What’s up. Meditay here. Let’s continue  the anatomy of the Central Nervous System.  

In this segment, we’ll cover the  anatomy of the 4th Ventricle,  

and then we’re going to cover the structures  and the cranial nuclei of the rhomboid fossa. 

So the central nervous system consists of two  parts: the encephalon and the spinal cord.  

The encephalon is then further  divided into specific parts. 

We have the brainstem, which consists of  the Medulla, Pons, and the Midbrain or the  

mesencephalon. We have the Cerebellum back here,  then the diencephalon and the telencephalon. 

Our focus in this video is going to be  the space between these three structures,  

called the 4th Ventricle, and the base of the 4th  Ventricle, called Rhomboid Fossa. Which is here. 

So in this video, we’re first going to  cover all the ventricles in the brain.  

Then we’re going to cover the border and  the communications of the 4th Ventricle.  

After that, we’re going to look at the Topography  of the Rhomboid fossa. Then cover the anatomical  

structures you see on the rhomboid fossa. And  then the Cranial nuclei and the cranial nerves  

associated with the rhomboid fossa. Then  I’ve made a little quiz at the end which  

might help you if you need to memorize. Alright, so we can start by replacing  

this picture with a more realistic one. Again, the fourth Ventricle is here,  

which lies between the Pons, Medulla,  and Cerebellum. Inferiorly, the fourth  

Ventricle is going to continue as the  central canal within the spinal cord. 

And superiorly, the fourth Ventricle will  continue upwards as the Aqueduct of the Midbrain,  

which is inside the mesencephalon. Let’s now look at the whole ventricular  

system within the brain to understand what we’re  learning. So if we continue upwards above the  

Aqueduct, we’ll find the Third Ventricle. And the  third Ventricle has communications upwards called  

the interventricular foramen, which communicates  with the Lateral Ventricles. So if we now  

look at the whole ventricular system from an  anterior perspective, it’ll look like this. 

So the Lateral Ventricles are here. They’re  called Lateral Ventricles because they’re located  

laterally. Then we have the third Ventricle,  which has three communications—two up here and  

one down here, which s the Aqueduct. And the  fourth Ventricle, with the Aqueduct up here,  

central canal down here, and two apertures located  on the lateral sides that go into the subarachnoid  

space. So these ventricles are spaces within the  Central nervous system that contains cerebrospinal  

fluid, which contains nutrients for the nerve  tissue, as well as removing waste products from  

them. The cerebrospinal fluid then flows through  these apertures on the 4th Ventricle, into the  

subarachnoid space. Then the CSF is filtered  into the venous system through the dural sinuses. 

So let’s go through that again once we understand  the structures of the 4th Ventricle. So let’s  

go back to this picture and zoom in. Now the 4th Ventricle consists of a  

roof called the roof of the 4th ventricle,  or tegmen ventriculi quarti, and a base,  

which is called rhomboid fossa. Let’s go  through the roof first and then talk about  

the rhomboid fossa. So the upper part of the  roof is formed by a plate of white matter that  

you’ll find between the superior cerebellar  peduncles called the superior medullary velum. 

SO here is the posterior view of the brain. When  we remove the Cerebellum. You’ll see the base  

of the 4th Ventricle, which is the rhomboid  fossa. On either side of the rhomboid fossa,  

you’ll see the cerebellar peduncles, which  contain fibers that run between the Cerebellum  

and all three parts of the brainstem. The inferior  cerebellar peduncle contains fibers that go to the  

Medulla. The middle cerebellar peduncle contains  fibers that go to Pons. The superior cerebellar  

peduncle contains fibers that go to the Midbrain.  So the superior medullary velum is located between  

these superior cerebellar peduncles, like this.  SO this is the superior part of the roof of the  

fourth Ventricle, the superior medullary  velum. It’s a thin plate of white matter. 

Now, the lower part of the roof is also formed  by a thin plate of white matter called the  

inferior medullary velum, which is between the  inferior cerebellar peduncles, as you see here. 

Between the inferior and the superior medullary  velum, there’s a pointy roof called Fastigium,  

which is the apex of the 4th Ventricle. Now. The lower part of the roof has a few  

more structures than the inferior medullary velum  that we need to address. And to understand them,  

we need to repeat our three layers of meninges. So the outermost layer is the dura mater.  

Underneath the dura mater, you’ll find the  arachnoid mater. Then underneath that again,  

you’ll find a veeery thin and delicate layer  covering the tissue of our central nervous system,  

called the Pia mater. And between  the pia mater and the arachnoid mater  

is a space called the subarachnoid space, which  is filled with Cerebrospinal Fluid, which again  

delivers nutrients for the brain and spinal  cord tissue, as well as removing waste products. 

So, where do you find cerebrospinal fluid?  You find them in the subarachnoid space  

and in the ventricles and canals  of our central nervous system. 

At the 4th Ventricle, there are apertures  or openings at which the cerebrospinal  

fluid flows from the ventricular system into the  subarachnoid space. And you see how the pia mater  

terminates at this aperture and then continues  above the aperture again? Once it terminates,  

the pia mater forms a membrane, which loops to the  inside of the inferior medullary velum, as you see  

here. This membrane is called Tela Choroidea Another structure you’ll find here, and that’s  

a structure you’ll see in all the ventricles of  the central nervous system. That is the Choroid  

Plexus. The choroid plexus is highly vascular,  and it produced the actual cerebrospinal fluid.  

So that was all the structures associated  with the roof of the 4th Ventricle.  

Now let’s go through the communications of the  4th Ventricle. And we’ll do that by looking  

at the 4th Ventricle from two perspectives. Remember, the 4th Ventricle communicates with  

the Third Ventricle through the Aqueduct  of the Midbrain and continues downwards  

as the central canal within the spinal cord.  Laterally, we’ll find two apertures called  

the Lateral apertures, or Foramina of Luschka,  which open into the subarachnoid space. Then  

there’s an opening we looked at earlier, called  the Median aperture or also referred to as foramen  

of Magendie. All of these apertures open into the  subarachnoid space to provide a flow for the CSF  

between the ventricles and the subarachnoid space. So that was all for the 4th Ventricle. Let’s now  

open up the roof and focus on the base of the  4th Ventricle, which is the rhomboid fossa. 

Now we’re looking at the  brainstem from a posterior  

perspective after removing the Cerebellum. So we can see Pons, the Medulla, and the Midbrain. 

And here is the rhomboid fossa. Now we divide  the rhomboid fossa into two main regions.  

The upper region is Pons. The lower region of  the rhomboid fossa is the medulla oblongata.  

Now topographically, the rhomboid fossa is  bordered by the peduncles. It’s bordered  

by the superior cerebellar peduncle and the  inferior cerebellar peduncle, as you see here. 

Let’s now isolate the posterior view  of the brainstem and focus on the  

anatomical structures of the rhomboid fossa. So first off, there’s a groove in the middle of  

the rhomboid fossa that divides the rhomboid fossa  into two symmetrical halves, called the median  

sulcus. So the median sulcus goes all the way  from the central canal of the spinal cord to the  

Aqueduct of the Midbrain. On either side of the  median sulcus, there are elevations we call the  

medial eminence. The motor nuclei of the brainstem  are usually situated underneath this eminence. 

Another thing you’ll see on the rhomboid  fossa is that the rhomboid fossa is crossed  

by myelinated nerve fibers called the  medullary stria, dividing the fossa into  

an upper part and a lower part. Usually, the upper  part is Pons, and the lower part is the Medulla. 

Above the medullary stria, you’ll find an  elevation on either side of the median sulcus,  

called the facial colliculus. These are elevations  made by fibers leaving the facial nucleus as they  

loop around the abducens nucleus, as you see here. Below the medullary stria, you’ll find the  

hypoglossal trigone, which is where the nucleus of  the 12th nerve is located, and the vagal trigone,  

which is the area where the vagal part of the  nucleus ambiguous is located. I’ll show you this  

later in this video when we go through the nuclei  of the cranial nerves in the rhomboid fossa. 

Alright, on the angle of the rhomboid  fossa, you’ll find the vestibular area.  

This is where the vestibular nuclei are situated  underneath, meaning this place is responsible for  

balance from the vestibular system. Then a little more superiorly,  

you’ll find the Locus Caeruleus, which are  composed of cells that produce norepinephrine.  

You’ll learn about the Locus caeruleus and  Norepinephrine system in physiology, but this  

system is related to the sleep-wake cycle, along  with attention and arousal and a few more as well. 

So that was all the anatomical structures of the  4th Ventricle. Take a mental picture of this for  

now, and let’s continue with the cranial  nuclei you’ll find in the rhomboid fossa.

In the rhomboid fossa, you’ll find the  cranial nuclei numbers 5 to 12. We’ll  

find the Trigeminal nerve, the abducent nerve,  the facial nerve, the vestibulocochlear nerve,  

the glossopharyngeal nerve, the vagus nerve,  accessory nerve, and the hypoglossal nerve.  

Let’s now go through all the nuclei  associated with each of these cranial  

nerves, starting with the Trigeminal nerve. The trigeminal nerve has two roots of nerves.  

One motor and one sensory. The  sensory part of the trigeminal nerve  

synapse with the Mesencephalic nucleus of the  trigeminal nerve. The trigeminal nerve is one  

of the biggest cranial nerves we have, and it’s  divided into three parts in the facial area.  

So the mesencephalic nucleus of the trigeminal  nerve receives proprioceptive information from  

the masticatory muscles, as well as from  mimetic muscles and the extraocular muscles. 

The next nucleus of the trigeminal nerve is  the principal nucleus of the trigeminal nerve.  

It receives information about touch, and  vibration, and some proprioception as well. 

The Spinal nucleus of the trigeminal nerve  is in the Medulla and the spinal cord.  

It receives information about pain and  temperature. The motor nucleus of the  

trigeminal nerve gives off fibers that innervate  the muscles of mastication and the anterior  

belly of the digastricus, the mylohyoid, and the  tensor tympani muscles. So that was this nerve, 

Next we have the 6th cranial nerve, the Abducent  nerve. The abducent nerve consists only motor  

fibers, coming from only one nucleus in the  rhomboid fossa. And that is the nucleus of the  

abducent nerve. The abducent nerve is responsible  for the abduction movement of the eyeballs. And it  

does that by innervating the lateral rectus  muscle of the eyeball. So that is this one. 

Next, we have the facial nerve. The facial nerve  consists of two roots. One motor root innervates  

the facial muscles and an intermediate nerve  that carries parasympathetic and sensory fibers.  

The motor root of the facial nerve comes  from the motor nucleus of the facial nerve.  

So fibers go out from this nucleus to innervate  all the facial muscles for facial expression,  

as well as some superficial muscles on the neck  region as well. The intermediate nerve carries  

sensory fibers and parasympathetic fibers. The  parasympathetic fibers come from a parasympathetic  

nucleus called the superior salivatory  nucleus, giving off parasympathetic innervation  

to glands in the facial region, like the lacrimal  gland and the submandibular sublingual gland. 

The sensory fibers of the intermediate nerve  synapse with the Nucleus of the solitary tract,  

which receives information about  taste. So that is the facial nerve. 

Next is cranial nerve number 8, which is the  vestibulocochlear nerve. The vestibulocochlear  

nerve consists of the vestibular nerve and the  cochlear nerve. So if you put them together,  

you get the vestibulocochlear nerve. So there  are 4 sensory nuclei for the vestibular part.  

These are the medial, lateral, superior ad  inferior vestibular nuclei. And there are  

two cochlear nerves, medial and lateral. Remember,  the inner ear consist of two parts—the vestibulum  

for balance and the cochlea for hearing. The  vestibular nerve transmits information about  

balance, motion, and acceleration. Inside the  semicircular canals of the vestibulum, there are  

cystals that detect movement, that stimulate the  receptors of the vestibular nerve. This nerve will  

then go to the vestibular nuclei. The cochlear  nerve transmits sound information from the organ  

of Corti by the movement of the endolymph  to the medial and lateral cochlear nucleus. 

So that is the vestibulocochlear nerve. Next is  the 9th cranial nerve, the Glossopharyngeal nerve. 

This nerve contains motor, sensory  and parasympathetic fibers.  

The parasympathetic fibers come  from the inferior salivatory nucleus  

that innervates salivary glands like the  parotid gland. The motor fibers come from  

the nucleus ambiguous that innervates  muscles of the pharynx and soft palate. 

The sensory fibers synapse with the nucleus  of the solitary tract, just like the facial  

nerve. It contains sensory information of  taste from the posterior third of the tongue. 

Next, we have the 10th cranial  nerve, the Vagus nerve. 

The vagus nerve also contains three  types of fibers, which are Motor,  

Sensory and parasympathetic fibers. The motor and  sensory parts of this nerve are also associated  

with the nucleus ambiguous and the nucleus of  the solitary tract. SO it innervates the muscles  

of the pharynx, soft palate, and Larynx. As well  as sensing taste from the posterior third of the  

tongue. The parasympathetic nerve fibers come  from the posterior nucleus of the vagus nerve.  

And this nerve is the only cranial nerve that  actually leaves the cranium, to innervate the  

organs of the thorax and the abdomen. Remember,  parasympathetic innervation is rest and digest.  

So the Vagus nerve reduces the heart rate,  increases gastric motility and absorption,  

and all of those things are related to the  parasympathetic function of the organs. 

Next, we have the 11th cranial nerve, the  accessory nerve. And notice I’ve added the  

spinal cord because this cranial nerve  is a little special. So the accessory  

nerve consists of two motor fibers. We have a  cranial root that comes from the part of the  

nucleus ambiguous located in the posterolateral  sulcus of the medulla oblongata. And it contains  

a spinal root that comes from the spinal nucleus  of the accessory nerve. This nucleus appears on  

the posterolateral sulcus of the spinal  cord, between C1 and C6 spinal segments.  

It ascends along the spinal cord and enters  the cranial cavity through the foramen magnum  

and joins the cranial root, to then form the  11th cranial never, which is the accessory nerve.  

The accessory nerve is primarily a motor  nerve that innervates the sternocleidomastoid  

muscles and the trapezius. It’s called an  accessory nerve because it forms a kind of a  

mixed system with the vagus nerve  and the glossopharyngeal nerve. 

The last nerve is the 12th cranial  nerve, called the hypoglossal nerve.  

It’s a motor neuron, of which the fibers come  from the nucleus of the hypoglossal nerve.  

These motor fibers go to the tongue and  innervate all the muscles of the tongue. 

So that was the 12th cranial nerve SO here, I’ve added all the nuclei of the rhomboid  

fossa with the corresponding cranial nerve that  synapses with them. And so this is where this  

video gets scary. I am going to make all these  cranial nerves disappear. And basically, if you  

hit pause. Could you go through from the beginning  and list you know which cranial nerves go with  

number 1? Which cranial nerve goes with number 2.  And what does number 3 do, and what does number  

4 do. This is a little challenging, but you know  once you get it right, you understand it. And so  

this was a video about the 4th Ventricle and the  rhomboid fossa. If you found this video helpful,  

please put a like, comment, share, whatever you  find convenient. And I hope this was helpful.