Circulation of the Cerebrospinal Fluid
The cerebrospinal fluid is a clear watery nutritional fluid containing sodium chloride, potassium, glucose, and various proteins. It fills the space between the arachnoid and pia mater of the central nervous system (CNS), the ventricles of the brain and the central canal of the spinal cord. It is secreted by the choroid plexuses lining the ventricles, and extends into the subarachnoid space of the brain, and into the central canal of the spinal cord. In addition to providing nutritional supply to the brain, the CSF also removes by-products of metabolism, and also serves to provide a fluid cushion to protect the brain from injury.
The CSF is very useful for clinical diagnosis, and samples are usually obtained from the subarachnoid space by passing a needle in the interval between the third and fourth lumbar vertebrae, or placing a needle into the cerebellomedullary cistern (Cisterna Magna). The former procedure is called a lumbar puncture, while the latter is called a cisternal puncture, in reference to their respective locations of application.
Cerebrospinal fluid is produced by vascular bulges, called choroid plexuses. The choroid plexuses are located in the lateral ventricles (along the line of the choroid fissure) and in the roof of the third and fourth ventricles. The vascular source of the choroid plexus differs between the lateral and third ventricles, and the fourth ventricle. The lateral and third ventricles are supplied anteriorly by the internal carotid and choroidal branches of the posterior cerebral artery, whereas the fourth ventricle, being much lower in position, is supplied by the inferior cerebellar arteries. The CSF is not passively filtered into the ventricles but actively secreted by the choroid plexus epithelium, which shares many characteristics with secretory and transport cells. The epithelial lining is a complex system: the capillaries are tortuous, presenting interdigitations, while the apical, secretory borders contain microvilli; the vessels are fenestrated underneath the epithelial lining, with tight junctions in the apical border of the epithelia, allowing only small molecules and fluid through. In this way, the choroid plexus is still able to maintain a Blood-CSF barrier, while still supplying fluid to cushion and nurture the brain.
Cerebrospinal fluid is constantly produced at a secretion rate of 0.35-0.40 mL.min-1, and replaced with an average volume of 150 mL in the ventricular system/subarachnoid space. Of this volume, about 125 mL is intracranial and 25 mL of this volume lies within the ventricles. The circulating CSF flows from the lateral ventricles through the third and fourth ventricles and into the subarachnoid space. From the fourth ventricle, the CSF either continues to the central canal of the spinal cord or leaves for the subarachnoid space by passing through the medial aperture (foramen of Magendie) and paired lateral apertures (foramina of Luschka) in the roof and lateral recesses of the fourth ventricle, respectively. The level at which the CSF enters the subarachnoid space is called the cerebellomedullary cistern. The fluid then flows through the subarachnoid space surrounding the brain and spinal cord. It finally leaves the subarachnoid space and CNS by entering the venous sinuses through the arachnoid villi. The arachnoid villi provide a valvular mechanism for flow of CSF into the bloodstream without permitting backflow of blood into the CSF.
The most significant disorder of the CSF anatomy is arguably hydrocephalus. Hydrocephalus may be caused by excessive production of CSF, by obstruction of its flow, or by impaired absorption through the arachnoid villi. Hydrocephalus is an abnormal enlargement of the head in children due to an abnormal increase in the amount of CSF. This creates an abnormal pressure within the cranium leading to degeneration of brain tissues. Hydrocephalus is classified as obstructive when there is an obstruction to its flow from the ventricular system to the subarachnoid space; or as communicating when such obstruction is not present. Obstructions are most likely to occur at narrow passages such as the interventricular foramen, the cerebral aqueduct, the median aperture, and lateral apertures of the fourth ventricle.
Lumbar punctures are a common procedure for assessing diseases and disorders relating to the CSF and meninges in both adults and infants. The procedure is performed with the patient in their maximal lumbar flexion. Marks are made between the top of the iliac crests to provide the indication of the join between L3 and L4, palpation is made to ensure correct placement of the needle. When a lumbar puncture procedure is undertaken the extracted CSF undergoes clinical tests, including visual and technical analysis. The presence of blood in the sample is obvious through its discolouration of the CSF. Further clinical testing allows for the identification of meningitis, encephalitis, Reye Syndrome, myelitis, and many other neurological disorders. Lumbar puncture sites are also used for to apply an epidural anaesthetic, for example during birth.