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Fig. 8 | Fluids and Barriers of the CNS

Fig. 8

From: Fluid and ion transfer across the blood–brain and blood–cerebrospinal fluid barriers; a comparative account of mechanisms and roles

Fig. 8

Schematic diagram of brain structures, CSF flows and perfusion pipette positions related to the perfusion studies and other investigations discussed in this section. Most of the CSF is produced by the choroid plexuses located in the lateral (L), IIIrd and IVth ventricles. Net CSF flow then normally proceeds through the cisterna magna (CM) to the subarachnoid spaces (SA), which for this purpose include the basal cisterns. Outflow from the brain occurs via a number of routes including perineural routes through the cribriform plate (cp), the arachnoid villi (av), perineural pathways at roots of nerves (nr) including those in the spinal cord, and, in addition, perivascular routes and dural lymphatics that are not shown [543, 544]. Any fluid secreted by the blood–brain barrier within the parenchyma can flow into CSF in the subarachnoid spaces or leave the brain by perivascular and perhaps perineural pathways without first mixing with the CSF that is sampled at the cisterna magna. Flows are investigated using a number of perfusion techniques. In ventriculo-cisternal perfusion, fluid is injected via a pipette or cannula at (a) and withdrawn at (b). For ventriculo-lumbar perfusion the withdrawal is at (c) while for ventriculo-subarachnoid perfusion at d. For spinal perfusion fluid is injected at (b) or (e) and withdrawn at (c). In non-communicating hydrocephalus as discussed in this review, the aqueduct connecting the IIIrd and IVth ventricles is blocked as indicated at (i). In hydrocephalus induced by injection of kaolin into the cisterna magna the block is at the cisterna magna and at its connections to the IVth ventricle and the subarachnoid spaces as indicated at (ii). The causative pathology in communicating hydrocephalus is unknown but outflow of CSF is somehow hindered (see Fig. 9). In kaolin induced hydrocephalus the major escape route for CSF is now thought to be along the spinal canal, through spinal parenchyma to the subarachnoid space and out via the nerve roots. In non-communicating hydrocephalus (point 3i) and possibly in communicating hydrocephalus (points 3iii) there is a route of escape of CSF from the lateral and IIIrd ventricles, indicated in the diagram as being from the IIIrd ventricle

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