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Hypoglossal nerve

Course, branches and nuclei of the hypoglossal nerve.

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Transcript

Hey there! This is Nicole from Kenhub, and welcome to this video tutorial on the anatomy of the hypoglossal nerve. So, the hypoglossal nerve is one of the twelve cranial nerves and has one main primary function which is to do exactly what you see here. Yes, this is the nerve which controls the majority of the muscles in your tongue.

So let me first give you an overview of what we’ll be discussing today in this video tutorial and we’ll be beginning by first describing the pathway and anatomy of the hypoglossal nerve taking into account its role in the innervation of the infrahyoid muscles as well as the innervation of the tongue including both intrinsic and extrinsic muscles. And finally, we’ll conclude with the clinical implications of damage to the hypoglossal nerve.

The hypoglossal nerve is the twelfth cranial nerve and carries only motor information, meaning that it does not convey sensory information from the tongue back to the brain. And if we look at the name hypoglossal, it can be divided into hypo- which means below and –glossal which means tongue, so the nerve’s role is to innervate the tongue from below.

As I mentioned at the beginning of this tutorial, the main function of the hypoglossal nerve is innervation of muscles which work to change the position and the shape of the tongue. It’s therefore involved in the process of mastication and swallowing and the formation of speech or phonation. It’s also involved in the innervation of infrahyoid muscles, also known as strap muscles, and these are also involved in speech and swallowing.

But let’s start with a brief overview of the pathway of the hypoglossal nerve.

So in this image, we can see a lateral view of a portion of the head and neck region, and to give you a quick overview, both the left and right nerves originate here in the brainstem. They then pass through the hypoglossal canal then move inferiorly, anteriorly, and finally superiorly to innervate the muscles of the tongue. So let’s break this pathway up and have a look at it step-by-step for a little bit more detail.

The nucleus of the hypoglossal nerve, which we can see here in green, is located in the medulla oblongata of the brainstem at the level of the hypoglossal trigone, which is a slight elevation in the floor of the inferior recess of the fourth ventricle. It then exits the skull through the hypoglossal canal which is indicated in green right about here when observed from a superior perspective. The hypoglossal canal is a bilateral structure, meaning that there’s one on each side, lateral to the foramen magnum, and it gives exclusive passage to the hypoglossal nerve alone, meaning no other blood vessels or nerves pass through the hypoglossal canal.

Once outside of the skull, the hypoglossal nerve descends vertically between the internal carotid artery and to the internal jugular vein before turning laterally close to the greater cornu of the hyoid bone. Along its course, the hypoglossal nerve then receives a branch from the cervical plexus and this branch contains fibers from the anterior rami of C1 and C2 spinal nerves, which we can see in this image here.

Importantly, these fibers do not combine with the hypoglossal nerve but just travel within the sheath of the hypoglossal nerve. Think of them as hitching a ride with the hypoglossal nerve to reach their targets which are the infrahyoid muscles. So, let’s now have a look at how the combination of these nerve fibers within the hypoglossal nerve are involved in the innervation of the infrahyoid muscles.

So the function of the hypoglossal nerve proper is to innervate the muscles of the tongue which we’ll explore in more detail shortly. Before that though, I want to take a look at these inferior branches just here, and this contained the fibers of the C1 and C2 spinal nerves which we looked at a moment ago and they’re continuous with fibers from the C2 to C4 spinal nerves forming a circuit known as the ansa cervicalis, which is to also be referred to as the ansa hypoglossi.

As the hypoglossal nerve travels down and passes between the internal carotid artery and the internal jugular vein, many of the nerves originating from the C1 spinal nerve but contained within the sheath of the hypoglossal nerve branch away as seen here, and this is known as the superior root or the radix of the ansa cervicalis.

As it descends, it gives off muscular branches which innervate the superior belly of the omohyoid muscle and the superior root of the ansa cervicalis also innervates the sternothyroid and the sternohyoid muscles, and this nerve is also sometimes referred to as the descending branch of the hypoglossal nerve. As the nerve loops down further inferiorly and posteriorly, it is continuous with the inferior radix of the ansa cervicalis which we can see in this image, and this root receives branches from the C2, C3, and sometimes C4 spinal nerves. It also innervates the inferior belly of the omohyoid muscle.

Getting back to the hypoglossal nerve after it’s branched to the ansa cervicalis, it continues anteriorly soon giving off another branch known as the nerve to the thyrohyoid muscle which, true to its name, provides motor nerve supply to the thyrohyoid muscle like we see in this image here. And the role of this muscle is to depress the hyoid bone and to elevate the larynx.

Now I’m just going to stop right here for a moment and take a breath, because this is going to get a little bit complicated. So, I want to mention here that the innervation of the thyrohyoid muscle is a somewhat controversial subject in the literature. So, you see, although the nerve to the thyrohyoid muscle is often seen as a direct branch of the hypoglossal nerve, it contains nerve fibers derived from the C1 spinal nerve, hence, you may see the innervation of this muscle as coming from either the hypoglossal nerve or the C1 spinal nerve.

To complicate matters even further, some textbooks may also describe the innervation of the thyrohyoid muscle as originating as the thyrohyoid branch of the ansa cervicalis instead of a direct branch coming off the hypoglossal nerve, and this is generally deemed to be a variation of the typical anatomy, but hopefully, this explanation might save you some confusion when studying for your final exams.

You’ll be glad to know that we’re going to be moving on from the thyrohyoid muscle now, and looking at the hypoglossal nerve at the sight of its main function, which is innervating the muscles of the tongue via its lingual branches. And there are intrinsic and extrinsic muscles of the tongue. Let’s start with the four intrinsic muscles.

So, now we’re looking at a coronal section of the tongue with the hypoglossal nerve highlighted in green once again, and as we just mentioned, the hypoglossal nerve innervates the intrinsic muscles of the tongue which change the shape of the tongue itself. And these muscles include the superior longitudinal muscle which is responsible for elevating the tip of the tongue, the inferior longitudinal muscle which shortens the inferior portion of the tongue, the transverse muscle which helps to narrow and lengthen the tongue, and the vertical muscle which helps to flatten the tongue.

So now that we’ve looked at the intrinsic muscles of the tongue, let’s take a quick look at the extrinsic muscles.

The extrinsic muscles of the tongue are muscles that are attached to bone and alter the position of the tongue in the mouth, and they include the styloglossus muscle which helps to retract and elevate the tongue, the hyoglossus muscle which helps to depress and retract the tongue, and the genioglossus muscle which is the main muscle responsible for sticking your tongue out when you don’t like something, and finally the palatoglossus muscle which works to elevate the dorsum or the back of the tongue.

It should be noted that the lingual branches of the hypoglossal nerve innervate all of these muscles except for the final one, the palatoglossus, which is innervated by the vagus nerve, also known as cranial nerve ten.

So, we’re coming to the end of our journey with the hypoglossal nerve, but there’s one last branch that I want to draw to your attention, and it’s this one here which is known as the nerve to the geniohyoid muscle which, as you guessed, innervates the geniohyoid muscle.

And this is another one of these slightly complicated nerves in the sense that although it physically is a branch of the hypoglossal nerve, it contains fibers originating from the C1 spinal nerve and just like we saw with the thyrohyoid nerve, and this is why you’ll often see the innervation of the geniohyoid muscle described as the C1 spinal nerve even though it physically appears to be a branch of the hypoglossal nerve.

Alright, with that, we’ve discussed the anatomy of the hypoglossal nerve, so let’s move on and have a look at some clinical implications of the hypoglossal nerve.

So as we talked about earlier, the main function of the hypoglossal nerve is to innervate muscles that move the tongue around the mouth as well as change the actual shape of the tongue, and this is important for tasks such as talking and swallowing. Therefore, lesions throughout the hypoglossal nerve will affect these functions. And lesions can be caused by many things like stroke, tumors, cuts, infections, and neurodegeneration, and this can result in a palsy of the tongue.

Patients with palsy of the hypoglossal nerve will present with a deviation of the tongue to one side depending on whether the left or right nerve has been affected and at what level. The muscles of the tongue may have started to waste away on the affected side and some muscle twitching may also be present.

So, to examine the function of the hypoglossal nerve, we normally ask the patient to stick their tongue out and if there’s damage to the nerve, the tongue will stick to one side. So, if there’s damage to an upper motor neuron, the tongue will curve away from the side of the lesion, and conversely if there’s lower motor neuron damage, the tongue will curve towards the same side of the lesion and, generally, muscle wasting and twitching is only observed in upper hypoglossal nerve lesions.

Muscle deviation can be present during hypoglossal nerve damage and, in this case, the damage can be assessed by asking the patient to poke the inside of their cheek and feeling how much pressure they can apply. Upon examination, you must also gauge whether the patient’s speech is slurred with the tongue sometimes being described as heavy or clumsy.

Okay, so let’s summarize what we’ve learned today.

So, the hypoglossal nerve – the twelfth cranial nerve – originates in the hypoglossal nucleus of the brainstem. It passes through the hypoglossal canal and receives branches from the C1 and C2 spinal nerve which are given off to the ansa cervicalis which is part of the cervical plexus.

The superior and inferior radices of the ansa cervicalis innervate muscles throughout the upper neck and the hypoglossal nerve continues to supply the intrinsic muscles of the tongue including the superior longitudinal muscle, the inferior longitudinal muscle, the transverse muscle, and the vertical muscles. It also supplies three of the four extrinsic muscles of the tongue which were the styloglossus, the hyoglossus, and the genioglossus muscles. And these muscle groups, of course, reposition and shape the tongue. And, finally, we saw the involvement of the hypoglossal nerve in the innervation of the geniohyoid muscle.

We also discussed some lesions of the hypoglossal nerve which can cause tongue muscle deviation, wasting, and twitching and to determine the presence of these lesions, you can ask the patient to stick their tongue out.

Alright, so that brings us to the end of our tutorial. Thanks for watching.

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