Cerebrospinal fluid (CSF) is a clear, transparent fluid that bathes the central nervous system (CNS), and is responsible for a plethora of roles that are vital to normal neuronal function. It has a viscosity similar to that of blood plasma, and much akin to this fluid, its composition is regulated within certain physiologic parameters. At any given time, there is roughly 150ml of CSF; daily production is approximately 500ml/day, indicating a total volume replacement of about 3 times/day.
The roles of the CSF include:
The brain is a heavy organ; its unsuspended weight is approximately 1400g. However, when floating within the CSF, its net weight is closer to 25g - this has the following advantages:
2) Removal of waste/maintanence of homeostasis
Neurones are extremely sensitive to deviations from normal extracellular fluid concentrations, such that fluctuations outside of these values may result in abnormal action potentials and overall cerebral dysfunction, including the lowering of seizure threshold. The continual circulation of CSF through the ventricular system, basal cisterns and subarachnoid spaces remove waste products into the venous return for excretion by the renal system.
The CSF affords the brain some mechanical protection from trauma, such that direct impact forces to the head are dissipated somewhat within the fluid. This may reduce cerebral contusion/coup-contrecoup injuries during acceleration and deceleration forces.
4) Aiding in the distribution of hormones
Whilst the cerebral vasculature is primarily responsible for the distribution of hormones within the CNS, the CSF also allows a secondary pathway for hormonal transport.
CSF is produced by the choroid plexus throughout the ventricular system, though it is not found within the aqueduct of Sylvius (cerebral aqueduct) or in certain parts of the lateral ventricles (frontal or occipital horns). The choroid plexus consists of a network of capillaries for which the barrier from the ventricular fluid is by way of modified ependymal cells (the cells that line the ventricular system) - this barrier is the blood-CSF barrier and is distinct from the blood-brain barrier.
The choroid plexus filters fluid from the blood into the ventricles, with further fine-tuning of CSF composition by way of active transport.
As aforementioned, CSF is produced in the ventricles, with the greatest volume being secreted by the choroid plexus within the lateral ventricles. Net fluid flow is in a pulsatile manner owing to changes in vascular pressures within the cardiac cycle, and it follows a specific pathway as a consequence of sites of proximal production and distal absorption. The presence of cilia throughout the ventricular system also aid unidirectional flow. In order to describe the transport of CSF through the brain and cord, let us begin with a description of the flow beginning in the lateral ventricles:
There are two lateral ventricles which are deep in the cerebral hemispheres just inferior to the corpus collosum. They are separated by a membranous tissue septum - the septum pellucidum. CSF produced within these ventricles will flow inferiorly to coalesce within the third ventricle via the foramen of Monro. From here, again flowing inferiorly, the pathway will be into the fourth ventricle via the aqueduct of Sylvius (or cerebral aqueduct). The fourth ventricle is posterior to the brainstem and anterior to the cerebellum, and it is from here that CSF flows into the central canal of the spinal cord, or into the basal cisterns and subarachnoid spaces via the foramen of Magendie (median aperture) or the foramen of Lushka (x2 - lateral apertures). The basal cisterns distribute CSF to the deeper structures of the brain, whilst the subarachnoid spaces ensure the cord and cerebral convexities are surrounded by CSF.
The rate of prodcution and absorption of CSF must be matched in order to maintain normal pressures of approximately 10mmHg. The site of CSF absorption is mainly into the superior sagittal sinus via the arachnoid granulations. These granulations are small projections of the meningeal layers into the lumen of the venous sinus such that CSF moves from the subarachnoid space into the venous return. The driving force behind this mechanism is fluid pressure: fluid pressures within the subarachnoid space are higher than within the venous system, and as such, fluid is driven out into the blood. Simple mechanics, therefore, explains why a venous sinus thrombus within the superior sagittal sinus will lead to higher venous pressures and no/reduced CSF absorption, resulting in a communicating hydrocephalus.
Intracranial pressure (ICP) is the sum of three components: blood, brain, and CSF. Owing to the fact that the skull is, in effect, a closed box, the sum of these components (that is, the ICP) must remain constant such that an increase in one variable will cause a decrease in one, or both, of the other variables. This is known as the Monro-Kellie doctrine.
The role of CSF in the normal dynamics of ICP is important. During times of increased cerebral blood flow, or in pathologies that increase the ICP (such as the presence of a tumour, haematoma, etc), the intracranial volume of CSF may decrease in order to prevent a pathological rise in ICP. Note that this compensatory mechanism can hold only to a certain point, and that continually increasing the volume of one component will eventually result in brain herniation and eventual death.
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