Video: Medulla oblongata: Hypoglossal nerve level

Hey everyone! This is Nicole from Kenhub, and welcome to our tutorial on the anatomy of the medulla oblongata at the level of the hypoglossal nerve.

So, we're going to begin this tutorial with some basic concepts such as how to orientate yourself when viewing brainstem sections and which anatomical landmarks you should try to identify in order to understand the level of the brainstem that you're looking at, and in our case, we'll discover which landmarks are unique to a section at the level of the hypoglossal nerve which, as you know, is the twelfth cranial nerve.

So, let's begin by forming a structure of what we'll be discussing today.

So initially we're going to be talking about the anatomical subdivisions of the brainstem to better orient ourselves. We’ll further locate the hypoglossal nucleus because this is a tutorial about the anatomy at the level of this nucleus and also the olivary nucleus because it is a very prominent and hard-to-miss structure even from a gross section perspective. At the end of our introduction, we'll be presenting the most important anatomical landmarks present at this level, which will later help us to explore the anatomy of this region.

While exploring the anatomical landmarks, we will break down the anatomy of this area into smaller regions or portions corresponding to significant landmarks. More specifically, we'll be talking about the anterior portion and this is where we'll find the pyramidal tract and adjacent nuclei such as the arcuate nucleus and its corresponding superficial accurate fibers. We'll also see that the inferior olivary complex is located in this portion and we'll also talk about the olivary nuclei, the olivary cerebellar tract, and the posterior accessory olivary nucleus.

We're then going to continue to the anterolateral portion and this is where we'll talk about the central tegmental tract and the lateral reticular nucleus, and after this region, we’ll talk about the midline, where we will discuss several important structures such as the medial lemniscus, the medial longitudinal fasciculus, and the nucleus ambiguus. Once we’re clear with the midline structures, we’ll move laterally again to the lateral portion, and once there, we'll talk about the spinal trigeminal nucleus and the spinal tract of the trigeminal nerve.

Finally, we'll present the nuclei present on the dorsal medulla and we'll start with the hypoglossal nucleus and nerve and then move laterally, highlighting the dorsal longitudinal fasciculus, the dorsal nucleus of the vagus nerve, the nucleus of the solitary tract, and the solitary tract itself, and the gracile and cuneate nuclei. And having discussed the gracile and cuneate nuclei, we’ll then talk about the internal arcuate fibers, which arise from these nuclei and the decussation of the medial lemniscus. We’ll then close our tutorial with the clinical vignette of Dejerine syndrome – a cluster of symptoms resulting from ischemia of parenchymal structures at the level of the hypoglossal nucleus.

Okay, so in this image, we're looking at a sagittal section of the brainstem through the midline in order to better orientate ourselves on the illustrations that will follow. In this illustration here, we can see the hypoglossal nucleus highlighted in green and remember that the basic anatomical subdivisions of the brainstem are the midbrain, the pons, and the medulla oblongata. The part of the brainstem that lies anteriorly contains the most number of cranial nerves, as you can see here, while the posterior part is the side where we find the floor of the fourth ventricle.

So we're looking now at a theoretical dissection at the level of the hypoglossal nucleus from a midsagittal perspective, and notice that we will still get to see a part of the fourth ventricle in our slide, so I hope this will help you to orientate yourselves when we start looking at horizontal slides. Additionally, we get to see our dissection line cross over many cranial nerve nuclei, but there is also another nucleus in the picture which will help us to define the front of our sections.

So, take a look at this big nucleus highlighted in green. This is no other than the olivary nucleus. Now that we've clarified some of the basic concepts about the level we'll be looking at for the rest of our tutorial, let's move on now to start discovering the horizontal section at the level of the hypoglossal nerve.

So, approaching a horizontal brainstem section can be pretty menacing at first glance and even more so when one tries to remember every single delicate structure at a specific level. So, for this reason, it's crucial to focus on large structures first because this will be easier to remember as reference points and landmarks later on.

For instance, anteriorly, we’ll use the pyramids or the pyramidal tract and the olivary nuclei as landmarks. The medial lemniscus is in the midline as you can see and the spinal trigeminal nucleus is situated laterally. As for all the nuclei present on the floor of the fourth ventricle, we won't refer to them individually just yet to avoid confusion, and for now, we’ll refer to the whole group of them as the central gray matter. The hypoglossal nerve nucleus is included in this central gray matter cluster and you can see the nerve highlighted in the following image.

So now that we've clarified the structures we’ll use as landmarks in the rest of our tutorial, let's have a look at the structures present near each one of those landmarks as well as their function.

So, we're now looking at the front of a horizontal medullary section showing the pyramidal tract highlighted in green, and as I mentioned earlier, we will use this tract as a landmark for the other structures to follow, and you can see here that the pyramidal tract is a paired structure. It consists of neural fibers which originated in the pyramidal cell layer of the cerebral cortex and terminates at various levels of the central nervous system. And here we can distinguish two large neural tracts which are considered portions of the pyramidal tract. One is the corticobulbar tract which contains fibers that terminate in the brainstem and the other is the corticospinal tract. The corticospinal tract terminates at various levels of the spinal cord and it displays a somatotopy with the most lateral fibers subserving many of the medial arm and neck movements.

Now that we’ve familiarized ourselves with where and what the pyramidal tract is, let's move on now to this flattened gray matter nucleus directly in front of the pyramidal tract. Over here, we see the arcuate nucleus. Take a look at it closely and notice that it is located directly anterior to the pyramidal tracts. This nucleus is believed to be a displaced pontine nucleus and is also proved to be involved in the control of breathing.

Two types of fibers arise from this nucleus. The first group is known as the stria medullaris of the medulla and note that there's also a stria medullaris in the thalamus, which is a completely different structure. The second group is known as the superficial arcuate fibers and both of these groups project mainly to the contralateral cerebellum through the inferior cerebellar peduncle. However, the first group, the stria medullaris, travels inside the medullary parenchyma to reach the contralateral peduncle. The second group – the superficial arcuate fibers – is clearly visible in our image and we'll highlight it shortly.

So now we're looking at the superficial arcuate fibers highlighted on the lateral sides of our medullary slice and remember that the majority of these fibers reach the contralateral cerebral peduncle so the fibers we're seeing highlighted on the left originated from the right nucleus and vice versa. Once these fibers emerge from an arcuate nucleus, they course superficially and medially towards the anterior median fissure of the medulla always anterior to the medullary pyramids. They then cross over at the anterior median fissure and continue coursing superficially at the contralateral side of the medulla where we can actually distinguish them as the bundle we have highlighted.

Finally, they head to the contralateral cerebellar peduncle. And don't worry if you're not seeing cerebellar peduncles at this medullary level. They're normally encountered at a higher medullary level. So, if we're seeing the superficial arcuate fibers here at this level, it must mean that they may also be travelling upwards as well as posteriorly to reach the inferior cerebellar peduncle at a higher level.

Before we begin our discussion on specific olivary nuclei, it's best if we first clarify some terms surrounding these nuclei. You may have already encountered the concepts of superior olivary nucleus and inferior olivary nucleus. The first concept, the superior olivary nucleus, is an area of gray matter in the pons involved in the processing of auditory signals. And as we can't see it in a medullary section, we won't refer to this nucleus any further.

The second concept, the inferior olivary nucleus, which is also known as the inferior olivary complex, is a collection of nuclei that can be found inside the oval-shaped body known as the olivary body in the medulla. The inferior olivary complex is closely associated with the cerebellum, so it is probably involved in sensorimotor processing and coordination as well as cognitive tasks. It has also been proposed that this nucleus encodes the timing of sensory input independently of attention or awareness.

The olivary body contains three nuclei – the main or primary olivary nucleus which is the same nucleus we've used as a landmark and two accessory nuclei, one medial between the primary nucleus and the pyramidal tract and one dorsal or posterior lying behind the dorsal aspect of the primary nucleus.

And we're now viewing the primary olivary nucleus of the inferior olivary complex. If you pay close attention to it, you'll notice it has a rather irregular shape that really stands out compared to the other gray matter nuclei in the medulla. It is dentated, and for those of you who've studied the cerebellum, this nucleus may actually remind you of the dentate nucleus of the cerebellum. For those of you who is seeing this nucleus for the first time, it may remind you of the outline of an oak leaf. Note that the hilum of this nucleus is directed medially.

Now shift your attention to all those fiber bundles that emerge from the medial margin of the nucleus. These fibers are collectively known as the olivocerebellar tract. If you watch these fibers closely, you'll see that they head towards the midline. This is where they decussate. From this point on, they join a group of fibers known as the internal arcuate fibers. They then head superiorly, posteriorly, and slightly upwards to enter the cerebellum via the inferior cerebellar peduncle at a higher medullary level.

Just posterior to the primary olivary nucleus, we also find another nucleus belonging to the inferior olivary complex. This is the posterior or dorsal olivary nucleus.

Now on the anterolateral slice, we see the central tegmental tract. This is a complicated tract which is found to contain multiple other tracts each serving different functions. The central tegmental tract is said to consist of reticular formation afferent and efferent fibers, ascending axons from the solitary nucleus which terminate in the ventral posteromedial nucleus of the thalamus, and descending fibers from the red nucleus of the midbrain to the olivary complex and to the spinal cord.

If we were looking at slices from higher levels of the brainstem, we would find the central tegmental tract very close to the midline and the medial lemniscus. However, as we're relatively low on the medulla, at this level, we find the central tegmental tract directly posterior and lateral to the inferior olivary complex. We can imagine some neural interaction going on at this level between the tract and the olivary complex particularly if we want to take into account the rubro-olivary components present in the central tegmental tract.

The lateral reticular nucleus can be found on the lateral side of the medulla oblongata at the level of the hypoglossal nerve and posterolateral to the inferior olivary complex. This nucleus receives inputs from both higher brain structures and from the spinal cord and relays those inputs to the ipsilateral cerebellum. It is believed to be involved in the regulation of motor function in cooperation with the cerebellum. However, the exact way this nucleus helps regulate motor function is uncertain.

Now we're going to be moving on to the midline of our brain slice and we'll be approaching anatomical structures starting from the midline and moving laterally. We'll begin with the medial lemniscus. The medial lemniscus is a white matter structure which at the level of the hypoglossal nerve is found in the midline behind the pyramidal tract and the decussation of the olivocerebellar tract. It consists of heavily myelinated axons which belong to the much larger pathway known as the posterior column – the medial lemniscus pathway.

The posterior column – medial lemniscus pathway transfers information regarding fine touch, vibration, two-point discrimination, and proprioception from the skin and joints to the ventral posterolateral nucleus of the thalamus. The medial lemniscus contains axons of second-order neurons of this pathway, which originate from either of two nuclei in the central gray matter region of the dorsal medulla, namely the gracile and cuneate nuclei. These nuclei project bundles of fibers which cross over the midline and form the contralateral medial lemniscus. We’ll refer to these nuclei later on in our tutorial. For now, however, let's highlight another interesting fact.

So, you should have figured out by now that we're looking at the level of the brainstem where the decussation of the lemnisci actually takes place. This is why the two medial lemnisci can be seen so near each other and so close to the midline. If you happen to explore higher levels of the brainstem, you'll realize that each medial lemniscus gradually moves away from the midline to the points where at the level of the midbrain it can be found at its lateral sides.

The medial longitudinal fasciculus is a small compact tract near the midline. It lies anterior or ventral to the central gray matter and is specifically anterior to what we'll find out to be the hypoglossal nucleus. At this level of the medulla, the medial longitudinal fasciculus is displaced dorsally by the decussation of the medial lemniscus and the pyramidal tract. However, even at higher brainstem levels, the medial longitudinal fasciculus is found very close to the midline and to the central gray matter. This tract specifically contains fibers that connect oculomotor nuclei to help integrate gaze patterns from higher centers to ocular motions. It also contains fibers from vestibular nuclei to help integrate ocular motions within the motion of the head.

Finally, it contains a descending white matter tract known as the vestibulospinal tract. The medial longitudinal fasciculus carries the vestibulospinal tract to the cervical spinal cord where it innervates the muscles of the neck and upper limbs.

Moving on laterally, we've now highlighted a small nucleus isolated within white matter tracts. This nucleus is known as the nucleus ambiguus. To locate this nucleus in the future, remember to look at the white matter posterior to the inferior olivary complex. The nucleus ambiguus is known to contain large neurons which innervate muscles of the soft palate, pharynx, and larynx, which are strongly associated with speech and swallowing. Some of these neurons are part of the glossopharyngeal nerve while others are part of the vagus nerve. There's also a group of parasympathetic neurons in the nucleus ambiguus, which are part of the vagus nerve and which innervates the heart.

Finally, moving on to the most lateral part of this portion, we're now seeing the spinal trigeminal nucleus highlighted. For those who are not very familiar with the trigeminal nerve, you should note that the nucleus of this nerve is very widely distributed across all the brainstem and the highest levels of the cervical spinal cord. The region of the trigeminal nucleus present in the medulla oblongata is known as the spinal trigeminal nucleus.

This part of the trigeminal nucleus receives information about crude touch, pain, and temperature from the ipsilateral face. In addition to the trigeminal nerve, the facial, the glossopharyngeal, and vagus nerves, it also conveys pain information from the areas to the spinal trigeminal nucleus. All the information conveyed to the trigeminal nucleus is carried by fibers in the spinal trigeminal tract. This tract lies anterolateral to the spinal trigeminal nucleus.

We’ll now be moving on to discuss the central gray matter of our brain slice. As you can see, there are many different nuclei as well as white matter tracts in this section. It's best if we approach this by looking at the structures near the midline and then moving on dorsally and laterally.

The first structure we'll be looking at is the nucleus of the hypoglossal nerve. As you can see, this nucleus is located very close to the midline which is also true for many other motor nuclei. The hypoglossal nucleus also lies dorsally to the medial longitudinal fasciculus. If you look closely, you'll notice the fibers of the hypoglossal nerve emanating from the nucleus.

Now speaking of the hypoglossal nerve, the hypoglossal nerve is the twelfth cranial nerve. As you can see, its neural fibers travel laterally to structures in the midline effectively separating the medial longitudinal fasciculus and medial lemniscus from the lateral sides of the medulla where the nucleus ambiguus and olivary complex reside. Notice how as they move closer to the surface of the ventral medulla, the fibers turn laterally to exit from the sulcus between the pyramidal tract and the olivary complex. And this sulcus is known as the anterolateral sulcus.

Next, we'll be moving on to the dorsal longitudinal fasciculus. So, what we're looking at is a white matter tract which as you can see is located in the dorsal brainstem tegmentum and very near to the floor of the fourth ventricle. You'll also notice that this fasciculus is located behind the hypoglossal nucleus and in front of the dorsal nucleus of the vagus nerve which we'll see next. This tract contains myelinated axons which enable the communication between the hypothalamus and autonomic nervous system nuclei through the central nervous system.

In the medulla, this tract is believed to innervate centers associated with the heart rate, blood pressure, and control of breathing as well as with the parasympathetic nuclei of the vagus nerve. In fact, we'll see that this tract lies very near to some vagal nuclei. This tract reaches the lower ends of the spinal cord innervating the thoracolumbar sympathetic centers and lumbosacral parasympathetic centers.

Just behind and a little lateral to the dorsal longitudinal fasciculus and just as close to the floor of the fourth ventricle, we will find the dorsal nucleus of the vagus nerve. As expected, this nucleus receives input from the hypothalamus via the dorsal longitudinal fasciculus, but also from the nucleus of the solitary tract. It mostly serves parasympathetic vagal functions in the gastrointestinal tract, lungs, and other thoracic and abdominal vagal innervations.

The nucleus of the solitary tract lies lateral to the dorsal vagal nucleus. These two nuclei are almost coextensive – meaning that if you find one of them in a horizontal section, you're almost certain to find the other one close by. This nucleus is also intimately related to a tract with the same name and it processes visceral sensory information from the pharynx, the esophagus, and the abdominal gastrointestinal tract. There's also evidence to suggest that these nuclei also processes information from the respiratory and the cardiovascular systems. It is believed to project to the thalamus and then to the cerebral cortex. Its output is carried to the ventral posteromedial nucleus of the thalamus. That's the nucleus associated with taste via the medial lemniscus. Axons from this thalamic nucleus then projects to the sensory motor cortex and the insular cortex.

The solitary tract lies ventral to its nucleus and lateral to the dorsal nucleus of the vagus and the dorsal longitudinal fasciculus. This tract, which can be observed through the length of the medulla oblongata, is composed of general visceral afferents from the vagus and glossopharyngeal nerves and gustatory fibers from the facial, glossopharyngeal, and vagal nerves. The solitary tract conveys all of these fibers to the homonymous nucleus.

We're now looking a little further laterally to the gracile nucleus, and this is one of the nuclei in the dorsal column – medial lemniscus pathway, the other being the cuneate nucleus. The gracile nucleus receives fibers from the homonymous tract, which as you may have imagined, lies in the posterior column of the spinal cord. As you may already know, the dorsal column conveys the sensations of fine touch and proprioception.

The gracile tract conveys these same sensations, but specifically from the lower body. The gracile nucleus processes this information and transfers it further up through the medial lemnisci to the ventral posterolateral nuclei of the thalamus, so it can be said to contain second-order sensory neurons.

This large nucleus next to the gracile nucleus is known as the cuneate nucleus. It serves a very similar function to the gracile nucleus, but for the upper body above the T6 level of the spinal cord to be precise except for the areas of the face and the ear. These last two areas are processed by the trigeminal nuclei.

So, the cuneate nucleus processes fine touch and proprioception perceptions from the upper body and limbs and conveys it through the medial lemniscus through the axons of second-order neurons to the ventral posterolateral nucleus of the thalamus.

So, I hope you remember us discussing the axons of second-order neurons of the gracile nucleus and cuneate nucleus traveling through the medial lemniscus to the thalamus. Well, these axons have to somehow get to the medial lemniscus. So, if you have imagined that this group of fibers connecting the cuneate nucleus to the midline and the medial lemniscus is the route these axons use, you are absolutely correct. What may be missed by simply looking at the picture is that this white matter pathway, which we will from now on refer to as the internal arcuate fibers, in fact, also contains the axons of the gracile nucleus.

So, to sum up, the internal arcuate fibers are the axons of second-order neurons from both the cuneate and the gracile nuclei which travel towards the midline of the medulla and cross over to form what is known as the decussation of the lemnisci.

So, we'll close this tutorial with an eponymous stroke syndrome which can occur at the level of the hypoglossal nerve. So, those of you who have studied the blood supply of the brainstem may remember that the medulla is supplied by branches of the vertebral arteries such as the anterior spinal artery which you can see highlighted on our second image as well as branches of the posterior inferior cerebellar arteries known as the posterior spinal arteries.

Now since the role of this tutorial is not to teach the blood supply of the medulla, we'll move on to discuss the stroke syndrome I promised.

Dejerine syndrome, it's also known as medial medullary syndrome, where the structure of the medullary midline dysfunctions or even dies out because of reduction or loss of blood supply. Blood supply is reduced in this case due to an obstruction of the anterior spinal artery. Don't, however, confuse it with the Dejerine-Roussy syndrome which is a stroke syndrome of the thalamus.

So now that we've clarified our terms and some basics of the vascular anatomy of the area, let's try to find the clinical presentation of the middle medullary syndrome based on the anatomy of our medullary slice.

If we look at the midline of the slice, we’ll see the pyramidal tracts, the olivocerebellar tracts, the medial lemniscus, and the medial longitudinal fasciculus as well as the nucleus and the fibers of the hypoglossal nerve. Essential infarct in the anterior spinal artery would give off bilateral motor and sensory symptoms. But such an infarct would be rare. A more common pathology would be the occlusion of a branch of the anterior spinal artery which is distributed to either the right or the left of the midline and we can see one of these lesions on the right side of the medulla.

Based on the anatomy of this region, we'd probably expect to find the following symptoms. The damage in the hypoglossal nerve would cause a deviation of the tongue to the side of the infarct on attempted protrusion due to the ipsilateral muscle weakness. The damage to the pyramidal tracts would then cause limb weakness on the contralateral side of the infarct since the pyramidal tracts have not completed their decussation at the level of the hypoglossal nerve. The damage to the medial lemniscus would cause a loss of discriminative touch, conscious proprioception, and vibration sense mostly on the contralateral side of the infarct.

Remember that we're at the level of the decussation of the lemnisci, so while the damage to the lemniscus would be present as a contralateral sensory defect, the damage to surrounding uncrossed internal arcuate fibers would be seen as an ipsilateral sensory defect. Finally, the damage to the medial longitudinal fasciculus would hardly present with a gaze defect at this medullary level as the oculomotor nuclei reside much higher in the brainstem. So, there probably be little or no ocular symptoms.

Okay, thanks for sticking with me throughout this tutorial. We've now finally come to the end, but, of course, before I leave you, let's summarize what we saw today.

So, we've decided to take a look at some structures in different regions of our horizontal slice in order to remember them better, and more specifically, we've decided to take a look at the anterior portion of our horizontal slice. In that region, we talked about the pyramidal tract which contains the descending axons of higher motor neurons and we presented the olivary nuclei which helps control movements independently of attention as well as associated tracts.

Before moving onto the midline, we briefly discussed some structures on the anterolateral portion of the medulla such as the central tegmental tract and the lateral reticular nucleus. Next, we discussed important structures in the midline highlighting the medial lemniscus carrying the axons of second-order neurons to the thalamus and the medial longitudinal fasciculus which at this level carries fibers to innervate muscles at the neck and the upper limbs.

We then discussed structures on the more lateral sides of our slice like the spinal trigeminal nucleus and the associated tract which receive and process information from the cranial nerves five, seven, nine, and ten, and in this section, we also talked about the nucleus ambiguus – a motor nucleus associated with both cranial nerves nine and ten.

We then moved on to discussing nuclei in the central gray matter and referred, of course, to the hypoglossal nucleus as being the most medial. We also talked about the dorsal nucleus of the vagus nerve, the solitary tract and its nucleus, as well as the gracile and cuneate nucleus, and these last two are parts of the posterior column – medial lemniscus sensory pathway.

Finally, we highlighted that at the level we're looking at, we can see the decussation of the medial lemnisci, and at this point, we looked at the internal arcuate fibers – the fibers which emerge from the gracile and cuneate nuclei – and head towards the midline to form the lemnisci.

Having discussed the anatomy of the medulla at the level of the hypoglossal nerve, we presented a rare clinical entity known as Dejerine syndrome to help solidify our knowledge on the anatomy and function of the distinct structures of the region.

And that brings us to the end of our tutorial. Thanks for watching and happy studying.