The Cerebrospinal Fluid (CSF)

The Cerebrospinal Fluid (CSF)

The cavity enclosing the brain and spinal cord has a volume of 1600 ml. The total volume taken up by the cerebrospinal fluid is 150 ml.

CSF is produced at a rate of 500 ml/day, about 21- 22 ml/hour. So there is a fairly rapid turnover each day.

Two thirds of the CSF is formed in the Choroid Plexuses in the Lateral, 3rd, and 4th ventricles. These are “fringes” of capillary tufts from the pia mater invaginating the walls of the ventricles. Additional amounts are secreted by all of the lining membrane surfaces of the ventricles and arachnoid membranes.

The secretion of cerebrospinal fluid occurs mainly by “active transport” of Na+ ions through the cells lining the outside of the choroid plexus. These sodium ions are closely followed by Cl- ions which follow along with H2O.

Composition of CSF vs. dialysed plasma:
Osmotic Pressure = plasma
Sodium = plasma
Chloride- 15% greater than plasma
Potassium- 40% less than plasma
Mg- 25% greater than plasma
Glucose- 30% less than plasma

Cerebrospinal Fluid Function
The primary function of the CSF appears to be mechanical. The central nervous system is suspended within a series of 3 membranous coverings, the meninges, that stabilize the shape and position of nerve tissue in two different ways during head and body movements.

First, the brain is mechanically suspended within the meninges, which in turn are anchored to the skull so that the brain is constrained to move in parallel with the head.

Second, there is a layer of CSF within the meninges. The buoyant effect of this fluid environment greatly decreases the tendency of various forces (such as gravity) to distort the brain. Thus a brain weighing 1500 grams in air effectively weighs less than 50 grams in its normal cerebrospinal fluid environment, where it is easily able to maintain its shape.

In contrast, an isolated fresh brain, unsupported by its usual surroundings, becomes seriously distorted and may even tear under the influence of gravity.
A second function of the cerebrospinal fluid occurs in concert with metabolism. There are no lymphatics within the central nervous system and the CSF largely performs the function carried out by lymph in other tissue. CSF is involved in the removal of the waste products of cerebral metabolism, primarily CO2, lactate and excess H+ ions. The CSF also serves as a route for the conduction of neuron-active peptides and hormones and also helps to provide a stable chemical environment for cerebral and spinal neurons.

Cerebrospinal Fluid and the “Blood- Brain Barrier”
The endothelial cells of the capillaries of the brain are joined closely together by so-called “tight junctions”. The membranes of adjacent cells are almost fused with each other. This causes a very low permeability (barrier) between the blood and brain and between the CSF and brain. This is a protective device.

This interface between the blood/brain and CSF/brain is highly permeable to H2O, CO2, O2, which are fairly harmless, and alcohol.

This barrier is only slightly permeable to Na+, Cl-, K+ and large organic molecules. This makes it very difficult to achieve effective concentrations of antibodies or non-lipid-soluble drugs in the brain or the CSF.

The Blood- CSF Barrier
However, the surfaces of the ventricles are lined with a thin, cuboidal epithelium membrane and the CSF on the outer surface of the brain is separated from the brain tissue by only a thin layer of pia mater. Both of these layers are extremely permeable. This means that almost all substances which enter the CSF can diffuse into the surface areas of the brain interstitial fluid. Substances within this interstitial fluid can diffuse in the other direction as well.

Circulation of the Cerebrospinal Fluid
The journey of CSF begins in the Lateral ventricles. The CSF moves through the Foramen of Monroe to the 3rd Ventricle. From there it moves through the Aqueduct of Sylvius down into the 4th Ventricle where it exits through the paired Foramen of Lushka and the Foramen of Magendie into the Cerebromedullary cistern and the cistern of the great cerebral vein. From here the CSF circulates up and around the cerebral hemispheres and exits through the Arachnoid Villi into the Superior Sagittal Sinus.

 

Some of the CSF moves from the cisterns into the subarachnoid space and circulates around the spinal cord. The CSF flows downward, posterior to the cord and upward, anterior to the cord.

Arterial pulsations cause a constant ebb and flow with a small net movement toward the sagittal sinus with each heartbeat. It requires several hours for the CSF to make its journey.
Absorption of the Cerebrospinal Fluid
The normal pressure of the cerebrospinal fluid is 10 mm Hg. In contrast, the normal pressure of interstitial fluids is -3 to -5 mm Hg.

Cerebrospinal fluid pressure is regulated almost entirely by the Arachnoid Villi. These villi are microscopic fingerlike projections of arachnoid membrane through the walls of the venous sinuses. Large conglomerates of these villi are known as “granulations” and are macroscopic, seen protruding into the venous sinuses.

These structures function like one-way valves to allow the CSF into the venous blood of the sinuses but not the blood in the opposite direction. These valves will open at a pressure of 1.5 mm Hg greater than the pressure of the blood within the sinuses.

Some of the cerebrospinal fluid is also taken up by lymphatics along the meningeal sleeves which enclose the spinal nerve roots. It has been concluded that alternate pathways exist for the drainage of CSF into the lymph.

CSF and Cranial Release Technique
One long- held premise of cranial- based healthcare has been that the flow and function of the cerebrospinal fluid can be negatively altered by distortion/dysfunction of the cranial bones. This has ramifications for the health of the brain and central nervous system and thus, for the health of the entire body.

“The brain and nervous system control and coordinate the function of all the tissues, organs and systems of the body.”
Guyton- “Textbook of Human Physiology”

It can readily be seen that the restoration of optimal movement of the cranial bones through the Cranial Release Technique will have positive effects in the restoration of health and the maintenance of overall well-being.

 

References:

Textbook of Human Physiology;  A. Guyton, M.D.

Physiological Basis of Medical Practice;  Best and Taylor

Textbook of Human Anatomy;  H. Gray, F.R.S.

Ciba Collection of Medical Illustrations, Vol. 1- Nervous System; F. Netter, M.D.

Pathologic Basis of Disease;  S. Robins, M.D., R. Cotran, M.D., V. Kumar, M.D.

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