Anatomy of the ventricular system

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Introduction0:00–0:37

An adult human brain weighs about 1.5 kg, but we don't really feel it weighing us down. That's because the brain is cushioned by cerebrospinal fluid, or CSF, which can be found within brain cavities called ventricles.
These cavities are involved in the production, transport, and removal of CSF. And they are connected to each other.
So as a whole, they are often referred to as the ventricular system of the brain. CSF doesn't only fill the ventricles, but also the subarachnoid space, which surrounds the brain and spinal cord.

Subarachnoid space0:37–1:23

This way, CSF cushions and protects the brain from head trauma, and it also provides buoyancy, so that the brain doesn't compress blood vessels and cranial nerve roots against the cranium.
It also provides protection against sudden intracranial pressure changes. CSF can also transport nutrients for nervous tissue, as well as remove metabolic waste products.
It can also transport hormones. And influence the brain's excitability by regulating its ionic composition.
Now, the ventricular system consists of 4 ventricles, 2 lateral ventricles, the 3rd ventricle, and the 4th ventricle. CSF flows from the lateral ventricles through the left and right interventricular foramina, also called the foramina of Monroe, to the third ventricle.

Ventricular system1:23–3:37

From here, it passes through the cerebral aqueduct to reach the 4th ventricle. CSF can then flow caudally into the central canal of the spinal cord.
The fourth ventricle also has two lateral apertures called the foramina of Lushka, and a median aperture called the foramen of Megendi, both of which allow CSF to reach the subarachnoid space.
The lateral ventricles are the largest. They occupy both cerebral hemispheres, and they're present in all four lobes.
This is best seen on coronal sections of the brain, cutting through the frontal lobe, the parietal lobe, and temporal lobes, and finally, the occipital lobe.
On a mid-sagittal section of the brain, you can see the 3rd ventricle centrally. Rostrally and superiorly, there is a depression that narrows into the interventricular foramen, which connects the third ventricle to the two lateral ventricles.
Inferiorly, the 3rd ventricle continues into the cerebral aqueduct, which is a narrow canal that passes through the midbrain and reaches the 4th ventricle, which is a pyramid-like cavity sitting dorsal to the brain stem and ventral to the cerebellum.
Caudally, the fourth ventricle continues to the central canal of the spinal cord, and just inferior to the cerebellum, there is the foramen of Megendi.
Laterally, at the level of the cerebellum, there are 2 foramina of Lushka. These foramina allow CSF to leave the 4th ventricle and enter the subarachnoid space.
The subarachnoid space has CSF filled dilations called subarachnoid cisterns. Some of these cisterns contain proximal parts of cranial nerves and blood vessels.

Subarachnoid cisterns3:37–5:25

On a mid-sagittal section of the brain, we can identify the main subarachnoid cisterns. Between the cerebellum and the medulla, there is the posterior cerebellum medullary cistern, also known as cisterna magna.
It receives CSF from the foramen of Megendi and can even be accessed for obtaining a CSF sample in some rare cases. On both sides of the cisterna magna, there are two lateral cerebellum medullary cisterns that can't be seen in this section.
They receive CSF from the foramina of Lushka and contain cranial nerves 7 and 8. Then, ventral to the pons, there's the pontocerebellar or pontine cistern, which contains the basilar artery and continues inferiorly into the spinal subarachnoid space.
Next is the interpeduncular or basilar cistern, which sits between the cerebral peduncles of the midbrain. Rostally, there's the chiasmatic cistern, or the cistern of the optic chiasma, which lies below the optic chiasm.
And lastly, the quadrigeminal cistern sits inferior to the caudal part of the corpus callosum and superior to the cerebellum.
It is also referred to as the cistern of the great cerebral vein, since it contains the great cerebral vein of galen. No surprises here.
Next, let's go over the choroid plexus. Now, CSF is produced by the epithelial cells of the choroid plexus contained in all four ventricles.

Choroid plexus5:25–7:09

The choroid plexus is a cauliflower-like structure that consists of many fringes and folds of vascular pia mater that protrude into the ventricles.
And surrounding the pia mater, there are cuboidal epithelial cells. These are actually modified ependyal cells, similar to those that line the inner surface of the ventricles.
The choroid plexus can be found protruding from the floor of the lateral ventricles and the roof of the 3rd and 4th ventricles.
As CSF flows from the ventricles to the subarachnoid space, it continues superiorly to reach the superior sagittal sinus, which is a channel between two layers of dura mater that holds venous blood.
The arachnoid mater forms small protrusions called arachnoid villa that penetrate the dura and enter the sinus. The subarachnoid space extends into these villa, which are therefore filled with CSF.
The CSF then passes through the thin lining of the villi and drains into the venous system to be recycled. Interestingly, the villa tend to group together and form arachnoid granulations.
You can even see the impressions of these granulations called the granular foveolae on the inferior surface of the calvaria.
OK, now let's look at each ventricle in a bit more detail. First up, the lateral ventricles.

Lateral ventricles7:09–7:35

Each of them is shaped like the letter C, and they each have a body or a central part, and three horns. The anterior horn, posterior horn, and inferior horn, also called the frontal, occipital, and temporal horns respectively.
To begin, there's the body of the lateral ventricle, and it can be found deep within the parietal lobe. The body of the lateral ventricle has a distinct roof, a floor, and a medial wall.

Lateral ventricle - Body7:35–9:49

On a coronal section of the brain, you can see that the roof is formed by the inferior surface of the corpus callosum. The floor, on the other hand, from medial to lateral, is formed by the fornix, choroid plexus, lateral part of the dorsal side of the thalamus, and the caudate nucleus.
Medially, the septum pellucidum extends from the corpus callosum to the fornix, separating the left and right lateral ventricles.
On more caudal coronal sections, the septum pellucidum slowly disappears as the corpus callosum and fornix come closer together.
Rostrally, the body connects to the interventricular foramina, which can be seen on a transverse section of the brain. Each interventricular foramen is bounded by the anterior column of the fornixrosrally and the anterior aspect of the thalamus caudally.
As we follow the body of the lateral ventricle caudally, we see that it stretches along the medial aspect of the body of the caudate nucleus.
And eventually becomes continuous with the posterior and inferior horns. On a mid-sagittal section of the brain, we can easily identify the corpus callosum and the septum pellucidum, extending from it to the fornix.
Caudally, the septum pellucidum becomes smaller as the roof and the floor of the lateral ventricles come closer together.
If we look at a three-quarter view of the brain and remove the cerebrum, we can see how the body of the lateral ventricle curves around the thalamus and caudate nucleus, and the corpus callosum is located superior to it.
Grostrally, the body is continuous with the anterior horn. Now, the anterior horn continues from the body at the interventricular foramen and extends into the frontal lobe.

Lateral ventricle - Anterior horn9:49–11:10

On a coronal section we can see that the anterior horn has a roof, a floor, and a medial wall. The roof is also formed by the corpus callosum, while the floor is formed by the head of the caudate nucleus.
The medial wall is formed by the septum pellucidum and the anterior columns of the fornix. Again, if we look at a three-quarter view of the brain and remove the cerebrum, we can appreciate the relationship between the lateral ventricle and the basal ganglia.
Here we can see how the anterior horn is above the head of the caudate nucleus and below the corpus callosum. As the rostral part of the corpus callosum curves inferiorly and caudally, it also forms the rostral border of the anterior horn and even a small portion of its floor.
On a transverse section at the level of the interventricular foramina, we can follow the anterior horn from the foramen along the medial side of the head of the caudate nucleus.
The posterior horn continues from the body at the posterior end of the thalamus. And extends into the occipital lobe.

Lateral ventricle - Posterior horn11:10–12:25

The posterior horn has a roof or a lateral wall, and a floor or a medial wall. On a coronal section, you can see the roof is formed by the fibers spreading laterally from the corpus callosum, called the tapedum.
Lateral to the tapedum, there are fibers of the optic radiation. The floor has a superior and inferior bump.
The superior bump, also known as the bulb of the posterior horn, is formed by the fibers of the corpus callosum called the forceps major, spreading posteriorly.
The inferior bump, also known as the calcar avis, is formed by the underlying calcine sulcus. On the transverse section, the optic radiation borders the posterior horn laterally, and on a mid-sagittal section, if we ghost the ventricles, we can see how the calorine sulcus reaches the posterior horn.

Lateral ventricle - Inferior horn12:25–14:00

Finally, the inferior horn continues from the body inferiorly and then rostrally extending into the temporal lobe. This horn also has a roof and a floor, both of which are visible on a coronal section.
The roof is formed by the tapedum, spreading laterally from the corpus callosum. On the medial and superior aspect, there's the tail of the caudate nucleus.
The lateral part of the floor is formed by the collateral eminence, which is an elevation caused by the collateral sulcus underneath it.
The medial part of the floor is formed by the hippocampus. The ventricular surface of the hippocampus is lined by the alveus, which is formed by the fibers of the neurons inside the hippocampal cortex.
Fibers of the alveus group together to form the fimbria, which are positioned medial to the hippocampus. On a transverse section, the hippocampus extends along the medial part of the floor of the inferior horn.
Rostrally, it expands and becomes a bit wrinkly, forming the pez of the hippocampus. Looking at a three-quarter view, we can see that the inferior horn is bordered rostally by the amygdaloid nucleus.
Also, the tail of the caudate nucleus on the roof extends all the way to the amygdala as well. OK, time for a quick break.

Quiz14:00–14:13

Can you remember the names of the three horns of the lateral ventricles? Great.
Now let's look at the 3rd ventricle, which is a very narrow cavity in the middle of the diencephalon. On a mid-sagittal section, the superior part of the lateral wall is formed by the thalamus, while the inferior part is formed by the hypothalamus.

Third ventricle14:13–16:51

The third ventricle is bordered rostrally by the anterior commissure and the lamina terminalis. On the anterior and superior part of the lateral wall, there's the interventricular foramen, while in the middle, there's the interthalamic adhesion, which is a bridge of tissue connecting the two thhalami.
The third ventricle also has small extensions that form recesses near adjacent structures. These include the optic recess between the lamina terminalis and the optic chiasm.
The infundibular recess towards the pituitary gland. The pineal recess towards the pineal gland.
And the suprapineal recess above the epithalamus. On a coronal section, you can see that the roof is formed by the choroid plexus.
The floor is formed by the hypothalamus. And laterally, there are the thalami.
On a transverse section, the third ventricle looks like a slit or cavity between the thalammi that continues into the lateral ventricles at the level of the interventricular foramen.
Now on a mid-sagittal section, the third ventricle connects inferiorly and caudally with the cerebral aqueduct, or the aqueduct of Sylvius.
The cerebral aqueduct stretches through the dorsal part of the midbrain inferiorly towards the 4th ventricle. This is the narrowest passage of the ventricular system that the CSF flows through, and it doesn't usually have choroid plexus.
On a coronal section, we can see it as a central opening or canal extending down the brain stem. On a transverse section of the midbrain, the cerebral aqueduct is positioned in the middle between the tectum posteriorly and the tegmentum anteriorly.
Surrounding it is the periaqueductal gray matter. Lastly, let's cover the 4th ventricle.

Fourth ventricle16:51–21:18

It has a dorsal wall or a roof. On a mid-sagittal section, the 4th ventricle looks like a tent.
Its roof has a central part called the apex or the fastigium that extends towards the cerebellum. From the apex, the superior medullary vellum extends superiorly.
This is a thin layer of white matter between the superior cerebellar peduncles. There's also an inferior medullary vellum that extends from the apex inferiorly.
This structure holds the choroid plexus of the 4th ventricle, and it also contains the foramen of Megendi in the midline, where most of the CSF leaves the 4th ventricle into the subarachnoid space.
The ventral wall or floor of the 4th ventricle, on the other hand. Is formed by the dorsal sides of the pons and the medulla.
In the midline, the median sulcus can be seen extending from the cerebral aqueduct rostrally to the central canal of the spinal cord caudally.
To better visualize the floor of the 4th ventricle, we should expose the dorsal surface of the brain stem. The floor is diamond or rhorhomboid shaped.
So it's called the rrhomboid fossa. The dorsal aspect of the pons, which is triangular shaped, creates the rostral half of the floor.
Laterally, there are the superior cerebellar peduncles. Running vertically down the midline is the median sulcus, and just parallel to it on each side, the sulcuslians.
Between the median sulcus and sulcuslians, there are two elevations called the medial eminences. Whose caudal parts enlarge to form the facial colliculi.
The facial colliculi are two bumps which are made up of axons from the facial nuclei that wrap around the abducens nuclei.
Cauddle to the superior cerebellar peduncles, we see the middle cerebellar peduncles. Now, on the dorsal aspect of the medulla oblongata, we can see that the rostral medulla creates the dorsal half of the floor of the fourth ventricle.
Laterally, there are the inferior cerebellar peduncles, which contain fibers that travel between the medulla and the cerebellum.
The ventricle or fossa tapers at its caudal aspect to a point called the obex. And near this caudal limit is the entrance to the central canal of the spinal cord.
In this region, CSF can travel from the 4th ventricle into the central canal. Lastly, let's look at the area post strema, which can be found on the dorsal surface of the medulla on the floor of the 4th ventricle just before the entrance to the central canal.
It holds the chemoreceptor trigger zone, or CTZ. That can initiate the vomiting reflex by sending input to the vomiting center in the medulla.
The CTZ doesn't have a well developed blood-brain barrier, which means that it can be exposed to chemical substances found in the blood and CSF.
These substances include toxins, hormones, alcohol and drugs such as chemotherapy drugs. The substances can in turn directly activate and initiate the vomiting reflex.
It also receives input from visceral afferent fibers coming from the GI tract via the vagus nerve. For example, chemotherapy drugs can stimulate receptors on vagal afferent nerve fibers in the bowel wall of the gastrointestinal tract.
Which ascend and trigger the vomiting reflex in the CTZ. Additionally, it also receives input from the vestibular system, which can explain car or seasickness.

Review21:18–22:34

All right, as a quick recap, the ventricular system produces CSF via the choroid plexuses and transports it to the subarachnoid space.
The lateral ventricles have a body in the parietal lobe and three horns, the anterior, posterior, and inferior horns that extend into the frontal, occipital, and temporal lobes respectively.
The CSF flows from the lateral ventricles through the interventricular foramina of Monroe to the 3rd ventricle that is located in the middle of the diencephalon.
The CSF then flows through the cerebral aqueduct of sylvius of the midbrain into the 4th ventricle found between the brain stem and the cerebellum.
The CSF then enters the central canal of the spinal cord, or the subarachnoid space through the median foramen of Megendi and two lateral foramina of Lushka.
From the subarachnoid space, the CSF gets recycled into the venous system via the arachnoid villi and granulations.