Clinical and experimental observation has established that the cerebrospinal fluid (CSF) is mainly formed by secretion from the choroid plexuses of the cerebral ventricles.
That formed by the plexuses of the lateral ventricle passes through the interventricular formina into the third ventricle. Since the fluid flows through the cerebral aqueduct into the fourth ventricle , which it leaves by the median and to lateral foramina (of Magendie and Luschka) of the 4th ventricle to reach the subarachnoid space covering the cerebrum and also the spinal cord.
The subarachnoid space
(SAS), which lies between the arachnoid membrane externally
and the pia mater internally, carries the flow from the
cerebral ventricles to its points of absorption. The inner
surface of the arachnoid and the outer surface of the pia are
covered with flattened mesothelial cells; these also cover the
numerous trabeculae, which bridge the SAS, and the nerves and
blood vessels which pass across it.
Cross section of brain covering showing relation between layers
The SAS is deepest at the base of the brain; its expansions constitute the various cisterns, the largest of which is the cerebello-medullary cistern or cisterna magna which lies between the cerebellum and medulla and extends downwards below the foramen magnum behind the spinal cord.
The SAS extends superficially over the whole surface of the brain and spinal cord. Every blood vessels entering or leaving the nervous system must pass across it. In so doing, it carries with it into the nervous system a sleeve of arachnoid immediately surrounding the vessel and a sleeve of pia mater more externally. Between the two lies an extension of the SAS, known as the perivascular or Virchow- Robin space which subdivides with each division of the vessel to terminate where the pia mater and arachnoid become continuos.
Probably, products of
metabolism and cell - containing inflamatory exudates pass from
the perivascular spaces to enter the CSF in the SAS, but must
cross the tight junction of the capillary endothelial cells which
largely constitute the blood/ CSF and blood/brain
barriers in order to do so. These barriers may be
damaged by inflammation and other processes and it is in the
perivascular spaces that cuffs of inflammatory cells are found in
inflammatory disorders of the NS.
Much work involving clinical observation in man and experimental studies in animals using studies of spinal drainage of isotopic exchange, of ventriculo-cisternal perfusion, Manometric infusion techniques, and radiographic contrast studies, most recently employing metrizamide washout to measure CSF bulk flow as well as biochemical investigation upon the composition of ventricular, cisternal, and spinal samples of fluid have shown that the fluid is largely formed by active secretion and transport via the choroid plexuses. Vesicular transport is probably also involved. The total volume of the fluid at any one time probably varies between 70-120 ml in different subjects and its rate of formation is about 0.35 ml per min.
It is thus replaced several times each day. There is also known to be a constant process of dialysis with exchange of various chemical constituents between the CSF and the Blood across the ventricular empendyma, the perivascular spaces and the arachnoid membrane at all levels. Large molecules fail to enter the CSF from the blood because of the interposition of the vascular endothelium (the blood CSF barrier) but there is a rapid exchange of small molecular weight substances between the CSF and the extracellular fluid of the brain and cord.
The fluid acts in some respect as a "sink" preventing the extracellular fluid of the brain from achieving a true equilibrium with the blood plasma. The composition of the ventricular CSF is very different from that of cisternal and spinal fluid, indicating that some components are added to the fluid across the spinal arachnoid.
After bathing the surface of the spinal cord and the base of the brain, CSF passes upward over the convexity of the hemispheres to be absorbed into the intracranial venous sinuses. Studies of bulk flow have shown that absorption place through the microscopic arachnoid villi, which are minute projections of the SAS into the lumen of the sinus. Electron microscopic studies demonstrated vacuoles within the cells of the villi which suggested that there is a dynamic system of transcellular channels or pores which allow the bulk outflow of CSF across the mesothelial barrier.
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