluated from functional MRI blood oxygen level-dependent (BOLD) contrast signals [Fultz et al. (501)]. The direction of coupling between the various oscillators is an exciting and growing field to determine the multiple processes underlying CSF flow (502).

FIGURE 19.

Physiological drivers of the glymphatic system. Flow in the glymphatic system is established by different physiological processes driving intracerebral pressure gradients. Top left inset: anterograde cerebral spinal fluid (CSF) flow in perivascular spaces of leptomeningeal surface arteries is driven by perivascular pumping. The distension of the arterial wall caused by the cardiac impulse wave pumps CSF forward in a pulsatile fashion, where increased CSF flow velocity occurs in phase with the R component of the ECG (systole) (21). Bottom left inset: the respiratory cycle changes intrathoracic pressure, which affects central venous pressure and also the cardiac cycle (respiratory bradycardia). This in turn affects venous pressure in brain as well as intracranial pressure (ICP). Intraventricular CSF flow is thus highly dependent on the respiratory cycle, where inspiration leads to a drop in venous blood volume in brain, thus causing increased flow of CSF into the ventricular system. Forced inspiration can increase this CSF flow rate (498, 515). In the glymphatic system, the venous pressure changes caused by the respiratory cycle are believed to have earlier and greater effects on perivenous spaces than periarterial spaces (495). This might impose a pressure gradient facilitating extracellular fluid flow toward the venous compartment during each inspiration. Top right inset: increased brain extracellular volume is associated with increased glymphatic function that occurs during sleep. The 60% increase in extracellular space volume that typically occurs during nonrapid eye movement (NREM) sleep causes a drop in hydraulic resistance that promotes fluid flow through the parenchyma (1). ECS, extracellular space. Bottom right inset: changes in arterial and arteriolar diameter caused by autoregulation of smooth muscle cell tone have been associated both with intraventricular CSF flow and brain clearance. In the 4th ventricle, increased CSF inflow deeper into the ventricular system has an anticorrelation with the blood oxygen level-dependent (BOLD) signal on functional MRI. This suggests that loss of cerebral blood volume due to vasoconstriction following the hyperemia induced by global increases in neural activity during NREM sleep increases CSF flow (501). Brain fluid clearance in mice has also been associated with vasomotion, both in association with 0.1-Hz neuronal oscillations occurring during wakefulness and during evoked functional hyperemia (439). PVS, perivascular space.

FIGURE 20.

Glymphatic activity and cerebrospinal fluid (CSF) egress during sleep and wakefulness. The glymphatic system is primarily active during deep sleep, when brain clearance more than doubles compared with wakefulness. This is likely due to an increase in extracellular volume fraction, which in rats is ∼14% during wakefulness but increases to 23% in a process dependent on the loss of noradrenergic signaling from the locus coeruleus (1). This could occur by a mechanism whereby perivascular space access is shut off during periods of high noradrenergic tonus, but then opens up for fluid influx during the low noradrenergic tonus of sleep. The rate of CSF egress from the subarachnoid space shows an inverse relationship with glymphatic activity, thus increasing during wakefulness and decreasing during sleep (523). Top row: the relationship between sleep and glymphatic activity. It is important to note that many details of the interconnection between the various stages of sleep and glymphatic flow remain to be determined. For example, the activity of the glymphatic system during rapid eye movement (REM) sleep is unknown. Bottom row: the critical importance of the extracellular volume fraction on glymphatic flow in the active state (left) and during inactivity [non-REM (NREM) sleep, right]. See sect. 12.10 for details on CSF egress. ISF, interstitial fluid.

FIGURE 21.

Glymphatic system and its role in clearance, nourishment, and volume transmission. The glymphatic system serves various physiological functions. By establishing extracellular fluid efflux, the glymphatic system clears the brain parenchyma of extracellular metabolites and waste products (1, 4). Furthermore, the influx of cerebrospinal fluid (CSF) to the brain along periarterial spaces also provides nourishment to the brain, whereby the glymphatic system distributes glucose to neurons and astrocytes, and delivers lipids and apolipoprotein E arising in the choroid plexus throughout the brain (436, 537, 538). Certain vitamins like folate and ascorbate are exclusively delivered to the brain parenchyma by CSF. Finally, the glymphatic system may play a role in local volume transmission, which refers to the transport of “spill-over” neurotransmitter from a synapse to neighboring non-postsynaptic cells. Glymphatic flow in the extracellular space would theoretically increase local volume transmission, as may occur during nonrapid eye movement (NREM) sleep, when large volumes of CSF wash through the neuropil. Interesting, NREM sleep is characterized physiologically by large scale synchrony of neuronal activity, which fits with the notion of widespread humoral action supported volume transmission, as opposed to the tight spatiotemporal coupling of neurotransmission during wakefulness. Global volume transmission refers to the brain-wide delivery of neuropeptides or hormones with CSF, arising in circumventricular organs or from the choroid plexus. CVO, circumventricular organs; AQP4, aquaporin 4.

FIGURE 22.

Overview of existing models evaluating intra-axial fluid transport. The two main types of models are perivascular pumping (left) and parenchymal fluid flow (right). Perivascular pumping models consist of an artery with a surrounding perivascular space (PVS). Most such models implement a concentric anulus in which the artery lies central to the larger PVS, but modifying the centricity and the shape of the PVS cross-section to better represent in vivo measurements greatly reduces the calculated hydraulic resistance (409). The size and porosity (or lack thereof) of the PVS also affect the hydraulic conductivity (21, 409) The velocity of the arterial pulsation (wave speed) divided by the frequency of the pulsation (heart rate) indicates the wavelength. Models have used wide ranges of wavelengths due to the difference in resting heart rates between mice and humans, and due to inconsistent use of pulse wave velocity measurements collected from different blood vessels and species (278). Including outflowing boundary conditions such as intracranial compliance (C) and cranial cerebral spinal fluid outflow resistance (R) modifies the volume flow rate through the system (554). Few studies have evaluated how vascular branching and bifurcations affect wave speed transmission and arterial rigid motions (411, 555). The majority of perivascular pumping models have found that a static pressure gradient (ΔP) across the PVS is required for arterial pulsations to drive net flow at a rate comparable to those measured in vivo (21, 282, 555, 556). Parenchymal fluid flow models primarily consist of a segment of brain tissue interposed between an artery and a vein. The majority of such models to date have not included the interposed capillary network, which could but present an interesting extension. The extracellular space (ECS) geometry can be idealized or reconstructed from serial electron microscopic images. The volume and tortuosity of the ECS is approximated using the alpha (α) and lambda (λ) terms measured from real time iontophoresis experiments. The viscosity and concentration of solutes in the extracellular fluid can also be defined in these models. Static pressure gradients from the arterial side to the venous side have been tested as possible drivers of parenchymal fluid flow.

FIGURE 23.

Reframing the contributions of advection and diffusion to glymphatic clearance. A: 2-dimensional representation of the glymphatic pathway, composed of a brain-wide network of perivascular spaces that branch along the cerebral vasculature. Cerebrospinal fluid (CSF) flows into the system (Q_in) through the large surface perivascular spaces of the cerebral arteries flowing at several microns per second (µm/s) and transports solutes like oxygen and carbon dioxide by a primarily advective process. As the flow pathways bifurcate and divide into daughter branches, glymphatic fluid flow slows down perhaps up to several nanometers per second (nm/s) near the capillaries in the tissue where diffusion would dominate transport of most solutes. The flow continues along the closed system toward the outflow routes and coalesces increasing in speed. The total outflow rate (Q_out) might not be exclusively composed of fluid coming from perivenous spaces but might also include white matter tracts and subependymal pathways. The sum of all these outflow pathways should theoretically equal the inflow rate (Q_in = Q_out). B: ions and small molecules (e.g., H₂O: 18 Da; Na: 23 Da; O₂: 32 Da; and K: 39 Da) can be transported quickly and efficiently across short distances via diffusion, while larger molecules (e.g., lactate: 90 Da, and glucose: 180 Da), peptides and protein aggregates (e.g., Aβ_1-40: 4.3 kDa; Aβ_1-42: 4.5 kDa; albumin: 66.5 kDa, Aβ oligomers: 100-200 kDa; and tau aggregates: 242 kDa) might rely more so on the advective modes of transport for clearance. C: glymphatic function is dependent on the interplay between effective advective transport via perivascular spaces and adequate access to the extracellular space. Dysfunction in glymphatic clearance could be due to variations in either or both of these processes. RBCs, red blood cells; AQP4, aquaporin 4.

FIGURE 24.

Extracellular fluid efflux and intracranial fluid egress. To distinguish between outflow of fluid from the brain parenchyma and outflow from the intracranial and spinal compartments to the peripheral circulatory systems, we have defined the following terms that we shall use categorically to distinguish between these two serially connected fluid outflow systems. Efflux: glymphatic efflux, also known as brain clearance, is the process by which extracellular fluid and its solutes in intra-axial compartments of the brain parenchyma transfers to cerebrospinal fluid (CSF) compartments or directly to egress sites. For example, glymphatic efflux includes the outflow of waste proteins in the brain extracellular space along major cerebral draining veins, possibly connecting with dural lymphatics and dural parasagittal spaces. Egress: this refers to the transport of CSF, extracellular fluid, solutes and cells from the central nervous system compartment to the periphery, e.g., the transport of CSF tracers from the subarachnoid space through dural lymphatics to the cervical lymphatic system.

FIGURE 25.

Cerebrospinal fluid (CSF) egress sites in the cranium. CSF can drain from the intracranial subarachnoid space via many different routes. In general, CSF egress proceeds either to the cervical lymphatic system or directly into the bloodstream. Drainage to the cervical lymphatic system can occur through several different structures. Perineuronal drainage, especially along the olfactory nerves, seems to be one of the most important egress pathways. CSF egress along the perineuronal routes can occur within the nerve axons, in the endoneurial fluid or in a subepineurial space, or even in the epineurial connective tissue. The perineurium is an epithelial barrier membrane sealed by tight junctions. Lymphatics associated with the cranial nerves can also direct CSF out of the cranium, as can the dural lymphatics. Present in dorsal and basal dura mater, the dural lymphatics take up macromolecules from the CSF. Morphological characteristics of the lymphatics in the basal dura mater hint that these vessels might be better conduits for lymphatic drainage than are the dorsal dural lymphatics. Nonvessel like structures designated as the parasagittal dura spaces that drain CSF have also been identified in dura mater. These spaces can convey CSF to the lymphatics in dura mater, and also to the arachnoid granulations. The only structures known to drain CSF directly to blood are the arachnoid villi/granulations, which are protrusions of the arachnoidea through slits in the dura mater. CSF egress along this pathway may occur by transcellular and paracellular routes.

FIGURE 26.

A reduction in glymphatic fluid transport has been demonstrated to occur in many neurological disorders as well as healthy aging. In healthy aging of mice, impaired brain clearance arises to due to loss of aquaporin 4 (AQP4) polarization, which abrogates the removal of extracellular amyloid-β (15). In neurodegenerative disorders such as Alzheimer’s disease and Parkinson’s disease, reduced glymphatic function has been linked to build-up of amyloid-β (450, 688) and α-synuclein, respectively (544, 689). In an ischemic stroke model in mice, spreading ischemic depolarizations cause massive vasoconstriction as well as an ionic shift that leads to increased cerebral spinal fluid (CSF) influx to brain, resulting in edema (469). In mouse and nonhuman primate models of subarachnoid hemorrhage, blood components including fibrin and fibrinogen occlude periarterial spaces and impair CSF influx (163). Both mild and moderate traumatic brain injury (TBI) have been associated with reduced glymphatic function in rodents. In human TBI, there is a pathological buildup of hyperphosphorylated tau, typically seen in perivascular areas (56, 690, 691). Potential biomarkers for TBI (S100-B, for example) are washed out of the brain parenchyma by the glymphatic system predominantly during sleep. As such, sleep disruption due to the standard clinical practice of administering repeated neurological exams could thus be impairing glymphatic function and interfering with the detection of blood-based biomarkers in TBI (692). A rodent model of noncommunicating hydrocephalus and human cases of normal pressure hydrocephalus have been associated with impaired glymphatic clearance of brain parenchyma (162, 164,165, 533, 693). RBC, red blood cells. SAH, subarachnoid hemorrhage; PVS, perivascular space; GFAP, glial fibrillary acidic protein; NSE, neuron-specific enolase.

FIGURE 27.

Technological approaches to imaging of glymphatic system. Different techniques used to image the glymphatic system plotted in a 3-dimensional plot depicting the typical qualities of each imaging modality in 3 different measures, namely temporal resolution, spatial resolution, and volume imaging. It should be noted that any of these imaging modalities can be adjusted in a manner changing their parameters. For example, increasing temporal resolution of functional MRI brings a loss in spatial resolution, as occurs in ultrafast magnetic encephalography, or is at the expense of limiting the brain volume imaged (407, 495). Histology provides the highest spatial resolution and affords brain wide imaging, albeit with a loss of precise temporal control, and at a cost of considerable workload. Furthermore, histological preparations all suffer from postmortem changes such as loss of extracellular volume, as well as preparation artifacts (shrinkage) that might confound the results. Tracers are traditionally delivered into cisterna magna or intrathecally (–553), but noninvasive approaches to imaging fluid transport are under fast development. CT, computed topography; Macro, macroscopic transcranial imaging; PET, positron emission tomography; MRI, magnetic resonance imaging; 2p, two-photon microscopy.