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

In this segment, we’ll cover the complete anatomy  of the Midbrain, also known as the mesencephalon

So remember, 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 Midbrain, which is here.

So in this video, we’re first going to cover the  external surfaces of the midbrain. Basically,

look at its topography and what structures you’ll  find from an anterior view and a posterior view.

Then we’re gonna slice up the midbrain  and look at the internal surface.

Basically, see how the grey matter and  white matter are arranged within it.

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.

From this view, we can locate Pons, the  Medulla, the cerebellum, and the spinal cord.

Now, if we remove a part of the cerebral Cortex,  you’ll be able to see the rest of the brainstem,

which is the mesencephalon, or the midbrain. So as you see from this picture,

the midbrain is located above Pons, in front  of the Cerebellum, and below the Diencephalon.

Now. Externally, your midbrain has two surfaces.  It has an anterior surface and a posterior

surface. Let’s now cover the typical morphology  of these two surfaces, starting with the anterior

surface first. And we’ll do that by looking  at the midbrain from an anterior view. So the

midbrain is here. The first thing we can do is to  remove the less significant surrounding structures

to make this easier. The majority of the anterior  surface of the midbrain is the Cerebral Peduncles.

These are two large peduncles that contain  tracts coming from the Cortex of your cerebrum

and are responsible for voluntary movement of  the body. I’ll show you later in this video

when we go through the cross-section. But  between these two peduncles, we have a pit

we call the interpeduncular Fossa. And within the  interpeduncular Fossa, you’ll find some structures

of the Diencephalon, which are not a part of your  midbrain, like the Hypothalamus and the Pituitary

gland. So again, they’re the Diencephalon and  our topic for the next video. But there are a few

structures here that are a part of the midbrain. And the first one is the posterior perforated

substance. This is a depression here on the  anterior surface of the midbrain, that contains

grey matter. And they contain small holes  here for blood vessels to go in and out from.

Then there is a groove called the  Ocuculomotor sulcus of the mesencephalon,

of which the third cranial nerve goes out, called  the oculomotor nerve. This nerve is called the

oculomotor nerve because it goes towards the eye  to innervate the extrinsic eye muscles that enable

the movement of the eyes. So that was all the  structures associated with the anterior surface.

Let’s now do the posterior surface. And to do  that, we’ll be looking at the midbrain from a

posterior view. Now from this view, we can  locate the Midbrain, Pons, and the Medulla.

So the first thing we can highlight is the  cerebral peduncles that we talked about earlier.

They are huge in comparison with the actual  midbrain, that is why you’re able to see them

peeking out from the sides from a posterior view.  But the most significant structures associated

with the posterior view, are the Tectal Plate, or  Lamina Tecti. And you’ll notice that on the tectal

plate, you’ll find four rounded structures. The  upper two are called the superior colliculus. And

this one Is associated with rapid and controlled  eye movements. Now how is it able to do so?

To understand that, we need to go through the  visual pathway. Within the retina of your eyes,

you have receptors for the 2nd cranial nerve,  the optic nerve. The fibers of the optic nerve

will go back, and then half of the fibers will  cross and form the optic chiasm. After that,

they will synapse with the Lateral geniculate  bodies, which is the metathalamus. We’ll go

through them when w go through the Diencephalon.  But From the Lateral Geniculate bodies, the fibers

will go back to the occipital lobe, which is  where you’ll find the primary visual Cortex.

When they go to the Cortex, that is when you’re  consciously aware of the things you see around

you. But fibers also go from the lateral  geniculate bodies to the superior colliculi,

through the Brachium of the superior colliculi,  which are these ones you see on the sides,

and then to the superior colliculi, which  will activate the tectospinal tract.

Which sends motor impulses for the  coordinated eyes and the neck muscles.

Then under that, we have the inferior colliculi,  Which is a part of the hearing pathway. So the

cochlear nerve receives impulses from the cochlea,  where sound is converted into nerve signals. The

cochlear nerve will then go to the cochlear nuclei  at the Rhomboid Fossa in Pons. And then they will

cross and form the trapezoid body, remember it’s  a structure of Pons. After that, fibers ascend,

they go up as the Lateral Lemniscus to synapse  with the inferior colliculi. From there,

impulses are sent through the Brachium of the  inferior colliculus to the medial geniculate body,

not the lateral this time. The lateral geniculate  body is for vision, medial is for hearing. From

the medial geniculate body, impulses go to  the primary auditory Cortex, which is in your

primary temporal gyrus. So that was these two. Another structure you’ll find on the posterior

surface is the Lateral sulcus  of the Mesencephalon. This is

the border between the cerebral peduncles  and the posterior surface of the midbrain.

Another thing you’ll find here is the trigone  of the lateral lemniscus, located in this area.

And then, below the inferior colliculus,  you’ll find the cranial nerve number 4 called

the trochlear nerve, which will turn towards the  anterior side to innervates the superior oblique

muscle of the eye. So that was everything  for he external surface of the midbrain.

Now let’s go ahead and cut the midbrain right  about here, and look at it from this perspective.

We’ll see this! This is at the level of the

superior colliculi. Structures are more or less  the same whether you make a cross-section at the

superior colliculus or the inferior colliculus,  but there are some slight differences. And I’ll

make sure to highlight them along the way. So anteriorly, we have the Cerebral Peduncles,

with the interpeduncular Fossa here in the middle.  And then there’s the aqueduct of the midbrain,

which remember is a part of the  ventricular system, connecting the

third ventricle with the fourth ventricle. So. The internal surface of the midbrain can

be divided into three regions. First is the  tectum of the midbrain, which contains the

colliculi. Then the tegmentum of the midbrain  in the middle. And then the cerebral peduncles.

Let’s now go through all the internal  structures, starting with the grey matter.

And just to remind you again, white  matter consist of myelinated nerve fibers.

So when we go through white matter, we talk about  tracts. Grey matter contain nuclei of neurons. So

structures in the grey matter are nuclei. So the  first grey matter structure we gonna talk about is

the red nucleus or nucleus ruber. These nuclei are  pale pink in color, due to the presence of iron,

as either hemoglobin or ferritin. These nuclei  are very characteristic of the midbrain. And

they contribute to the extrapyramidal pathway  of coordinating voluntary muscle control.

We’ll talk about this later in this video but  remember, from the lobes of the Cortex. The

corticopontine tract will descend and synapse  with the pontine nuclei of Pons. After that,

fibers will go to the cerebellum as the  pontocerebellar tract. Then fibers will go

from the cerebellum to the red nucleus as the  cerebellorubral tract. And then down towards

the spinal cord as the rubrospinal tract. To  coordinate and support voluntary muscle movements,

mainly flexor muscles. Tracts may even come  directly from the Cortex to the nucleus ruber,

but this right here is the most important  pathway when you talk about these nuclei.

After that, we have the Substantia Nigra.  Substantia nigra is actually made up of

two distinct regions. The substantia nigra pars  compacta and the substantia nigra pars reticulata.

When we talk about substantia nigra, we  generally mean pars compacta because that’s

the most significant one. The pars compacta  is very dark colors due to the large number

of dopamine neurons producing neuromelanin. The substantia nigra is considered a part of

your basal ganglia. The basal ganglia what that is  they’re a group of grey matter nuclei found within

your brain. We’ll talk about that when we talk  about the internal structures of the hemispheres,

but the basal ganglia consist of  the caudate nucleus, the Putamen,

Globus pallidus. There are two, external to the  left and internal to the right. Then the Thalamus,

but not the whole Thalamus, only the ventral  anterior and ventral lateral part of the Thalamus

is considered a part of your basal ganglia. Then  there are the subthalamic nuclei and then the

substantia nigra as well. You might find other  names when you talk about the basal ganglia,

like the striatum. Whenever you’re talking about  striatum then you’re talking about the Putamen

and the Caudate nucleus together. And when you  put the Putamen and the Globus Pallidus together

you’re talking about the Lentiform nucleus. Now. You’re probably wondering what the

heck I’m talking about and why I’m mentioning  them. The basic motor function, is coordinated

by the cerebral Cortex, right?  Primarily the primary motor area.

Whenever you decide to consciously  move a limb, your primary motor cortex

will send motor tracts along the spinal cord to  engage muscles necessary to do so through the

corticospinal tract. But in order for this  motor plan to be able to go to the muscles,

you need to kind of have a  communication with the basal gangion.

SO imagine for a second we’ve combined  all the basal ganglia structures into

a purple bulb here. So the primary motor  area have to communicate their motor plan

with the basal ganglia. The basal ganglia take  that motor plan and modify it in a particular way,

and send it back to the cerebral Cortex to send  now the proper motor plan to start movement,

stop movement or modulate the movement. Beautiful.  Now let’s get back to the substantia nigra. So

the Substantia nigra is therefore a part of a  larger group of structures that start movement,

stop movement and modulate movement, through the  nigrostriatal pathway using dopamine. I won’t

go in detail into that, but if for instance  a disease happen that causes the substantia

nigra to have less dopamine neurons. What do you  think will happen? You’ll get Parkinsons Disease.

And this will give the typical TRAP symptoms,  which are Tremor, Rigidity, Akinesia and Postural

instability. All of those, because the basal  ganglia is not able to function well. Awesome.

I hope the substantia nigra gave a little more  sense now. Understanding that will give you a good

starting point in studying its physiology. Alright Our next grey matter nuclei depend on which

layer you’re looking at. If we make a  cross-section at the superior colliculi,

we’ll see the nucleus of the oculomotor nerve and  the posterior accessory nucleus of the oculomotor

nerve. So the oculomotor nerve will travel  towards the anterior surface towards the eye,

to innervate the extraocular muscles to move your  eyes. And the posterior accessory nucleus will

send fibers together with the oculomotor nerve  to give a parasympathetic innervation to the

eyes. So that is at the superior colliculi level. Then at the Inferior Colliculi level, there’s the

nucleus of the trochlear nerve. Remember, we have  the trochlear nerve going out at the posterior

surface, which turns anteriorly, goes towards the  eyes, and supplies the superior oblique muscle.

Another nucleus you’ll find at the level of the  Inferior colliculus is the mesencephalic nucleus

of the Trigeminal Nerve. Ok, let’s repeat the  nuclei of the trigeminal nerve at the Rhomboid

Fossa. These are the Mesencephalic nucleus of  the trigeminal nerve. So here is the trigeminal

nerve. The mesencephalic nucleus takes in sensory  information from mainly muscles of mastication.

Other nuclei are the Principal nucleus of  the trigeminal nerve, which senses touch

and vibration. Spinal nucleus which senses  pain and temperature, and motor nucleus of

the trigeminal nerve, which provide motor  innervation for the muscles of mastication.

These are the nuclei of the trigeminal nerve, but  the mesencephalic nucleus of the trigeminal nerve

is the only one extending upwards to the  inferior part of the midbrain, and that’s why

you see it at that level. So that’s these. Then we have the reticular formation,

which is essential for the vital functions and  balance. And we have the periaqueductal grey

substance, or the central grey substance, which is  associated with eliminating, or decreasing pain.

So that was all for the grey matter of the  midbrain. Now let’s do the White matter of

the midbrain. White matter in the CNS is  distributed as either ascending tracts,

which are sensory tracts. And descending tracts,  which are motor tracts. We’ll do the ascending

tracts first and then do the descending ones. The first one is the medial lemniscus. The medial

lemniscus is an ascending tract you’ll  find throughout the whole brainstem.

So here is the cross-section of  the spinal cord, the medulla,

and Pons. Do you remember that sensory  fibers came from the Lower parts of the body,

which ascend as Gracile fascicle, and sensory  fibers that came from the upper parts of your

body, which ascend as Cuneate fascicle? They sense  conscious proprioception and mechanoreceptors. And

I say conscious because they take their fibers  all the way up to your cerebral Cortex. If it

were unconscious, then it would take its fibers  to the cerebellum or other subcortical structures.

They will ascend to the gracile and cuneate nuclei  in the medulla. Then fibers there will leave as

either the external arcuate fibers or internal  arcuate fibers. The internal arcuate fibers

cross to the other side. Then they will ascend as  the medial lemniscus, which is what you see here

in the midbrain. They will ascend and go to the  primary somatosensory area in the cerebral Cortex.

So that is the Medial Lemniscus. Next to the medial lemniscus,

we have the spinal lemniscus. Ok. So again  here’s the cross-section of the medulla and Pons.

On the cross-section of Medulla, you’ll  find two tracts called the anterior

and the lateral spinothalamic tracts. At some  point between the Pons and the Medulla, these

tracts will join together and form the spinal  lemniscus. And the spinal lemniscus will ascend

through Pons and the mesencephalon and then to  the primary somatosensory area in the Cortex.

They’re responsible for conscious  sensory input of Pain temperature,

Pressure, and touch. So that is this one. Next, we have the Trigemnical Lemniscus.

The trigemnical lemniscus comes from the  trigeminal ganglion, which is a part of the

trigeminal nerve, the 5th cranial nerve. It  receives sensory input from the facial area,

and send their axons towards Pons. In  Pons, these fibers will cross to the

other side and ascent through the midbrain to  the primary somatosensory area as well. Awesome.

Then we have a tract we went through a little  earlier in this video, the Lateral Lemniscus.

Which was part of the hearing pathway. So the  cochlear nerve receives impulses from the cochlea,

where sound is converted into nerve signals.  The cochlear nerve will then go to the cochlear

nuclei in Pons. Then they will cross and form the  trapezoid body of Pons. After that, fibers ascend,

they go up as the Lateral Lemniscus to synapse  with the inferior colliculi. From there,

impulses are sent through the Brachium of the  inferior colliculus, to the medial geniculate

body, and then to the primary auditory Cortex,  which is in your superior temporal gyrus. Cool.

That was all of our sensory  tracts in the mesencephalon.

Now let’s do all the descending tracts we find  here. The descending tracts of the mesencephalon

are arranged along the Cerebral peduncles and the  tegmentum of the Midbrain. We’ll do the tegmentum

first and then go over to the cerebral peduncles. The first one is a tract that comes from the

superior colliculus. And that tract will decussate  as the posterior tegmental decussation, to then

descend as the tectospinal tract. Remember when  we went through the superior colliculus, where we

said that the superior colliculus is a part of the  visual pathway? Where you have receptors for the

optic nerve at the eyes. The fibers of the optic  nerve will go back, and then half of the fibers

will cross and form the optic chiasm. After that,  they will synapse with the Lateral geniculate

bodies, which is the metathalamus. From the  Lateral Geniculate bodies, the fibers will go

back to the occipital lobe so that you’ll be able  to consciously perceive what you see around you.

But fibers also go from the lateral geniculate  bodies to the superior colliculi. And from the

superior colliculi, fibers are sent down as the  tectospinal tract. And when the superior colliculi

are engaged, it usually just engages the neck  muscles based on what you see. So that’s this one.

So, we talked about the posterior  tegmental decussation. And since we

have a posterior tegmental decussation, we’ll  also find an anterior tegmental decussation!

Which is formed by the rubrospinal  tract just as it leaves the red nuclei.

Remember, from the Cortex, the corticopontine  tract will synapse with the pontine nuclei of

Pons. After that, fibers will go to the cerebellum  as the pontocerebellar tract. Then they will go to

the nucleus ruber as the cerebellorubral  tract. And then, from the nucleus ruber,

the rubrospinal tract will cross and then descend  towards the spinal cord and then to muscles to

coordinate and support voluntary muscle movements. Another tract you’ll find in the midbrain is,

it’s sort of logical, but at some point in the  midbrain, you’ll find the cerebellorubal tract

as it goes towards the nucleus ruber. So  we might as well include this one too.

Then after that, we have the reticulospinal  tract, which is part of the balance and posture

system. They come from the reticular formation  inside the brainstem. The Reticular system is

responsible for Sleep, alertness, cardiovascular  control, breathing, and all of those vital things.

But they’re also responsible for motor control  like your balance and posture through the

reticulospinal tract. So that is this one. Then we have a tract called the medial

longitudinal fasciculus, which descends  towards the spinal cord and is present only

in the cervical segments. This tract coordinated  involuntary movements of the head, neck, and eyes

through synapses between the cranial nerves 3, 4,  6, and 11. So that was all the descending tracts

in the tegmentum of the Midbrain. Now let’s do the  descending tracts within the Cerebral Peduncles.

These tracts are very easy to recognize  as they all come from the cerebral Cortex.

Hence the name cerebral peduncles. So the first descending tract is

our famous Corticospinal tract. The corticospinal tract originates from

the pyramidal cells of the primary motor area. And  since they come from the pyramidal cells, we call

this a pyramidal motor tract which descends down  to the spinal cord to innervate skeletal muscles.

Alongside the corticospinal tract, you’ll  find corticonuclear tracts as well.

It descends in the same areas as the corticospinal  tract. But the corticonuclear tracts are

responsible for the voluntary control  of muscles located in the head and neck.

Then we have the corticopontine tract. And I’ve  mentioned this like two other times in this video,

but it’s important to know it.  Remember the corticopontine tract

descends within the cerebral peduncle to synapse  with…. The Pontine nuclei of Pons. After that,

they go to the cerebellum as the pontocerebellar  tract and then back to the mesencephalon as the

cerebellorubral tract to  synapse with the nucleus ruber.

From which the rubrospinal tract will descend to  skeletal muscles to support voluntary movements.

The term corticopontine tract is a little  inaccurate to use in this sense. Because one tract

will arrive from each lobe of the brain and then  descend. They’ll all have the same pathway, it’s

just the origin point that differ. So if the tract  starts from the Cortex of the frontal lobe, it’s

called frontopontine tract. If it starts from the  occipital lobe or parietal lobe or temporal lobe,

it’ll be called occipitopontine, parietopontine,  or temporopontine tract. Notice that none of these

come from the pyramidal cells of the primary  motor cortex, that’s why they’re referred

to as extrapyramidal tract. They don’t initiate  movement, but they support the voluntary movement.

So, that was all the descending tracts  of the cerebral peduncles. That was all

the white matter of the mesencephalon. Now, I made this table with all the grey

and white matter we just went through, along  with a little description here on the right.

I am going to make all the names of the nuclei and  tract disappear. And can you, based on the picture

and the description, tell m what is the name of  number 1? What is the name of the number 7 and

so on. If you can do that, then you pretty much  have a good understanding of the mesencephalon.

If you found this video helpful, please  put a like, comment, share, whatever

you find convenient to you. My next video will be about the Diencephalon.