Cerebral Spinal Fluid - Anatomy & Physiology
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Cerebrospinal Fluid Function
Cerebrospinal fluid (CSF) surrounds the brain as well as the central canal of the spinal cord. It helps cushion the central nervous system (CNS), acting in a similar manner to a shock absorber. It also acts as a chemical buffer providing immunological protection and a transport system for waste products and nutrients. The CSF also provides buoyancy to the soft neural tissues which effectively allows the neural tissue to "float" in the CSF. This prevents the brain tissue from becoming deformed under its own weight. It acts as a diffusion medium for the transport of neurotransmitters and neuroendocrine substances.
CSF Production and Constituents
CSF is a clear fluid produced by dialysis of blood in the choroid plexus. Choroid plexi are found in each lateral ventricle and a pair are in the third and fourth ventricle. Further production also comes from the ependymal cell linings and vessels within the pia mater.
Edendymal cell production of CSF is via ultrafiltration of blood plasma and active transport across the ependymal cells. The ependyma is connected via a series of tight junctions preventing molecules passing between cells. The ependyma also sits on a basement membrane to provide support to the ependymal cells and provide further protection against blood perfusion. In areas of the brain where there are choroid plexi, the endothelium of the plexus vessel sits immediately adjacent to the basement membrane of the ependymal cells. Of the total CSF production, 35% is produced within the third ventricle of the brain, 23% via the fourth ventricle and 42% from general ependymal cell filtration.
CSF has a very low protein constituent, with only albumin being present together with a very low level of cellularity. The biochemistry of CSF includes high concentrations of sodium and chloride and very high concentrations of magnesium. Concentrations of potassium, calcium and glucose are low.
CSF Circulation
Once produced, CSF is then circulated from the choroid plexus through the interventricular foramina into the 3rd ventricle, and then through the cerebral aqueduct (aqueduct of Sylvius) into the 4th ventricle before flowing through the cerebromedullary cistern down the spinal cord and over the cerebral hemispheres. Most CSF escapes from the ventricular system at the hindbrain foramen of Luschka. CSF then flows down the length of the spinal cord in the subarachnoid space. Large amounts are drained into venous sinuses through arachnoid granulations in the dorsal sagittal sinus. Arachnoid granulations contain many villi that are able to act as a one way valve helping to regulate pressure within the CSF.
Any disruption to this flow can result in increased pressure building up within parts of the CSF system which can cause compression of neural structures surrounding the area of increased pressure. Clinical examples of this are hydrocephalus and syringohydromyelia.