The brain ventricles or ventriculi cerebri in Latin, are four cavities embedded in the brain system and filled with the cerebrospinal fluid (CSF) (1).
If we want to answer the question: “Why are cerebral ventricles there where they are?”, we will have to address some evolutionary references. Simply put, the brain ventricles are there because of the evolutionary processes, which in turn are related to the adaptation of organisms to environmental conditions.
If this existing brain placement was not suitable for the human being, it would not be located there where it is but elsewhere on or in the body.
In this article, we will talk about the structure, content, importance, function, and deformations of the brain ventricles.
Structure and position of the Cerebral Ventricles
The elongated brain and bridge form a rhombic pit, forming the bottom of the fourth brain ventricle, ventriculus quartus, and its roof, like a vault, forms the cerebellum. The fourth ventricle continues into the narrow canal of the spinal cord.
At the anterior end of the bridge, the fourth ventricle continues into a narrow tube that runs into the midbrain and is also called the water system of the midbrain (1).
This “water system” anteriorly expands to form the third ventricle, ventriculus tertius. The lateral walls of the third cerebral ventricle form paired gray nucleus structures – the hill (thalamus) and the suburbs (hypothalamus).
Inside both brain hemispheres there is a lateral ventricle, ventriculus lateralis. Both of these ventricles are connected to the third ventricle by an intercellular (interventricular) opening.
The lateral ventricles attach to the middle of the cerebral hemisphere and extend far into the forehead and the occipital region and into the temporal lobe (1).
Cerebrospinal Fluid and its Function
According to the classical theory, the cerebrospinal fluid is formed predominantly within the choroid segment, which is attached to the ventricular wall.
It travels through the lateral ventricles (chambers) and through the interventricular openings to the third ventricle, then through the cerebral aqueduct to the fourth, from which it enters the subarachnoid space through lateral and median openings, and is then absorbed through the arachnoid fissures into the veins of the dura.
Most of it, about 100 mL, is formed in the choroidal mesh that is located in all four chambers and the rest, approximately 50 mL, is formed in other CNS spaces.
These spaces include ependymal cells lining the ventricles of the brain, but the arachnoid membrane, despite being able to secrete peptides and proteins, does not produce liquor.
The formation can be divided into two phases: passive filtration of the fluid from the choroidal capillaries happens in the first stage, followed by active secretion via the choroid epithelium in the second (2).
The first step takes place depending on the pressure difference between the plasma and the interstitial fluid of the choroid, so that, in cases of increased pressure of the cerebrospinal fluid, for example in hydrocephalus, the formation of the same is reduced.
The second step takes place with the help of protein transporters located in the epithelium of the choroid. Due to the energy-favorable gradients for Na+ (basally enters the cell) and K+, Cl-, HCO3- (apically exits the cell), a net flow of fluid and ions from the plasma, through the cell, into the CSF is generated.
Liquor formation, among other things, is influenced by the growth factors and neuropeptides by regulating the fluid exchange through the ependymus.
Interactions between the ANP, AVP, and FGF2 and their membrane receptors, located on the apical membrane of the cell, reduce the formation of CSF via neuroendocrine effects on the choroid.
As the levels of ANP, AVP, and FGF2 are mainly under central control, such regulation is largely independent of the plasma composition or peripheral factors. Passing through the ventricles, the CSF exits through the central orifice and lateral openings of the fourth ventricle, thus reaching the subarachnoid area or outer CSF (2).
Unlike other organs that meet their metabolic needs through blood, the brain, due to the presence of the blood-brain barrier, does so through the cerebrospinal fluid. That is why liquor contains vitamins, peptides, nucleosides and growth factors.
In the mature and old age, many of these nutrients do not reach the neurons in sufficient quantity. Due to fibrosis of the interstitium in the choroid and oxidative damage to the epithelium, the transport of vitamins B and C is impaired.
The metabolic waste of the brain is removed through an interstitial fluid that goes to the blood capillaries or into the ventricular system and can be recovered from the ventricular system by active reabsorption through the epithelium of the choroid back into the interstitial fluid and then into the blood capillaries or goes through the already mentioned absorption into the venous system (2).
As the brain has no lymphatic capillaries, apart from the task of eliminating catabolic products, the cerebrospinal fluid also plays a role in the removal of macromolecules. To perform this function properly, it needs constant updating.
The creation rate is calculated as the volume ratio the liquor generated within 24 hours and the volume of space it fills. This figure decreases in old age, but also in Alzheimer’s disease, for two reasons: it reduces the ability of the choroid secretion, that is, to create the CSF, and increases the ventricular space.
Due to the slow flow, the liquor decreases the purity and ability to “flush out” the ventricles, resulting in an unfavorable chemical environment of the brain tissue, that is, the accumulation of potentially toxic peptides and metabolites in the CSF.
Starting from the fetal period, through adulthood to the old age, the choroid and CSF are actively involved in the construction, maintenance, and repair of the brain. By maintaining the homeostasis, the cerebrospinal fluid ensures the proper functioning of the neural networks. In diseases or aging, its slow formation and inadequate circulation results in inappropriate microenvironment and impairment of motor and cognitive functions (2).
Hydrocephalus is the enlargement of the brain ventricles or cavities by the excess cerebrospinal fluid. Hydrocephalus is not a single disease, but a name for a set of conditions characterized by an increased amount of CSF.
It is most common in infants and children, with a frequency of about 1 in 500 live births, although it can also occur in adults, most often due to brain tumors, after head trauma or meningitis.
An increase in the amount of the cerebrospinal fluid most commonly occurs due to impaired circulation and absorption, and much less frequently due to increased formation of the cerebrospinal fluid.
The result is an increase in the volume of fluid in the closed cranial cavity, which can lead to damage of the brain tissue due to direct pressure of the overflowing brain cavities, as well as due to impaired and inadequate circulation.
Depending on whether the underlying disorder is in the circulation of the cerebrospinal fluid or its absorption, hydrocephalus is divided into two basic types:
- Obstructive or the non-communicative type – In this type of hydrocephalus, there is a barrier to the flow of the cerebrospinal fluid through the brain chambers. It may happen due to narrowing of the canal that connects the third and fourth brain ventricle, as a result of some hereditary diseases and congenital anatomical anomalies, as a result of newborn meningitis, bleeding in the region of the brain cells prone to prematurely born babies, after some pregnancy infections, etc.
- Non-obstructive or communicative type – This occurs due to inadequate absorption of the CSF. The causes include bleeding into the brain chambers and subarachnoid space in premature babies, bacterial meningitis (tuberculosis or pneumococcal), as well as some infections in pregnancy.
The clinical image of hydrocephalus depends on the child’s age, the cause that led to the disorder, as well as the duration and rate of pressure increase in the skull. The biggest diagnostic problem is finding the cause of hydrocephalus.
A multidisciplinary approach is required, i.e. the cooperation between specialist doctors in different fields: neuro-pediatrician, ophthalmologist, neurosurgeon, etc., as well as the application of sophisticated medical diagnostic tools. Unfortunately, despite all efforts, the cause often remains unknown.
In most cases, obstructive hydrocephalus requires the installation of the so-called “shunt” – silicone “tubes” with a built-in “valve”, one end of which is placed in the lateral cerebral ventricle and the other most commonly in the abdominal cavity (then we refer to the ventriculoperitoneal shunt) with the aim of eliminating excess fluid and thereby preventing pressure increase in the cranial cavity.
Non-obstructive or communicable hydrocephalus usually does not require surgical treatment but only regular clinical monitoring (ultrasound, accompanied by tracking the psychomotor progress).
Thanks to the brain ventricles and the presence of liquor, the brain “floats” in the cranial cavity, instead of moving freely through it under the influence of gravity, which would be quite risky. Cerebrospinal fluid protects sensitive brain tissue from the effects of mechanical force, alleviates it, contains many hormones and nutrients, and eliminates harmful metabolism products.
- Korzh V. Development of brain ventricular system. Cell Mol Life Sci. 2018 Feb;75(3):375-383. doi: 10.1007/s00018-017-2605-y. Epub 2017 Aug 5. PMID: 28780589; PMCID: PMC5765195. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5765195/
- Jones HC. Cerebrospinal Fluid Research: A new platform for dissemination of research, opinions and reviews with a common theme. Cerebrospinal Fluid Res. 2004 Dec 10;1(1):1. doi: 10.1186/1743-8454-1-1. PMID: 15679934; PMCID: PMC544966. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC544966/