Cerebrospinal fluid (CSF) is the body fluid that constantly flows around the brain and spinal cord in what is known as the inner and outer CSF spaces. It is a system of interconnected cavities. The CSF renews itself up to four times a day in a continuous process of production and reabsorption. An important primary function is to protect the brain against concussions. The extent to which nutritional and other metabolic processes play a role in neural tissue has not yet been conclusively investigated.
What is cerebrospinal fluid?
Cerebrospinal fluid – as it is fully called – surrounds the cerebrum, diencephalon and spinal cord in special cavities that communicate with each other like communicating tubes. The cavities can be divided into an inner and an outer CSF space. The inner CSF space is formed by so-called ventricles, which are partially lined with a plexus of veins, the choroid plexus, from which fluid is constantly being formed and released into the ventricles. To keep the volume and pressure of the circulating CSF constant, the normally clear fluid diffuses into the venous blood via special villi (arachnoid villi) on the walls of the outer CSF space and is carried away by veins for further processing. The production and resorption rates must reach the same values. A special blood-cerebrospinal fluid barrier prevents venous blood from entering the outer CSF space via the arachnoid villi. Below the skullcap, the CSF envelops the cerebrum in a thin layer between the two soft meninges – comparable to a gel-padded inner protective helmet.
Composition
CSF-also called cerebrospinal fluid-is a normally crystal-clear, colorless fluid that contains only isolated cells and has a glucose concentration of 2.7 to 4.8 mmol/l, well below normal blood levels. The protein content, with values around 0.15 to 0.45 g/l, is also below that of blood serum, whose protein content is more than two hundred times higher. The CSF is generated in the inner CSF space in the walls of the four ventricles and flows slowly through special junctions (foramina) to one of the outer CSF spaces, ultimately re-entering the bloodstream via the arachnoid villi. The inner CSF space consists of the two lateral ventricles in the cerebrum with a distinct geometry with anterior, posterior and inferior horns, as well as a middle section, the third ventricle in the diencephalon and the fourth ventricle, which runs further below in the rhombencephalon or rhombus brain. The fourth ventricle communicates with the external cerebrospinal fluid space through a total of three openings through which cerebrospinal fluid can pass into the external cerebrospinal fluid space.
Function and Tasks
Probably the most important task of the cerebrospinal fluid is its mechanical-hydraulic protective function of the brain. The outer cerebrospinal fluid space is of particular importance in this regard. Below the skullcap, the cerebrospinal fluid circulates between the two soft meninges, the pia mater and the arachnoid mater, forming a kind of gel cushion that protects the brain – especially the cerebrum – from shocks acting on the head or the skullcap. Since the brain is largely surrounded by CSF, it virtually floats, so that the “imprinting surface” of the brain to compensate for gravity or other acceleration is evenly distributed in any direction and protects the brain from punctual and unilateral mechanical pressure that could lead to serious consequences. The extent to which CSF contributes to the supply of nutrients or other physiologically active substances to neurons has not yet been conclusively determined. The relatively high reproduction rate of the cerebrospinal fluid could be an indication that the CSF absorbs and removes degradation products from the metabolism of the nerve cells. CSF is also the source substance for the perilymph in the inner ear (scala tympani) and in the vestibular organs (scala vestibuli). Perilymph resembles CSF in its electrolyte composition, and the outer CSF space communicates with the perilymphatic spaces via the ductus perilymphaticus.
Complaints and diseases
Complaints and diseases directly related to cerebrospinal fluid are present when the rate of reproduction and reabsorption of cerebrospinal fluid are out of balance. Disturbances in the balance within the CSF may be caused by disease or the disturbances themselves may cause other diseases. In the case of an increase in the total volume of circulating CSF, the pressure of the fluid in the CSF spaces increases with serious consequences. Excessive fluid increase can be caused by cerebral edema, inflammatory processes in the brain as well as, for example, by a severe oversupply of vitamin A (hypervitaminosis A). The increase in pressure can also be caused by brain tumors, which take up space as a result of their physical dimensions and thus increase the pressure. A shift in equilibrium due to disruption of CSF outflow, or reabsorption, is also known to cause increased pressure in the CSF spaces. Decreased CSF outflow can be caused, for example, by a congenital malformation, adhesion of the meninges, or disruption of the transition from the internal to the external CSF spaces. The most important indication of increased CSF pressure, or intracranial pressure, apart from general symptoms such as headache and vomiting, is the development of edema in the tissue of the optic nerve papilla. In advanced stages, there is eye muscle paralysis, dizziness, and respiratory and consciousness disorders, which can lead to coma. If the outflow of cerebrospinal fluid is permanently disturbed, a so-called hydrocephalus may develop due to the increased pressure in the cerebrospinal fluid spaces. This is a serious condition that can often be attributed to developmental disorders and to genetic defects.
