Midbrain Anatomy Overview and Functions

What's up? Medite here. Let's continue the anatomy of the central nervous system. In this segment, we will cover the complete anatomy of the midbrain, which is also known as the mesencephalon. 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. So 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. Then I made a little quiz at the end which might help you if you need to memorize. So we can start by replacing this picture with a little more realistic one. From this view, we can locate the 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 the 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 are the cerebral peduncles. These are two large peduncles that contain tracts coming from the cortex of the cerebrum and are responsible for voluntary movement of the body. I'll show you this later in this video when we go through the cross section of the midbrain. 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 the midbrain, like the hypothalamus and pituitary gland. So again, they're the diencephalon. in 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 oculomotor sulcus of the mesencephalon, of which the third cranial nerve goes out from, called the oculomotor nerve. This nerve is called the oculomotor nerve because it goes towards the eyes to innervate the extrinsic muscles that enables 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 are the cerebral peduncles that we talked about earlier. And they are huge in comparison with the actual midbrain. That's why you're able to see them peeking out from the sides when you look at the midbrain from a posterior view. But the most significant structures associated with the posterior view are the tectil plate or lamina tecti. And you'll notice that on the tectil plate, you'll find 4 rounded structures. The upper two are called the superior colliculus. And they are associated with the rapid and controlled eye movements. Now how are they 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 second cranial nerve, the optic nerve. These fibers of the optic nerve will go back, and then half of the fibers will cross to form the optic chiasm. After that, they will synapse with the lateral gyniculate bodies, which are the metathalamus, we'll go through them when we go through the diencephalon. But from the lateral gyniculate bodies, these fibers will go back to the occipital lobe which is where you'll find the primary visual cortex. So 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 gilliculate body 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 eye and 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 the sound is converted into nerve signals. The cochlear nerve will then go to the cochlear nuclei at the rhomboid fossa in the pons, and then they will cross and form the trapezoid body. Remember it's a structure of pons. After that, fibers will ascend. They go up as the lateral lemma discus to synapse with the inferior colliculi. From there, impulses are sent through the brachium of the inferior colliculus, to the medial geniculate bodies, not the lateral this time. The lateral geniculate body is for vision. Medial is for hearing. From the medial geniculate body, impulses will go to the primary auditory cortex, which is in your superior 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 lemuniscus, located in this area. And then, below the inferior colliculus, you'll find the cranial nerve number 4, called the Choclear nerve, which will turn around to the anterior side to innervate the superior oblique muscle of the eye. So that was everything for the 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. So this is at the level of the superior colliculi. Structures are more or less the same, whether you make a cross section at the level of the superior colliculus or at the level of the inferior colliculus. But there are some slight differences, and I'll make sure to highlight these differences along the way. 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 there's the tegmentum of the midbrain in the middle. And then the cerebral peduncles. Now let's go through all the internal structures, starting with the grey matter. And just to remind you again, white matter consists of myelinated nerve fibers. So when you go through the white matter, we talk about tracts. Grey matter contains nuclei of neurons. So structures in grey matter are nuclei. So the first grey matter structure we're gonna talk about is the red nucleus, or Nucleus Ruberi. These nuclei are pale pink in color due to the presence of iron as either hemoglobin or ferritin. These nuclei are a very characteristic part of the midbrain, and they contribute to the extrapyramidal pathway of coordinating voluntary muscle control. I will 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. But tracts may even come directly from the cortex to the nucleus rubid. But this right here is the most important pathway when talking 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 substantia nigra pars reticulata. When we talk about the substantia nigra, we generally mean pars compacta, because this is the most significant one. The pars compacta is very dark in color due to the large numbers of dopamine neurons producing neuromelanin. The substantia nigra is considered a part of the basal ganglia. And the basal ganglia, what that is, is that they're a group of grey matter nuclei found within your brain. And we'll talk about that when we talk about the internal structures of the hemispheres. But the basal ganglia consists of the caudate nucleus, the putamen, globus pallidus, there are two globus pallidus, there's globus pallidus externus to the left, and internus to the right. Then there's the thalamus, but not the whole thalamus, only the ventral anterior and the ventral lateral part of the thalamus is considered a part of the basal ganglia. Then there's the subthalamic nuclei, and then the substantia nigra as well. And you might find other names when we talk about the basal ganglia, like the striatum. And whenever you're talking about the striatum, you're really talking about the putamen and the caudate nucleus together. And when you put the putamen and the glubus 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 ganglion. 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 with the basal ganglia. 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 happened that causes the substantia nigra to have less dopamine neurons, what do you think will happen? You'll get Parkinson's disease. And this will give you 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 level 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 eyes 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 a nucleus of the trochlear nerve. Remember, we have the trochlear nerve going out at the posterior surface, which turns anteriorly and 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. Okay, so let's repeat the nuclei of the trigeminal nerve at the rhomboid fossa. These are the mesencephalic nucleus of the trigeminal nerve. So here's the trigeminal nerve. The mesencephalic nucleus take part 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 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. That's why you see it at the level of the inferior colliculus. So that's these. Then we have the reticular formation, which are essential for the vital functions and balance. And we have the Periaqueductal Gray Substance, or the Central Gray Substance, which are associated with eliminating or decreasing pain. So that was all for the gray matter of the midbrain. Now, let's do the white matter of the midbrain. White matter in the central nervous system are distributed as either ascending tracts, which are sensory tracts, and descending tracts, which are motor tracts. So we'll do the ascending tracts first, and then the descending tracts. The first one is the medial lemniscus. The medial lemniscus is an ascending tract you'll find throughout the whole brainstem. So here is a cross section of the spinal cord, the medulla, and the pons. Do you remember that sensory fibers came from the lower parts of the body which ascend as the Grazil fascicle, and sensory fibers that came from the upper parts of the body, which ascend as the Cunate 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 ascend to the gracile and cuneate nuclei in the medulla. Then fibers will leave as either external arcuate fibers or internal arcuate fibers. The internal arcuate fibers will cross to the other side, then they will ascend as the medial lemnoscus, which is what you see here in the midbrain. They will ascend to 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. So again, here's the cross section of the medulla and pons. On the cross section of the medulla, you'll find two tracts called the anterior and lateral spinothalamic tracts. At some points between the medulla and pons, 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. And they're responsible for conscious sensory input of pain, Temperature, pressure, and touch. So that's this one. Next we have the Trigeminal Lemniscus. The trigeminal 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 sends their axons towards the pons. In pons, these fibers will cross to the other side and then ascend through the midbrain to the primary somatosensory area as well. Awesome. Then we have a tract we went through earlier in this video, the lateral lemniscus, which remember was 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 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. So that was all 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. And we'll do the tegmentum first. and then go over 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, and 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 upper. optic chiasm. After that, they will synapse with the lateral geniculate bodies. 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 around you. 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 nucleus. Remember, from the cortex, the corticopontine tract will synapse with the pontine nuclei of PANS. After that, Fibers will go to the cerebellum as the pontocerebellar tract. Then they will go to the nucleus rubur as the cerebellorubral tract. And then from the nucleus rubur, the rubrospinal tract will cross and then descend towards the spinal cord and then to the muscles to coordinate and support voluntary muscle movement. Another tract you'll find in the midbrain is, it's sort of logical, but at some point of the midbrain, you'll find the cerebellorubral tract as it goes towards the nucleus rubur. So we might as well include this one too. Then after that, we have the reticulospinal tract, which is a part of the balance and poster 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 poster, through the reticulospinal tract. So that's this one. Then we have the tract we call the medial longitudinal fasciculus, which descends towards the spinal cord and is present only in the cervical segments of the spinal cord. And this tract coordinates involuntary movement of the head, neck and eyes through synapses between the cranial nerves number 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 towards the spinal cord to innervate skeletal muscles. Alongside the corticospinal tract, you'll find the corticonuclear tract as well. It descends in the same area as the corticospinal tract, but the corticonuclear tract are responsible for the voluntary control of muscles located in the head and neck. Then we have the corticopontine tract, and I mentioned this like two other times in this video. But it's important that you know it. Remember, the corticopontine tract descends within the cerebral pinnacles 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 rubir, from which the rubrospinal tract will descend to the 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. And they will 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 the frontopontine tract. If it starts from the occipital lobe, or the parietal lobe, or the temporal lobe, it's called the 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 tracts. They don't initiate movement, but they support the voluntary movement. So that was all the descending tracts of the cerebral peduncles. And that was all the white matter of the mesencephalon. Now I made this table with all the grey matter and the white matter we just went through, along with a little description here on the right. Now I'm going to make all the names of the nuclei and tracts. disappear, and can you, based on the picture and the description, tell me what is the name of number 1, what is the name of 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, share, comment, whatever you find convenient to you. The next video is going to be about the diencephalon.

What's up? Medite here. Let's continue the anatomy of the central nervous system. In this segment, we will cover the complete anatomy of the midbrain, which is also known as the mesencephalon.

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.

So 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.

Then I made a little quiz at the end which might help you if you need to memorize. So we can start by replacing this picture with a little more realistic one. From this view, we can locate the 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 the 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 are the cerebral peduncles. These are two large peduncles that contain tracts coming from the cortex of the cerebrum and are responsible for voluntary movement of the body. I'll show you this later in this video when we go through the cross section of the midbrain.

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 the midbrain, like the hypothalamus and pituitary gland. So again, they're the diencephalon. in 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 oculomotor sulcus of the mesencephalon, of which the third cranial nerve goes out from, called the oculomotor nerve. This nerve is called the oculomotor nerve because it goes towards the eyes to innervate the extrinsic muscles that enables 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 are the cerebral peduncles that we talked about earlier. And they are huge in comparison with the actual midbrain.

That's why you're able to see them peeking out from the sides when you look at the midbrain from a posterior view. But the most significant structures associated with the posterior view are the tectil plate or lamina tecti. And you'll notice that on the tectil plate, you'll find 4 rounded structures.

The upper two are called the superior colliculus. And they are associated with the rapid and controlled eye movements. Now how are they 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 second cranial nerve, the optic nerve. These fibers of the optic nerve will go back, and then half of the fibers will cross to form the optic chiasm. After that, they will synapse with the lateral gyniculate bodies, which are the metathalamus, we'll go through them when we go through the diencephalon. But from the lateral gyniculate bodies, these fibers will go back to the occipital lobe which is where you'll find the primary visual cortex.

So 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 gilliculate body 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 eye and 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 the sound is converted into nerve signals. The cochlear nerve will then go to the cochlear nuclei at the rhomboid fossa in the pons, and then they will cross and form the trapezoid body. Remember it's a structure of pons. After that, fibers will ascend.

They go up as the lateral lemma discus to synapse with the inferior colliculi. From there, impulses are sent through the brachium of the inferior colliculus, to the medial geniculate bodies, not the lateral this time. The lateral geniculate body is for vision.

Medial is for hearing. From the medial geniculate body, impulses will go to the primary auditory cortex, which is in your superior 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 lemuniscus, located in this area.

And then, below the inferior colliculus, you'll find the cranial nerve number 4, called the Choclear nerve, which will turn around to the anterior side to innervate the superior oblique muscle of the eye. So that was everything for the 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. So this is at the level of the superior colliculi. Structures are more or less the same, whether you make a cross section at the level of the superior colliculus or at the level of the inferior colliculus.

But there are some slight differences, and I'll make sure to highlight these differences along the way. 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 there's the tegmentum of the midbrain in the middle. And then the cerebral peduncles.

Now let's go through all the internal structures, starting with the grey matter. And just to remind you again, white matter consists of myelinated nerve fibers. So when you go through the white matter, we talk about tracts.

Grey matter contains nuclei of neurons. So structures in grey matter are nuclei. So the first grey matter structure we're gonna talk about is the red nucleus, or Nucleus Ruberi.

These nuclei are pale pink in color due to the presence of iron as either hemoglobin or ferritin. These nuclei are a very characteristic part of the midbrain, and they contribute to the extrapyramidal pathway of coordinating voluntary muscle control. I will 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. But tracts may even come directly from the cortex to the nucleus rubid. But this right here is the most important pathway when talking 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 substantia nigra pars reticulata. When we talk about the substantia nigra, we generally mean pars compacta, because this is the most significant one.

The pars compacta is very dark in color due to the large numbers of dopamine neurons producing neuromelanin. The substantia nigra is considered a part of the basal ganglia. And the basal ganglia, what that is, is that they're a group of grey matter nuclei found within your brain.

And we'll talk about that when we talk about the internal structures of the hemispheres. But the basal ganglia consists of the caudate nucleus, the putamen, globus pallidus, there are two globus pallidus, there's globus pallidus externus to the left, and internus to the right. Then there's the thalamus, but not the whole thalamus, only the ventral anterior and the ventral lateral part of the thalamus is considered a part of the basal ganglia. Then there's the subthalamic nuclei, and then the substantia nigra as well.

And you might find other names when we talk about the basal ganglia, like the striatum. And whenever you're talking about the striatum, you're really talking about the putamen and the caudate nucleus together. And when you put the putamen and the glubus 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 ganglion. 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 with the basal ganglia. 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 happened that causes the substantia nigra to have less dopamine neurons, what do you think will happen? You'll get Parkinson's disease. And this will give you 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 level 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 eyes 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 a nucleus of the trochlear nerve. Remember, we have the trochlear nerve going out at the posterior surface, which turns anteriorly and 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. Okay, so let's repeat the nuclei of the trigeminal nerve at the rhomboid fossa. These are the mesencephalic nucleus of the trigeminal nerve.

So here's the trigeminal nerve. The mesencephalic nucleus take part 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 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. That's why you see it at the level of the inferior colliculus. So that's these. Then we have the reticular formation, which are essential for the vital functions and balance.

And we have the Periaqueductal Gray Substance, or the Central Gray Substance, which are associated with eliminating or decreasing pain. So that was all for the gray matter of the midbrain. Now, let's do the white matter of the midbrain.

White matter in the central nervous system are distributed as either ascending tracts, which are sensory tracts, and descending tracts, which are motor tracts. So we'll do the ascending tracts first, and then the descending tracts. The first one is the medial lemniscus.

The medial lemniscus is an ascending tract you'll find throughout the whole brainstem. So here is a cross section of the spinal cord, the medulla, and the pons. Do you remember that sensory fibers came from the lower parts of the body which ascend as the Grazil fascicle, and sensory fibers that came from the upper parts of the body, which ascend as the Cunate 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 ascend to the gracile and cuneate nuclei in the medulla. Then fibers will leave as either external arcuate fibers or internal arcuate fibers.

The internal arcuate fibers will cross to the other side, then they will ascend as the medial lemnoscus, which is what you see here in the midbrain. They will ascend to 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. So again, here's the cross section of the medulla and pons. On the cross section of the medulla, you'll find two tracts called the anterior and lateral spinothalamic tracts.

At some points between the medulla and pons, 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. And they're responsible for conscious sensory input of pain, Temperature, pressure, and touch. So that's this one.

Next we have the Trigeminal Lemniscus. The trigeminal 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 sends their axons towards the pons.

In pons, these fibers will cross to the other side and then ascend through the midbrain to the primary somatosensory area as well. Awesome. Then we have a tract we went through earlier in this video, the lateral lemniscus, which remember was 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 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. So that was all 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. And we'll do the tegmentum first. and then go over 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, and 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 upper. optic chiasm. After that, they will synapse with the lateral geniculate bodies. 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 around you. 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 nucleus. Remember, from the cortex, the corticopontine tract will synapse with the pontine nuclei of PANS. After that, Fibers will go to the cerebellum as the pontocerebellar tract. Then they will go to the nucleus rubur as the cerebellorubral tract. And then from the nucleus rubur, the rubrospinal tract will cross and then descend towards the spinal cord and then to the muscles to coordinate and support voluntary muscle movement.

Another tract you'll find in the midbrain is, it's sort of logical, but at some point of the midbrain, you'll find the cerebellorubral tract as it goes towards the nucleus rubur. So we might as well include this one too. Then after that, we have the reticulospinal tract, which is a part of the balance and poster 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 poster, through the reticulospinal tract. So that's this one. Then we have the tract we call the medial longitudinal fasciculus, which descends towards the spinal cord and is present only in the cervical segments of the spinal cord.

And this tract coordinates involuntary movement of the head, neck and eyes through synapses between the cranial nerves number 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 towards the spinal cord to innervate skeletal muscles. Alongside the corticospinal tract, you'll find the corticonuclear tract as well.

It descends in the same area as the corticospinal tract, but the corticonuclear tract are responsible for the voluntary control of muscles located in the head and neck. Then we have the corticopontine tract, and I mentioned this like two other times in this video. But it's important that you know it.

Remember, the corticopontine tract descends within the cerebral pinnacles 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 rubir, from which the rubrospinal tract will descend to the 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.

And they will 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 the frontopontine tract. If it starts from the occipital lobe, or the parietal lobe, or the temporal lobe, it's called the 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 tracts. They don't initiate movement, but they support the voluntary movement.

So that was all the descending tracts of the cerebral peduncles. And that was all the white matter of the mesencephalon. Now I made this table with all the grey matter and the white matter we just went through, along with a little description here on the right.

Now I'm going to make all the names of the nuclei and tracts. disappear, and can you, based on the picture and the description, tell me what is the name of number 1, what is the name of 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, share, comment, whatever you find convenient to you.

The next video is going to be about the diencephalon.

Transcript for:Midbrain Anatomy Overview and Functions

Transcript for:
Midbrain Anatomy Overview and Functions