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As you probably know by now, the brain is composed of numerous interconnected structures that all work together in tandem to produce the experience of being alive.
For humans, our brain has a few upgrades when compared to the rest of the known Animal Kingdom, in that our cognitive abilities are highly developed, providing us the ability to form a rational thought, and many other thought-driven skills.
Part of what gives us this unique functionality is the complexity of our brain morphology and physiology. The human brain is made up of three main sections: the cerebrum, cerebellum, and the brainstem.
Within the cerebrum (the largest portion of the brain) and the brainstem (the structure that connects the brain to the spinal cord) are structures known as cerebral ventricles.
The Ventricular System
When referred to altogether, the brain’s ventricles are called the ventricular system. There are four distinct ventricular spaces: the lateral ventricle, the third ventricle, the cerebral aqueduct, and the fourth ventricle.
Within each of them is a small structure called the choroid plexus. This is responsible for the production of cerebrospinal fluid, or CSF. CSF is a colorless fluid that performs many different functions, the most notable of which are the suspension of the brain within the skull and regulation of the neuronal environment surrounding the brain.
This structure is essential to maintaining the physical and physiological safety of the brain. As previously noted, the choroid plexus is the location in which the production of CSF takes place. It achieves this by the use of its specialized ependymal cells.
Before we get too far into that, though, let’s first layout how the choroid plexus is situated. Along with its population of ependymal cells, the choroid plexus is a network of capillaries that are not only central to brain safety and development but also important in the maintenance and function of the central nervous system in its entirety.
It can be found in all ventricles except the cerebral aqueduct and resides specifically within the innermost layer of the meninges, a membranous lining that envelopes and protects the central nervous system. Now, back to those ependymal cells that were mentioned a moment ago.
The CSF produced by these ependymal cells – a type of neuroglial cell that composes the epithelial tissue of the choroid plexus, ependyma – has two critical roles to play for both the central and peripheral nervous system. As mentioned previously, CSF provides a cushion of sorts and support for the brain and spinal cord.
In the event that there are abrupt, jarring movements, the presence of the fluid helps to buffer – or hopefully, prevent – a potential impact between the brain and the skull, and protect the integrity of the spinal cord.
Think of the CSF as the soft lining on the inside of a bike helmet. The lining is not going to stop an impact from happening, but at least, if you end up getting stopped short by a rock and flying head-first off of your bike into oblivion, the fall will not be as terrible as it could’ve been.
Additionally, CSF circulates nutrients and removes wastes from the central nervous system. As an emphasis on the necessity of this fluid, it has been found that an underproduction of CSF can stunt brain growth, whereas an overproduction could possibly lead to a condition known as hydrocephalus, where the accumulation of excess CSF puts increasing amounts of pressure on the brain and spinal cord.
The second major function of the CSF is to work hand-in-hand (or meninge in meninge?) with the blood-brain barrier to prevent harmful substances in the blood from making their way into the cerebrospinal fluid. This second barrier is called the blood-cerebrospinal fluid barrier.
There is so much more than the choroid plexus is responsible for, but that’s another story for another time. For now, we’ll focus on the larger system in which these structures are located, the ventricular system.
Choroid plexuses are located in all but one of the ventricles: the lateral, third, and fourth ventricles.
The lateral ventricles are collectively the largest ventricular space in the brain. There is one per hemisphere, both home to the cerebral cortex, basal ganglia, hippocampus, olfactory bulb, and basal forebrain.
These ventricles are C-shaped and can be found within the cerebrum, of course, and surrounded by the basal ganglia and corpus callosum.
Each lateral ventricle has five parts: the frontal (anterior) horn, body, atrium (also known by the much cooler name, the collateral trigone), temporal (inferior) horn, and occipital (posterior) horn. They are positioned side-by-side (remember, there is one per cerebral hemisphere) and partially separated by the septum pellucidum.
For the lateral ventricles, the choroid plexus can be found in the body, atrium, temporal horn, and interventricular foramen.
The lateral ventricles communicate with the third ventricle by the flow of CSF through the interventricular foramen of Monro.
This ventricle is long and thin and is located in the midsagittal plane of the diencephalon. Its choroid plexus stretches from the lateral ventricle down into the interventricular foramina (same as interventricular foramen of Monro, there are a few different ways to say it.)
These foramina contain their own choroid plexus as well. This ventricular space is surrounded by the thalamus, hypothalamus, and the lamina terminalis (a thin layer of gray matter that plays a critical role in the regulation of sodium excretion from the brain).
The flow of CSF continues on from the third ventricle to the fourth via the cerebral aqueduct which also goes by another ancient Roman-sounding name, the aqueduct of Sylvius.
The cerebral aqueduct, or the aqueduct of Sylvius, forms the connection between the third and fourth ventricles. And that’s about it.
At last! The fourth ventricle, the last in line and the ventricle with more than just one fancy name associated with it. This ventricle is shaped like an “elongated pyramid,” stretching up into the cerebellum and down into the pons and medulla.
From the fourth ventricle, things get a little more intense. CSF flows from this ventricular space into a space called the cisterna magna through any of three openings (here come the fancy names): the foramen of Magendie or one of the two foramina of Luschka.
The positioning of the cisterna magna is one of several subarachnoid cisterns and is also known as the cerebellomedullary cistern. No, this doesn’t mean that there are spiders in your brain – what this term means is that the cisterna magna is a compartment of sorts within the subarachnoid space (the middle layer of the meninges) in which CSF pools.
This, along with the other cisterns, is essential to CSF circulation and can cause serious problems if it becomes blocked. Cisterns are also crucial in that may have important blood vessels and cranial nerves passing through them.
The Flow of Cerebrospinal Fluid Through the Body
As you’ve learned, the journey of CSF begins in the choroid plexus, where it is made by the ependymal cells. This colorless fluid is absolutely essential to the maintenance of the human nervous system, and, as you’ve seen, flows through and around the brain, to be distributed through the brainstem, down the spinal cord, to the rest of the body.
Cerebrospinal Fluid Circulatory Pathway
The route CSF follows to be distributed throughout the body is known as the Cerebrospinal fluid Circulatory Pathway. So far you know that CSF flows from the lateral ventricles, into the third ventricle, passing through the cerebral aqueduct and into the fourth ventricle. From the fourth ventricle, CSF is dumped into the cisterna magna… what comes next?
After waiting around in the cisterns, CSF is absorbed by the blood vessels to flow over the surface of the brain and return to the bloodstream. From here, it is taken to the kidneys and liver, and filtered just like any other bodily fluid. (Note that absorption of the CSF occurs through the lymphatic system as well.)
When flowing through the ventricular system, CSF is not limited to staying within the ventricular space. In fact, it is allowed to flow directly from the ventricles straight into the brain tissue around them. This way, it enters cisterns separate from the cisterna magna. It is thought that the brain tissue into which the CSF is flowing is not actually absorbing the fluid, but simply providing another route by which it can travel.
The fluid that is absorbed by the lymphatic channels flows along the myelin sheath of nerves traveling through the brain stem and spinal cord.
The choroid plexuses of the human body produce about 500ml (0.125gal) of CSF every day in the constant loop of replacing CSF that has been absorbed.
The ventricular system has shown itself to be vital in the safety and continued standard functionality of both the central and peripheral nervous systems. Such small structures – a bunch of empty spaces, really! – are essential to human survival.
The wonders of human anatomy and physiology never cease to inspire scientists around the world as they continue to discover the functions and purposes of each part of the human body.