This is a representation of the production, circulation, and reabsorption of CSF, the ventricles of the brain, and the subarachnoid spaces around the brain, enlargements of which are called cisterns.
The ventricles of the brain are lined with a layer of cells known as the ependyma. In certain loci within each of the ventricles, the ependymal cells and the pia meet, thus forming the choroid plexus, which invaginates into the ventricle. Functionally, the choroid plexus has a vascular layer, i.e., the pia, on the inside, and the ependymal layer on the ventricular side. CSF is actively secreted by the choroid plexus. The blood vessels of the choroid plexus are freely permeable, but there is a cellular barrier between the interior of the choroid plexus and the ventricular space — the blood-CSF barrier (B-CSF-B). The barrier consists of tight junctions between the ependymal cells that line the choroid plexus. CSF is actively secreted by the choroid plexus, and an enzyme is involved. The ionic and protein composition of CSF is different from that of serum.
Choroid plexus is found in the lateral ventricles (see Figure 20A), the roof of the third ventricle, and the lower half of the roof of the fourth ventricle. CSF produced in the lateral ventricles flows via the foramen of Monro (from each lateral ventricle) into the third ventricle, and then through the aqueduct of the midbrain into the fourth ventricle. CSF leaves the ventricular system from the fourth ventricle, as indicated schematically in the diagram. In the intact brain, this occurs via the medially placed foramen of Magendie and the two laterally placed foramina of Luschka (as described in the previous illustrations) and enters the enlargement of the subarchnoid space under the cerebellum, the cerebello-medullary cistern, the cisterna magna. The cisterna magna is found inside the skull, just above the foramen magnum (see Figure 18).
CSF flows through the subarachnoid space, between the pia and arachnoid. The CSF fills the enlargements of the subarachnoid spaces around the brainstem — the var ious cisterns (each of which has a separate name). The CSF then flows upward around the hemispheres of the brain and is found in all the gyri and fissures. CSF also flows in the subarachnoid space downward around the spinal cord to fill the lumbar cistern (see Figure 1, Figure 2C, and Figure 3).
This slow circulation is completed by the return of CSF to the venous system. The return is through the arachnoid villi, protrusions of arachnoid into the venous sinuses of the brain, particularly along the superior sagittal sinus (see Figure 18). These can sometimes be seen on the specimens as collections of villi, called arachnoid granulations, on the surface of the brain lateral to the interhemispheric fissure.
There is no real barrier between the intercellular tissue of the brain and the CSF through the ependyma lining the ventricles (at all sites other than the choroid plexus). Therefore, substances found in detectable amounts in the intercellular spaces of the brain may be found in the CSF.
On the other hand, there is a real barrier, both structural and functional, between the blood vessels and the brain tissue. This is called the blood-brain barrier (BBB), and it is situated at the level of the brain capillaries where there are tight junctions between the endothelial cells. Only oxygen, carbon dioxide, glucose, and other (select) small molecules are normally able to cross the BBB.
The CSF flows down around the spinal cord and into the lumbar cistern. Sampling of CSF for clinical disease, including inflammation of the meninges (meningitis), is performed in the lumbar cistern (see Figure 1, Figure 2C, and Figure 3). The CSF is then analyzed, for cells, proteins, and other constituents to assist or confirm a diagnosis.
The major arteries of the circle of Willis travel through the subarachnoid space (see Figure 58). An aneurysm of these arteries that "bursts" (discussed with Figure 59A) will do so within the CSF space; this is called a subarach-noid hemorrhage.
Hydrocephalus has been discussed with the previous illustration.
P = Pituitary gland Ventricles
LV = Lateral ventricle
3 = 3rd ventricle
Aq = Aqueduct of midbrain
4 = 4th ventricle
Cp = Choroid plexus
Sc = Spinal cord
Cc = Central canal
Foramen of Magendie CSF cisterns
Qc = Quadrigeminal cistern Cm = Cisterna magna
Ss = Superior sagittal S = Straight Ag = Arachnoid granulation
C = Cerebellum
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