Video: Cranial nerve nuclei

Hey everyone! This is Nicole from Kenhub, and welcome to our tutorial on the anatomy of the cranial nerve nuclei. So we’re going to begin this tutorial with an introduction on what the cranial nerve nuclei actually are and we’re also going to be making use of a grouping of nuclei based on their function. We’re also going to be looking at a brief overview of the anatomy of the brainstem – that is the midbrain, the pons, and the medulla; and that overview will roughly present the cranial nerve contents in each segment of the brainstem.

Once we’re through this introduction, we’re going to cover each cranial nerve nuclei cluster in detail following an orderly fashion from cranial nerve two to cranial nerve twelve. And I know you're wondering why I said cranial nerve two to cranial nerve 12 and not cranial nerves 1-12, so I'll just say briefly that it's because cranial nerve one, or the olfactory nerve, originates from the olfactory bulb, which is located in the forebrain, therefore we won't be talking about that today. Then, we’ll generally present each nuclei cluster starting with the nerve first and then delving deeper to discuss its nuclei. We’ll then present the nuclei of each nerve grouped as sensory or motor when that grouping is applicable.

Once we’re done presenting each cranial nerve nuclear cluster, we’ll also talk about some important nuclei of the extrapyramidal system present in the brainstem. And finally we’ll try to make a clinical correlation to what we’re going to be learning, and we’re going to be doing that by discussing the pupillary reflex and a hypothetical clinical case where that reflex is absent.

Okay, so we’re currently looking at a sagittal section of the midbrain where we can see cranial nerve nuclei highlighted in various colors. The cranial nerve nuclei are collections of cell bodies which can be distinguished to have either a motor or sensory function based on whether they project their axons to the periphery or back to the nervous system. The nuclei are collections of cell bodies in close proximity to one another which allows signals to be relayed between distant locations.

So, in our image here, we have motor nuclei in blue and this give rise to axons that later on bundle up to become motor nerves and to control specific muscles, and we have over here our sensory nuclei in pink, and they receive fibers conveying sensory information. Generally, we can identify three types of cranial nerves, and these are motor cranial nerves which are our cranial nerves three, four, six, eleven, and twelve; and we also have sensory cranial nerves and these are our cranial nerves two and eight; and finally, we have mixed cranial nerves which include cranial nerve five, seven, nine, and ten.

Now you might be wondering as how some cranial nerves can have both sensory and motor function when the nuclei can only have one or another. So the answer to that is that cranial nerves which are both motor and sensory will have a separate nucleus for each component, and in addition to that, you’ll probably notice later on that some cranial nerves may share the same nucleus. And finally do note that cranial nerve nuclei are also bilateral, so there’ll be one of each on the right and the left.

The cranial nerve motor nuclei are further grouped according to their target tissues – that is, where the axons of the attached cranial nerves are sent. So, remember that motor nerves travel from the central nervous system to somewhere further away and these can be referred to as efferent. Cranial nerve nuclei with motor functions can be grouped in the following way – general somatic efferents (GSE) which innervates skeletal muscles, special visceral efferents which innervates the musculature of the pharyngeal arches, and general visceral efferents which innervate visceral smooth muscle.

Similarly, the cranial nerve sensory nuclei are grouped according to the tissues from which they receive impulses, and these nerves travel from distant tissues to the central nervous system and so can be referred to as afferent. And the sensory nuclei can be grouped into general somatic afferents which convey pain, touch and temperature; special somatic afferents which convey the special senses of vision, hearing and balance; general visceral afferents which conducts sensory impulses usually pain or reflex sensations from the viscera, glands and blood vessels to the central nervous system; and special visceral afferents which carry the special senses of smell which is olfaction and taste which is gustation.

Okay, so before we dig down into the anatomy of the cranial nerve nuclei, it will be useful to have a brief overview of the anatomy of the brainstem where all the cranial nerve nuclei reside. So, here we are looking at the brainstem from behind in the posterior view, and you might remember that the brainstem is divided into three distinct regions and these are the midbrain, the pons and the medulla, and each of these regions contain several cranial nerve nuclei. At this point, we should also briefly highlight the thalamus because we’ll need to refer to that region again in our discussion on the optic nerve.

So now we’re looking at a sagittal section through the entire brainstem from the left-hand side, and the midbrain which is also known as the mesencephalon is highlighted here, and it contains three cranial nerve nuclei and other nuclei associated with these cranial nerves. And these nuclei are the oculomotor nucleus and the trochlear nucleus as well as the mesencephalic nucleus of the trigeminal nerve. The other nuclei will be discussed in detail later in a section dedicated to midbrain structures.

In the pons, we find the trigeminal motor nucleus and the sensory nucleus, the abducens nerve nucleus, the facial nerve nucleus as well as the vestibulocochlear nuclei which is near the medulla. And finally in the medulla, things get a little bit more complicated. Over here, we can see that the glossopharyngeal nerve, the vagus nerve, and the accessory nerve, and the hypoglossal nerve arise from the medulla, but some of these nerves share similar nuclei.

So, remember that cranial nerve nine and cranial nerve ten are mixed nerves, so they both have a motor and sensory nuclei. So if we’re looking at the sensory nuclei of cranial nerves nine and ten, these nerves are the nucleus of the solitary tract and the spinal trigeminal nucleus. The motor nuclei of the glossopharyngeal nerve are the inferior salivatory nucleus and the nucleus ambiguus while the motor nuclei of the vagus nerve are the nucleus ambiguus and the dorsal nucleus of the vagus nerve.

The nucleus of the accessory nerve is also present in the medulla and this is the spinal accessory nucleus as is the hypoglossal nerve nucleus. And both of these nuclei are motor nuclei.

So we’re going to start with the optic nerve and its relevant nuclei, and as you can see, we’ll discuss the lateral geniculate nucleus which is not a brainstem nucleus and the superior colliculus. So, first of all, let’s clarify that the optic nerve doesn’t come off the brainstem. It actually synapses at the thalamus and particularly at the lateral geniculate nucleus which is the structure highlighted in this slide.

The thalamus and the lateral geniculate nucleus are located just above the brainstem. Here we have a superior view of the brain in transverse section at the level of the thalamus with the brainstem underneath out of view. You might remember that the optic nerve connects the retina to the thalamus. To be more specific, it synapses at the lateral geniculate nucleus with the thalamus which is the region highlighted in green.

You may remember that many retinal neurons, the axons of which travel inside the optic nerve synapse at the lateral geniculate nucleus. The lateral geniculate nucleus then projects posteriorly to the occipital cortex via the optic radiation, and this tells us that the lateral geniculate nucleus plays a role in the integration of visual input. But what about those axons of the optic nerve that don’t synapse at the lateral geniculate nucleus, I hear you ask. Well, we’re going to shift our view to look at a posterior view of the brainstem to give you just a hint.

So, once again, we’re looking at the lateral geniculate nucleus, but this time, we are back in a posterior view and as we can see, it is connected to another structure via a thin tract, and this structure is called the superior colliculus. But before we move on to further discussing the superior colliculus, let’s first recap what we’ve learned about the lateral geniculate nucleus.

So we know it’s a thalamic nucleus which, as we saw, is located posteriorly, inferiorly, and laterally to the main thalamic mass. It mainly receives input from the retina as well as the superior colliculus and it projects its output to the occipital cortex via the optic radiation, and it functions as a thalamic relay station for visual signals.

So moving on now to the superior colliculus. Once again, we’re looking at the brainstem from a posterior position with the superior colliculus highlighted in green, and you might also notice that we’ve moved from nuclei within the thalamus to the nuclei within the midbrain. So, the superior colliculus receives retinal input from the fibers of the optic nerve which bypasses the lateral geniculate nucleus. It would be overly complex to analyze all the inputs and outputs of the superior colliculus. So, for now, let’s just stick to the superior colliculus being a control center for eye movements and a multisensory integration.

So the superior colliculus is mostly studied for its involvement in eye movement and specifically in fixation, smooth pursuit and saccades. However, several findings do suggest that it is also involved in combined head and eye movements which would be impossible if this nucleus did not integrate information from other sensory systems.

So having completed the discussion of this nucleus, we are now ready to move on to the nuclei of the next cranial nerve which is the oculomotor nerve and nuclei. And in this section, we’re going to first talk about the nerve and then its two motor nuclei which are the main oculomotor nucleus and the accessory oculomotor nucleus.

Okay, so the oculomotor nerve which is also known as cranial nerve three is what we’re going to be talking about here, and you can see it emerging from the anterior surface of the midbrain. So, if you look a little bit closely at the midbrain parenchyma, you’ll notice that the oculomotor nerve is formed by joining the fibers from two different nuclei. This anatomical observation is also reflected on a functional level since the fibers originating from each nucleus have a distinct function.

Now since this is a tutorial on cranial nerve nuclei, we’re not going to move into further detail about the anatomy of the oculomotor nerve after it emerges from the brainstem, but you are definitely free to check out our tutorial on this topic if you want to learn more. So, don’t forget to go to kenhub.com and check that out. But for the moment, let’s move on to the main oculomotor nucleus.

So the oculomotor nerve and nuclei highlighted here in green are predominantly motor in function. The main oculomotor nucleus contains the cell bodies of neurons with a general somatic efferent function and controls all eye movements except for eye abduction and internal rotation of the eye, and we’ll look at this nucleus from a different angle before we move to the other oculomotor nucleus to extract some additional information about its anatomy.

So, once again, we’re observing our brainstem from a posterior view with the left oculomotor nucleus highlighted in green, and we’re looking at the superior colliculus from this angle a short while earlier. So, as you can see, both oculomotor nuclei are found in the midbrain at the level of the superior colliculus, and these nuclei are paired structures – meaning they are present on both the left and the right side.

So, there’s a second smaller oculomotor nucleus that I want to talk about today, and this lies just medial to the other one, and this little nucleus has got two names. Some people call it the accessory oculomotor nucleus, while others refer to it as the Edinger-Westphal nucleus. And we’re looking at this nucleus from a posterior perspective, and as you can see from this view, the accessory oculomotor nucleus is also located in the midbrain at the level of the superior colliculus and also a little bit medially to the main oculomotor nucleus.

So we’re now showing you the nucleus from a sagittal view – that is, from a lateral to medial perspective – so that we can remind you that it gives off a functionally distinct set of fibers. And these are general visceral efferent fibers.

The accessory oculomotor nucleus is a parasympathetic nucleus and gives rise to nerves that innervates the iris sphincter and the ciliary muscle, and in doing so, it controls pupillary constriction, more accurately known as miosis and pupillary accommodation.

Okay, so now, we’re done with cranial nerve number three, let’s move on to talk about cranial nerve number four, which is the trochlear nerve, and the trochlear nerve has a single motor nucleus. The trochlear nerve is a general somatic efferent nerve and this means that it innervates skeletal muscles. So the trochlear nerve only has one muscle that it innervates and this muscle is the superior oblique muscle of the eye and it’s the muscle responsible for the internal rotation of the eye.

So as you can see, we’re looking at the posterior aspect of the brainstem and what we’re interested in are these two structures over here, and if you look at them closely, you’ll notice that they look very similar to the superior colliculi but they’re just situated below them so we naturally call them the inferior colliculi. And we can see that each trochlear nerve exits from the posterior aspect of the midbrain just below the inferior colliculus on each side. Interesting fact: If you take a close look at our image, you’ll see that the trochlear nerve is the only nerve that emerges from the posterior aspect of the brainstem.

Now let’s look at our trochlear nucleus. So one of the first things you might notice about this nucleus is that it is contralateral to the corresponding trochlear nerve and as you can see, the fibers originating from this nucleus cross over inside the midbrain parenchyma. So damage to the trochlear nucleus would actually affect the contralateral eye whereas damage to all other cranial nerves manifest ipsilaterally. And as you can see, this nucleus is found at the level of the inferior colliculi and below the oculomotor nuclei. And we can see this anatomical relationship more clearly if we shift to a sagittal view.

So this is a less crowded view where we can also see the cerebral aqueduct and in addition to what we’ve already seen in the posterior view of the brainstem, this view also allows us to discover the relationship between the trochlear nucleus and the fibers to the cerebral aqueduct. And as you can see, the trochlear nucleus is located anteriorly to the aqueduct, and its fibers move around the aqueduct before they cross over.

Okay, so done with the trochlear nerve, now let’s move on to talk about the trigeminal nerve nuclei which is a pretty complicated set of nuclei. As always, we’re going to be starting with the nerve and then we’ll talk about the trigeminal ganglion and then we’ll talk about its single motor nucleus and its three sensory nuclei – the mesencephalic nucleus, the principal sensory nucleus, and the spinal trigeminal nucleus.

Okay, so the trigeminal nerve is the fifth cranial nerve and once again, we’re back in our posterior view of the brainstem, and we can see the trigeminal nerve emerging from the pons. If you look at the trigeminal nerve on the right, we can see that it divides into three branches, while on the left, it is only depicted as a single branch. Moreover, the intraparenchymal fibers from the right are colored red, while on the left, they’re colored blue. And you might already be guessing that this is because the trigeminal nerve is a nerve with mixed function which is absolutely correct. And the trigeminal nerve carries both motor and sensory fibers bilaterally. The trigeminal nerve also has multiple nuclei which we’re going to go through in turn, but first let’s start with the trigeminal ganglion.

Okay, so the cell bodies of first order sensory neurons of the trigeminal nerve reside in the trigeminal ganglion, which is the structure you see highlighted here and it’s also known as the semilunar ganglion, because it looks a little bit like a crescent moon. There’s another name for it though and that’s the Gasserian ganglion. You can see that after its ganglion, it subdivides into three branches – the ophthalmic branch, the maxillary branch, and the mandibular branch of which only the mandibular branch contains motor fibers. We will trace trigeminal fibers back to their nuclei starting with the motor fibers since they’re a little bit more simple.

So if we trace the trigeminal motor fibers back to their nucleus, we’re going to end up in the middle of the pons at the trigeminal motor nucleus. So, this nucleus innervates the muscles of the first branchial arch, namely the muscles of mastication, tensor tympani, the tensor veli palatini, the mylohyoid, and the anterior belly of the digastric muscle. From a sagittal point of view, we can see that the trigeminal motor nucleus is located near the top of the pons and at around the same level as with the pontine or principal sensory trigeminal nucleus. And we’ll now move on to the sensory nuclei of the trigeminal nerve.

So if you look closely at the areas highlighted as the sensory trigeminal nuclei, you’ll notice that these areas span all the length of the brainstem and even the beginning of the spinal cord. So to ease the understanding of this vast nucleus, we can subdivide it into three nuclei and, of course, it would be easier to remember that there’s a trigeminal sensory nucleus for every region of the brainstem but unfortunately only one is named after its respective location, and that’s the mesencephalic nucleus of the trigeminal nerve which you can see right here.

The next nucleus which is located at the pons is known as the main or the principal sensory nucleus of the trigeminal nerve, and we’ll see why a little bit later in this tutorial. And the last nucleus occupies most of the medulla and it stretches down to the spinal cord. And this one is known as the spinal trigeminal nucleus.

So now that we’ve clarified some basics about these nuclei and their location, let’s move on to discuss in details about each one of them.

So the first nucleus we’re going to be discussing in detail is the mesencephalic trigeminal nucleus, and as you can see, this nucleus can be identified from the level of the inferior colliculus and further below, and the main function of this nucleus is proprioception from the face which is the feeling of the position of facial muscles in space. From a sagittal point of view, we can confirm that this nucleus spans most of the midbrain from the level of the inferior colliculus and further down into the brainstem.

Next, we see the main nucleus of the trigeminal nerve highlighted in green, and this is a nucleus comprising of second order sensory neurons, the cell bodies of which reside in the caudal pons. And if we look at the sagittal view, we’ll clearly see that this is about the same level as the motor trigeminal nucleus. It receives information about discriminative sensation and light touch of the face as well as conscious proprioception of the jaw via first order neurons of cranial nerve five. Then the most of the sensory information crosses the midline and travels to the contralateral ventral posteromedial nucleus of the thalamus via the anterior trigeminothalamic tract. So, information from the oral cavity travels to the ipsilateral ventral posteromedial nucleus of the thalamus via the dorsal trigeminothalamic tract.

The last of the trigeminal nuclei we’ll discuss is the spinal trigeminal nucleus which you can now see highlighted in green, and this nucleus receives information about crude touch, pain, and temperature from the ipsilateral face. And what’s special about this nucleus is that despite it being named the trigeminal nucleus, it also receives input from the facial, glossopharyngeal, and vagus nerves especially regarding the sensation of pain. And like the principal trigeminal nucleus, this nucleus also projects to the ventral posteromedial nucleus of the thalamus via the anterior trigeminothalamic tract. A significant portion of this nucleus can be found at the medulla, but we can also see this nucleus spanning from as high as the mid pons to as low as the spinal cord.

Alright, let’s move on now to talk about our sixth cranial nerve, the abducens nerve, as well as its motor nucleus.

So as I mentioned before, the abducens nerve is the sixth cranial nerve and as you can see, we have shifted to a sagittal view to better understand some anatomical details regarding the abducens nerve. So as you can see, the abducens nerve emerges from the inferior pontine sulcus and the abducens nerve carries general somatic efferent fibers and controls the lateral rectus muscle of the eye, which is responsible for eye abduction, and the nucleus of the sixth cranial nerve lies in the pons.

Okay so in this image, you can see the nucleus highlighted and note that it has a close relationship to the fourth ventricle. And I also want you to notice that there are several other structures near the abducens nucleus. One of these structures is a group of fibers which loop around the abducens nerve nucleus and these are motor fibers of the facial nerve, which we’ll talk about a little bit later in the tutorial in just a minute, and these fibers loop around the abducens nucleus to give a pretty big bump on the dorsal surface of the pons due to the presence of the nucleus and the large nerve bundle. And because they’re motor fibers of the facial nerve, this is known as the facial colliculus. And the facial colliculus forms part of the floor of the fourth ventricle.

Of here, we have switched to a posterior view where we can see the abducens nucleus again highlighted in green and the facial nerve fibers as they loop around the nucleus, and we can also see the facial colliculus protruding from the floor of the fourth ventricle.

Okay so I just mentioned the facial nerve and now we’re going to be talking a little bit more about it. It’s a pretty complicated nerve, so we’re going to spend a little bit more time here than we have for the other ones talking about it. And as always, we’ll first present some key facts about the nerve and then talk about its two motor nuclei – the facial nerve nucleus and the superior salivatory nucleus – and finally, we’ll talk about its sensory nuclei, the solitary nucleus and the principal sensory trigeminal nucleus.

So the facial nerve is known as cranial nerve seven, and this is the nerve which also emerges from the inferior pontine sulcus. And if you look on the left of the image, you’ll notice that it contains fibers illustrated in blue on the left and red on the right. And the blue fibers on the left are general somatic efferent fibers which innervate the muscles of facial expression and general visceral efferent fibers which innervate the lacrimal gland and the mucosal glands of the nose, the palate, and the pharynx.

The red sensory fibers are special visceral afferent fibers and these convey the sensation of taste from the anterior two-thirds of the tongue and the oral cavity. Both motor and sensory fibers travel within the facial nerve bilaterally, and now that we’ve clarified the function of the facial nerve, let’s discuss some more interesting facts about its nuclei and its anatomy.

So if we look again on the left side of the brainstem, we’ll see the efferent fibers of the facial nerve emerging from two distinct nuclei and you can imagine that one of them contains the bodies of general somatic efferent fibers while the other contains the cell bodies of special visceral efferent fibers, and while we’re only looking at the motor component on the left side, these structures are present on both the left and the right.

So what you see highlighted is the facial nerve nucleus, and this nucleus contains the cell bodies of general somatic efferent neurons and is the one that controls the muscles of facial expression, and it’s these fibers that loop around the abducens nerve that we just talked about and cause the formation of the facial colliculus.

Okay so we’re looking once again at a sagittal view of the brainstem where the facial nerve nucleus is still highlighted in green, and here again we can see facial nerve motor fibers looping around the abducens nerve. And we can also see that these fibers are joined by motor fibers from another nucleus, and this is the inferior salivatory nucleus.

And finally we can also see that the facial nerve is formed by the joining of motor fibers and sensory fibers and we’re going to be talking about these sensory fibers a little bit later on. For now, we’re just going to stick with our next motor nucleus which is the superior salivatory nucleus.

This one contains the cell bodies of visceral efferent fibers, and is the nucleus that innervates the lacrimal gland and the mucosal gland of the nose, palate and pharynx. Due to it innervating the lacrimal gland, this nucleus has also been given the name lacrimal nucleus.

So we can learn a lot about the anatomical location of this nucleus if we looked at it from a sagittal point of view as we are now and in this image, we can see that this nucleus is located in the lower pons near the pontomedullary junction. Using a similar approach on the right, we’ll now take a look at the afferent fibers carried by the facial nerve after they enter the brainstem and we can see part of them coursing superiorly and terminating in a familiar location. And if you remember the name, then you’re top of the class. I hope you remembered that it was the principal sensory trigeminal nucleus.

We can see this relationship more clearly if we look at the sensory fibers of the facial nerve from a sagittal perspective. And here we see how some sensory fibers end up in the principal sensory terminal nucleus. The rest of the afferent fibers of the facial nerve become a tract and terminate at this nucleus, and the tract is known as the solitary tract, while the nucleus is known as the nucleus of the solitary tract. And we’re also going to look at the solitary tract and the nucleus from a lateral perspective, and we can see here how it receives sensory fibers from the facial nerve as well as other cranial nerves.

The solitary tract and nucleus receive special visceral efferent fibers which convey the sensation of taste from the anterior two-thirds of the tongue and the oral cavity, and as you can see, the solitary tract and the nucleus receive fibers from many other cranial nerves besides the facial nerve, and these are namely the glossopharyngeal nerve and the vagus nerve. And we’ll refer to the solitary tract and the nucleus again while discussing the glossopharyngeal and vagus nerves, and ultimately review its inputs and functions during our discussion of the vagus nerve.

Okay let’s talk about the vestibulocochlear nerve. So the vestibulocochlear nerve is a sensory nerve. We’re going to discuss the nerve first and then talk about the vestibular nuclei and the cochlear nuclei in a bit more detail.

So let’s move on to the vestibulocochlear nerve. So this is the eighth cranial nerve and this is also highlighted in green in our image, and as its name suggests, it carries sensory information from the vestibule and the cochlea. So the part of the nerve arising in the cochlea transfers auditory information while the vestibular part which arises from the vestibulum contains information about our sense of balance. And there are two distinct sensory fiber groups within the vestibulocochlear nerve, one for the sensation of sound and one for the sensation of balance, and this means that the vestibulocochlear nerve carries special somatic sensory fibers.

So let’s now move on and talk about the nuclei associated with the vestibulocochlear nerve.

So I want to point out that there are two types of nuclei associated with the vestibulocochlear nerve, and these are, of course, the vestibular nuclei which we are seeing highlighted in this side and the cochlear nuclei which we’ll see later on. So in this image we can see that the vestibular nuclei span an area from the mid-pons to the superior regions of the medulla oblongata and we’ll switch to the sagittal view to review some additional information about the vestibular nuclei cluster.

So we can see the vestibular nuclei spanning from the mid-pons to the medulla, but we can also see that they’re located on the floor of the fourth ventricle. The vestibular nuclei receives information about the sense of balance and the angular and linear acceleration transferred by the vestibular part of the vestibulocochlear nerve. As you can see, the vestibular nuclei are presented as a single structure, but in reality the cluster we are highlighting consists of four distinct sub-nuclei, and these are the medial, the lateral, the superior, and inferior vestibular nuclei.

We’re not going to go much into further detail about the sub-nuclei besides telling you their names because these nuclei are presented and discussed in our very extensive tutorial on the vestibulocochlear nerve. So, we may not be able to see any output coming off the vestibular nuclei, but we should take note of at least three important groups of fibers.

The first terminates in the contralateral cerebellum and helps regulate balanced motion, the second terminates in the midbrain tegmentum where it helps regulate ocular movements in accordance to the changes in head motion, and the last forms the vestibulospinal tract which terminates in the anterior horns of the spinal cord with many different levels and helps regulate posture.

Okay moving on to the cochlear nuclei, and we should first of all note that there are two cochlear nuclei in each side of the brainstem, namely an anterior or ventral cochlear nucleus and a posterior or dorsal cochlear nucleus, and these nuclei are located posterolaterally on the brainstem and they receive and process information about the sense of hearing transferred by the cochlear part of the vestibulocochlear nerve.

So we’re currently looking at the anterior cochlear nucleus which, of course, is highlighted in green, and this nucleus receives information about sound from the vestibulocochlear nerve. This information is distributed tonotopically on the nucleus and I can hear you asking what that means. Well, that means that different regions of the nucleus correspond to different sound frequencies which sounds pretty cool to me. Lower frequencies captured at the apex of the cochlea stimulate the most ventral or anterior aspect of the nucleus while higher frequencies captured at base of the cochlea stimulates the most dorsal portion.

And now we’re highlighting the posterior cochlear nucleus. Unlike the anterior cochlear nucleus, this nucleus does not receive information only from the vestibulocochlear nerve but also from the auditory cortex, the superior olivary complex, and the inferior colliculus. So the site of architecture and the neurochemistry of the posterior cochlear nucleus is similar to that of the cerebellum, a concept which makes many speculate that this nucleus is involved with more complex auditory processing rather than merely transferring information. And fibers originating from this nucleus terminates at the inferior colliculus as well as at the superior olivary complex. And this nucleus is also believed to play a significant role in sound localization.

Alright, so let’s talk about the glossopharyngeal nerve.

The glossopharyngeal nerve is as we mentioned a mixed nerve and we’re going to first present the nerve structure and then its two motor nuclei and its two sensory nuclei – the motor nuclei being the inferior salivatory nucleus and the nucleus ambiguus and the sensory nuclei being the spinal trigeminal nucleus and the solitary nucleus.

Okay, so remember that the glossopharyngeal nerve is cranial nerve nine and if we look at the left of our image, we’ll see the motor fibers and the nuclei associated with the glossopharyngeal nerve whereas if we look to the right, we can see the sensory fibers and the nuclei. So we can assume that the glossopharyngeal nerve is both motor and sensory, and you can imagine that this is a nerve involved in many motor and sensory functions, and indeed the glossopharyngeal nerve not only contains both motor and sensory fibers, many of these fibers belong to different functional groups.

For example, it contains both somatic sensory and visceral sensory fibers as well as somatic motor and visceral motor parasympathetic fibers. But to keep things clear and simple, we’re going to be referring to these functions at the same time. We’ll be presenting the respective nuclei, so let’s start with the motor nuclei of the glossopharyngeal nerve.

So we can see that the motor fibers of the glossopharyngeal nerve converge and given off by two major nuclei. The first one which is highlighted in this image is the inferior salivatory nucleus and you may remember its counterpart just above it, the superior salivatory nucleus from our discussion on the facial nerve, and if we look closely at our image, we can see that the inferior salivatory nucleus resides in the pons just above our pontomedullary junction. Now, let’s move on to its function.

The inferior salivatory nucleus contains the bodies of the general visceral efferent fibers contained in the glossopharyngeal nerve, and these fibers supply the parotid gland via the otic ganglion and they regulate salivation.

So we’re now highlighting the other nucleus emitting fibers which are contained in the glossopharyngeal nerve, and this is the nucleus ambiguus and it contains the bodies of the special visceral efferent fibers of the glossopharyngeal nerve.

So the nucleus ambiguus contains somatomotor fibers which travel in the glossopharyngeal nerve and innervates the stylopharyngeus muscle. However, you may also notice that the nucleus ambiguus gives rise to motor fibers which do not form part of the glossopharyngeal nerve. In fact, it contains many neurons, the fibers of which course through the vagus nerve but we’ll talk about these and their function on our discussion of the vagus nerve.

As far as the anatomy of the nucleus ambiguus is concerned, we can definitely say that it occupies space in the higher medulla. When we look at it from a sagittal perspective, we will notice that the nucleus ambiguus is located slightly posteriorly to another important nuclear complex, the inferior olivary nuclear complex.

In this image, we can see that the sensory fibers of the glossopharyngeal nerve divide and some of them end up in the already familiar spinal nucleus of the trigeminal nerve which is now highlighted and this nucleus receives general somatic sensory fibers from the tonsils, the pharynx, the middle ear, and the posterior third of the tongue. It then processes information from these fibers and relays it to the thalamus via the ventral trigeminothalamic tract.

And the other nucleus we can see receiving sensory fibers from the glossopharyngeal nerve is the nucleus of the solitary tract. In our image, we can see both the solitary tract and the nucleus highlighted in green and you can see that the solitary tract and the nucleus contain fibers from various nerves. The input of the glossopharyngeal nerve to the system consists of special visceral sensory fibers transferring information about taste from the posterior third of the tongue as well as baroreceptor signals from the carotid bodies located at the carotid sinus.

Okay, so we’re now up to the vagus nerve and its nuclei and we can see that it has two motor nuclei and two sensory nuclei – the motor nuclei being the nucleus ambiguus and the dorsal nucleus of the vagus and the sensory nuclei being the solitary nucleus and the spinal trigeminal nucleus. So even though we’ve had some pretty complex nuclei and nerves, I would say that this is perhaps one of the most complex cranial nerve regarding its nuclei. But don’t be worried despite all of these, we can break down all this information about the anatomy and the functions of the vagal nuclei for you to better understand. And as you can see, the motor aspect of the vagus nerve is presented on the left side of our image, while the sensory aspect is presented on the right.

Okay so if we look closely at the nuclei at the left, you’ll be able to see two nuclei meeting vagal fibers, and these are the nucleus ambiguus which we already know and the dorsal nucleus of the vagus nerve. And you might imagine that one of them is a somatic motor nucleus while the other is more associated with the visceral autonomic motion. So if we have a look to the right, we’ll also notice that the sensory fibers from the vagal nerve end up in two distinct sensory nuclei, and these two we already know, one of them is the nucleus of the solitary tract, and the other one is the spinal trigeminal nucleus.

So exciting news, this is the last time we’ll be discussing many nuclei we’ve already seen associated with more than one cranial nerve, but before we move on, let’s discuss another interesting topic regarding the vagal nuclei.

So as you can see, the vagal nuclei create an elevated area on the floor of the fourth ventricle, and this area is known to be the vagal trigone. Let’s now have a closer look at each nucleus starting with the motor nuclei and moving on to the sensory.

So we’re currently looking at the nucleus ambiguus and we’ve already referred to the nucleus ambiguus in our discussion on the glossopharyngeal nerve. So you might remember that it contains the cell bodies of special visceral efferent neurons, and this is also true about its contribution to the fibers contained in the vagus nerve. Therefore the nucleus ambiguus gives rise to the special visceral efferent neurons, also known as branchial efferent motor fibers of the vagus nerve, however, it also contains a group of neurons which give rise to preganglionic parasympathetic fibers that innervate the heart, and in addition to vagal and glossopharyngeal fibers, we can see that the nucleus ambiguus gives rise to some fibers of another nerve, the accessory nerve.

Before we move on, let’s just take a look at the dorsal nucleus of the vagus and as you can see, this is a medullary nucleus. The truth is that we can only realize why this nucleus has the word dorsal in its name if we look at it from a sagittal perspective, so let’s do a little switch and have a look at it from that perspective.

So this nucleus is located at the floor of the fourth ventricle, just like the vestibular nuclei but a little bit more medially. It mostly serves parasympathetic vagal functions in the gastrointestinal tract, lungs, and other thoracic and abdominal vagal innervations, and remember that the cell bodies for the preganglionic parasympathetic vagal neurons that innervate the heart reside in the nucleus ambiguus.

Alright, let’s now move on to the sensory nuclei of the vagus nerve. So we already know that solitary nucleus as a sensory nucleus of the facial nerve and of the glossopharyngeal nerve and it’s also a sensory nucleus of the vagus nerve. Let’s first review what we already know about its input and functions before we move on to discuss its role in the processing of vagal sensory input.

So we’ve already talked about how the solitary tract and the nucleus receive and process taste information from the anterior two-thirds of the tongue, from the facial nerve and from the posterior third of the tongue, from the glossopharyngeal nerve and there’s also vagal special sensory input regarding the sense of taste from a small area on the epiglottis. We also know that the nucleus of the solitary tract also receives information from the carotid body via the glossopharyngeal nerve and to that we should add the facts that it receives input from the sinoatrial node via the vagus nerve.

Finally, chemically and mechanically-sensitive neurons of the general visceral afferent pathway with endings located in the heart, lungs, airways, gastrointestinal system, pharynx and liver via the glossopharyngeal and vagus nerve send information via the solitary nucleus. So we can safely say that the solitary nucleus and tract are associated with special and general visceral sensation.

Okay so let’s now review the spinal trigeminal nucleus while adding some important information regarding its association to the vagus nerve, and remember that this nucleus receives information about deep touch, pain and temperature from the ipsilateral face. In addition to the trigeminal nerve, we can see that it receives fibers from the facial, the glossopharyngeal, and the vagus nerves. And these nerves convey pain information from their respective areas to the spinal trigeminal nucleus. Thus the spinal trigeminal nucleus receives input from cranial nerves five, seven, nine, and ten. This nucleus then projects to the ventral posteromedial nucleus in the contralateral thalamus via the anterior trigeminothalamic tract where the sensory information is then processed.

Okay, now let’s discuss the accessory nerve and its nucleus.

So we finally reached cranial nerve eleven. So the nerve we highlighted we now know is the accessory nerve. And this is the nerve that supplies the sternocleidomastoid and the trapezius muscles. So we know that it contains some general somatic efferent fibers and you can see that these fibers originate in two nuclei, one we already know to be the nucleus ambiguus and the other nucleus which we know to be the spinal accessory nucleus. It lies within the cervical spinal cord in the posterolateral aspect of the anterior horn and is a motor nucleus. Both these nucleus and the nucleus ambiguus contain general somatic efferent fibers which control the sternocleidomastoid muscle and the trapezius muscle. And from a sagittal perspective, we can clearly see that most of the nucleus continues below the medulla into the cervical spinal cord.

Then finally, let’s move on to the hypoglossal nerve, which we can see here in this sagittal section and this is the twelfth cranial nerve. So we’re looking at it from the left hand side and it innervates all the extrinsic and intrinsic muscles of the tongue except for the palatoglossus which is innervated by the vagus nerve.

So the hypoglossal nerve is a nerve with solely general somatomotor function and the nerve arises from the hypoglossal nucleus in the brainstem as well as a number of small rootlets. The nucleus of the hypoglossal nerve is located about the same level as the nucleus ambiguus and the dorsal nucleus of the vagus nerve, and as you can see, it is close to the midline.

In the open medulla, it is visible as what is known as the hypoglossal trigone, a raised area medial to the vagal trigone protruding slightly into the fourth ventricle. And indeed if we switched to a sagittal view, we’ll discover that the hypoglossal nucleus is very near the floor of the fourth ventricle which makes sense for this nucleus to have a corresponding surface area, the one we named the hypoglossal trigone. And by further comparing the two images, we can see that this nucleus lies medially to the dorsal nucleus of the vagus.

Now you might be thinking that we’re over having discussed all the cranial nerve nuclei residing in the brainstem and indeed we are over discussing those nuclei, however, there is another important system present in the brainstem, and this is the extrapyramidal system, the parts of which are the red nucleus and the olivary complex.

This nucleus is located in the midbrain anterior to the oculomotor nuclei, and the red nucleus receives many inputs from the contralateral cerebellum and the ipsilateral motor cortex. The majority of its output is known as the rubro-olivary tract and connects the red nucleus to the ipsilateral inferior olivary nuclear complex. The rest of the output constitutes the rubrospinal tract which crosses the midline and ends at several levels of the contralateral side of the spinal cord.

The red nucleus is a subcortical center for extrapyramidal motor function. It controls aspects of gait not directly controlled by the cortex such as crawling in babies and arm movement during regular walking.

And the very, very last complex we’re going to be talking about is the inferior olivary complex, which is located in the medulla oblongata. And as we already discuss in our previous slide on the red nucleus, the inferior olivary complex receives the fibers of the rubrospinal tract and emits fibers which travel to the cerebellum through the inferior cerebellar peduncle and it is closely associated with the cerebellum, meaning that it is involved in control and coordination of movements, sensory processing and cognitive tasks likely by encoding the timing of sensory input independently of attention or awareness.

Okay, now we finally finished talking all of the nuclei and that of the cranial nerves. Now let’s move on to talk a little bit about some clinical notes.

So in this section, I want to talk about the pupillary light reflex in order to understand some clinical relevance of the brainstem nuclei.

So this reflex arc is triggered when the light hits the retina. The signal is then transferred from the retina through the optic nerve to the pretectal area, the region at the back of the midbrain and very close to the accessory oculomotor nucleus. And neurons in the pretectal area signal the accessory oculomotor nucleus which in turn activates and sends out signals through the oculomotor nerve to the iris sphincter to cause pupillary constriction which clinically is known as miosis. Thus exposure of retina to increased amount of light causes a constriction of the iris by the activation of the accessory oculomotor nucleus.

But what if there were a patient whose iris would not respond to increased amount of light? Well, we can imagine that a problem would be possible in any segment of the reflex arc from the retina to the optic tract to the midbrain to the oculomotor nerve, but for the sake of simplicity, we’re going to only be examining the possibility of a midbrain lesion although in a real situation, one would have to probably rule out every other localization possible with proper examination.

But for this tutorial, the logic is simple. If there were a midbrain lesion, we would expect to have findings from any structure present in the midbrain, and in this tutorial we talked about some midbrain nuclei such as the oculomotor nucleus, the red nucleus and the mesencephalic trigeminal nucleus, that we should also point out that there are other structures like ascending and descending tracts present in the midbrain.

Okay so we’re going to examine further this imaginary patient only to find out that his right eye looks down and out and the contralateral side of his body seems to move a little bit oddly. Specifically, his left arm doesn’t seem to synchronize with his left leg so what would an enterprising medical student think about this? So regarding the eye, this must mean that the predominant muscle tone driving the eye to this direction is that of the abducens muscle. So there’s got to be an oculomotor nerve palsy.

But what about the odd motions of the contralateral side of the body? Because earlier we said that the arm does not synchronize with the leg, so the question is, what nucleus is responsible for arm-leg coordination while walking? Well, if you’re thinking that this is the red nucleus, then you are indeed correct.

So summing up, we have a possible lesion affecting the red nucleus, the main oculomotor nucleus, and the accessory oculomotor nucleus. And what we described is actually known as Claude syndrome and we saw that it is characterized by oculomotor nerve palsy and contralateral ataxia.

Okay, so now we’ve reached the end of the tutorial, I just want to go through a very brief summary because it’s a pretty long tutorial and you’ve probably forgotten a lot of what we already discussed today. So let’s just talk through some of the things we talked about.

So in this tutorial, we saw the anatomy and various functions of the cranial nerve nuclei. Initially, we talked about the lateral geniculate nucleus and the superior colliculus as parts of the visual system. In the midbrain, we also talked about the anatomy and function of the oculomotor nerve, its nuclei as well as the anatomy and function of the trochlear nerve and its nuclei as well as the red nucleus.

We then moved to the pons where we extensively discussed not only the pontine trigeminal nuclei but all the nuclei of the trigeminal nerve, the mesencephalic nucleus in the midbrain, and the spinal trigeminal nucleus. In the pons, we also discussed the abducens nerve, its nucleus and the unique intraparenchymal course of its fibers looping around the facial nucleus. And in our discussion of the facial nerve, we also mentioned the superior salivatory nucleus which is a part of the autonomic nervous system.

We then moved on to the vestibulocochlear nerve and nuclei, and we discussed the vestibular nucleus which spans from the mid pons to the upper medulla and the cochlear nuclei residing in the same region. Next, we started discussing nerves of the nuclei of which are sometimes mutually shared and mostly reside in the medulla and these were the glossopharyngeal nerve and the vagus nerve and the shared nuclei were the spinal trigeminal nucleus, the solitary tract nucleus as well as the nucleus ambiguus.

And in the medulla, we also encountered the nuclei of the hypoglossal nerve and we discovered that only a part of the accessory nerve arises from this region because most of this nerve arises from a spinal nucleus from the cervical spinal cord. Then we discussed some nuclei which may not contribute to the formation of a single nerve but are parts of the system which regulate general somatic motion – the extrapyramidal system. And these nuclei were the red nucleus and the inferior olivary complex, which we saw are strongly interconnected.

And finally, we talked about the pupillary light reflex and how a lesion in the brainstem can give symptoms from many structures at the same level by discussing Claude syndrome.

That’s it for today. Thanks for watching and happy studying!