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Facial nerve

Nuclei, course and branches of the facial nerve.

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Transcript

Hey everyone! It's Nicole from Kenhub, and today, we're going to be talking about the facial nerve.

So, the facial nerve is a highly complex and sometimes difficult nerve to grasp due to it having multiple functions and a fairly intricate path. And, of course, in this tutorial, we're going to take a closer look at its anatomical course but first off, we're going to start by looking at this image of a schematic of the facial nerve. And, of course, we're looking at a human head from a lateral left view with some of its internal organs visible and the facial nerve here highlighted in green. And this, of course, is going to be our base image throughout the tutorial and when we need to, we'll bring in another image to illustrate our point. But let's get cracking on the tutorial and first, I'm just going to give you a brief overview of what we're going to be talking about.

So, mainly, we're going to be talking about the anatomy of the facial nerve and, of course, we're going to break that down medially to peripherally into an intracranial portion and intratemporal and an extracranial portion – and just take a note that the extracranial portion can also be referred to as the branchial motor portion. We're also going to be talking about the brainstem nuclei associated with the facial nerve and, finally, we'll end with talking as we always do about some clinical correlations.

But before we get cracking onto that, I just want to talk a little bit more about the facial nerve and some of its functions. So just sticking with our image of the facial nerve on the right here, you can see that the facial nerve belongs to a set of neural structures called the cranial nerves. And these peripheral nerves are named due to their course through the cranium which is the part of the skull that houses the brain and the brainstem.

The facial nerve is also often referred to as cranial nerve seven as it's the seventh cranial nerve out of twelve and it's often abbreviated as CN VII as you can see on the left. It's also a bilateral nerve, that is, there's one on each side of the body, and the facial nerve serves many different functions associated with the head and neck including general sensation to a small area of the ear, the special sensation of taste to the anterior two-thirds of the tongue as well as some motor components from the parasympathetic innervation which innervates the salivary glands, the mucus-secreting glands of the nasal and oral cavities and the lacrimal glands. And, finally, the nerve provides branchial motor function to the muscles of facial expression and to the other small muscles associated with the ear.

So now that we know all of that, let's move on to talk about the anatomy of the facial nerve. And the first section, of course, that we are going to talk about is the intracranial portion which is also the most medial part or the part that arises directly from the brainstem nuclei.

So in this image which we are zooming in to a little bit, you can, of course, see the intracranial part highlighted in green and you can also see the brainstem highlighted in blue which is where the facial nerve arises specifically the pontomedullary junction which I've now highlighted for you in blue. And, in this image, we can also see the internal acoustic meatus encircled in blue which is where the facial nerve enters the temporal bone.

Let's just briefly take a minute to flip the brain and have a look at its inferior aspect so we can give you a bit of a deeper tour around these parts, and in this image, you can see the inferior view of the brain and the brainstem showing the twelve paired cranial nerves projecting from the central nervous system or otherwise known as the brain. And the facial nerve which I'm pointing out with my blue arrow, of course, emerges from each side of the brainstem between the abducens nerve which is the sixth cranial nerve and the vestibulocochlear nerve which is the eighth cranial nerve at the pontomedullary junction which I'm now outlining in blue. And from there, it will make its way anterolaterally towards the internal acoustic meatus which again we mentioned before.

And if you look at the zoomed image of our transverse image of the cranium from a superior view so looking down into the internal surface of the skull from the top as if we've lopped off the top of the person's head, you can see the internal acoustic meatus highlighted in green. And as we mentioned, it passes through here along with the vestibulocochlear nerve. And just to give you some text along with that, you can see on our left here that the intracranial part of the facial nerve arises from the pontomedullary junction and it travels anteromedially to the internal acoustic meatus.

The other thing that's important to note when we're talking about the intracranial part of the facial nerve is that it actually has two roots and you can see the first one of these on the right here highlighted in green – the motor root – and the second root that it has is the sensory root, which is also known as the intermediate nerve, and they travel together through the internal acoustic meatus as you can see on the right in the breakout.

Moving on now, of course, as once the roots of the facial nerve pass through the internal acoustic meatus, they enter the petrous portion of the temporal bone, and this marks the start of what is known as the intratemporal part of the facial nerve and that's marked here in green on our image. And in this part, the facial nerve has a close relationship with the middle ear and gives off various structures and branches to support a number of different functions. And these structures and branches are the geniculate ganglion, the greater petrosal nerve, the pterygopalatine ganglion, the chorda tympani, the submandibular ganglion and the nerve to the stapedius. So let's begin with the geniculate ganglion.

So, in this image, we can see the geniculate ganglion highlighted in green ghosted through a lateral view of the head showing its rough location in the temporal bone. And as we mentioned on the previous slide, the intratemporal part of the facial nerve arises within the petrous part of the temporal bone which we can see in the zoom and we can see it outlined in blue. And, of course, this image is not exactly aligned as we're working in three dimensions but the geniculate ganglion would normally be aligned with this bone. And as the facial nerve courses through the temporal bone, it takes a turn to enter the facial canal which is a canal within the cranium and at the tip of this turn is the location of a structure called the geniculate ganglion which houses the cell bodies for the sensory component of the facial nerve. And if we bring back our image to the internal acoustic meatus, you can see the facial canal dotted in blue and the geniculate ganglion hanging out right about here. The geniculate ganglion then gives off an important neural branch called the greater petrosal nerve and this nerve transmits parasympathetic and taste axons.

Let's take this image of the greater petrosal nerve and zoom in a little bit to examine its course. So, once the nerve arises in the geniculate ganglion which I've highlighted for you in blue, the greater petrosal nerve passes out of the temporal bone which is this bone just there and then runs along the floor of the middle cranial fossa until it reaches the foramen lacerum which we can see just here. And if we bring in our image of a superior view of a transverse section of the skull, we can see the foramen lacerum highlighted in green.

Once it's through the foramen, the greater petrosal nerve is then joined by the deep petrosal nerve which you can see in this image, and the deep petrosal nerve is a branch of the internal carotid plexus that contains postsynaptic sympathetic axons. As these two nerves meet, they form the nerve to the pterygoid canal or the Vidian nerve, which you can see is this little green highlight here in this magnification, and passes through the pterygoid canal to reach the pterygopalatine ganglion which I've highlighted for you in blue.

Let's now take a closer look at the pterygopalatine ganglion which in this image is highlighted in green. And the pterygopalatine ganglion is found hanging from the maxillary division of the trigeminal nerve. So, here in this image, we've highlighted the trigeminal nerve and if we pass through the trigeminal ganglion in blue, we can see the maxillary nerve highlighted in blue. The pterygopalatine ganglion is located within the pterygopalatine fossa which is an osteological feature that can be seen on this lateral view of the skull. The pterygopalatine ganglion houses the cell bodies of the postganglionic parasympathetic nerve fibers and is the site of synapses between the pre- and postganglionic neurons. The pterygopalatine ganglion then distributes via its branches to supply parasympathetics to the palatine mucosa, the nasal cavity and the taste fibers which travel within the Vidian nerve to various areas of the palate.

The pterygopalatine ganglion also sends postganglionic parasympathetic fibers to the maxillary division of the trigeminal nerve which we can see here in green on our right which then passes them onto its zygomatic branch and, via the communicating branch with the lacrimal branch of the ophthalmic nerve, supplies the lacrimal gland. I'm just going over this again so don't worry if you didn’t catch it the first time. I'm going to draw a few things for you to make things a little bit easier to see.

So, the first thing that I'm going to draw is the zygomatic branch of the maxillary nerve and over here, we have the ophthalmic nerve drawn in a thick blue line and coming off that ophthalmic nerve is the lacrimal branch which is just there. And so the next thing I want to draw for you is the communicating branch which runs between the zygomatic branch and the lacrimal branch and you can see it doing exactly that just here. And the last image is of the lacrimal gland.

So, if we were going to trace a pathway of the parasympathetic supply of the lacrimal gland from the pterygopalatine ganglion, we would start off at the pterygopalatine ganglion which I've labeled just here and then we would follow it up through its sensory root and I've labeled this as the sensory root of the maxillary nerve. The supply would then move up through the zygomatic branch which I've labeled for you just here and then travel up through the communicating branch between the zygomatic and the lacrimal branches then move along the lacrimal branch like this and it would finally end up in the lacrimal gland which I've labeled for you as well. One small note on the lacrimal gland, as you can see, this gland is located in the superolateral aspect of the orbit where it secretes a watery fluid that prevents the eye from drying out.

Now, let's go back to the geniculate ganglion for a second, and from the geniculate ganglion, we're going to follow the facial nerve after its turn in the facial canal. So, as you can see in this close-up during the course through the facial canal, the facial nerve runs along the medial wall of the middle ear which is marked by green in our image. And just to orientate you, we're now looking at a coronal section of the right temporal bone in ear and while on the middle ear, we can see that the facial nerve gives off two branches and these are the chorda tympani which is now highlighted in blue and the nerve to the stapedius which I've now dotted out in blue. And just to reiterate that text, you can see that's as we mentioned, the facial nerve gives off two branches in this region – the chorda tympani and the nerve to the stapedius.

And over the next couple of slides, we're going to talk about these two branches, so let's just start with the chorda tympani. And the chorda tympani, of course, arises from the facial nerve and then passes through the middle ear between the two ossicles – that is, between the incus and the malleus – before passing anteriorly out of the temporal bone through the petrotympanic fissure – and I've just dotted that for you in blue. The chorda tympani holds the axons for taste to the anterior two-thirds of the tongue and the preganglionic parasympathetic axons which supply the submandibular and sublingual glands. And we'll talk a little bit more about these later on in the tutorial.

But, first, I just want to take a couple of minutes to explain a little bit more about how the chorda tympani innervates its organs. So, if you have a look at the image on the right here, you can see the chorda tympani ghosted in green passing anteroinferiorly though, normally, this nerve would be running deep to the infratemporal fossa so we've just drawn it in for illustration's sake. And as we follow it, we can see it joining the lingual branch of the mandibular division of the trigeminal nerve which I've drawn for you in blue. And from this point, the chorda tympani axons do what we would call a piggyback onto the lingual nerve as it courses towards the anterior aspect of the tongue and this explains how the facial nerve supplies taste to the anterior two-thirds of the tongue.

But there's one more question I want to ask you. How does the chorda tympani supply parasympathetics to the glands? We're going to answer that question in this next slide. And as we follow the lingual nerve, we can see in this image that the submandibular ganglion is a little ganglion that hangs off it – and I'll just draw in the lingual nerve again in the breakout so you can have a clear image of what that looks like. And this ganglion houses the synapses between the pre- and postganglionic parasympathetic axons which then move on to stimulate the submandibular and sublingual glands to produce saliva in the floor of the mouth.

And, of course, I want to talk briefly about the nerve to the stapedius. And if you remember – and this small nerve branches from the facial nerve within the facial canal to supply the stapedius muscle which is highlighted here in green and whose function is to reduce the oscillation of the stapes to dampen the sound, and we'll just draw the nerve in the dotted blue line as it comes off from the facial nerve.

Just before finishing up about the facial nerve, let's have a chat about where the facial nerve ends up. So after branching off to the chorda tympani and the nerve to the stapedius, the facial nerve continues out of the temporal bone via the stylomastoid foramen which we can see here in green in this inferior view of the skull. And this leads us to our next segment where we will discuss the main motor component and a small sensory area of the facial nerve. And, of course, I'm referring to the extracranial portion of the facial nerve otherwise known as the branchial motor section.

So as we mentioned a bit earlier on in the tutorial, the major motor components of the facial nerve which as we mentioned are the segments that arise and exit the skull from the stylomastoid foramen are referred to as the branchial motor component. And this is because during the embryonic development, the facial nerve innervates all the structures derived from the second branchial arch. The term "branchial" originates from the gills of fish who also express this developmental arch pattern of the head and neck. However, in humans, which are whom we're interested in in this tutorial, the facial nerve supplies the muscles of facial expression, the auricular muscles which I've drawn for you in blue on this image, the stylohyoid muscle and the posterior belly of the digastric. And, of course, we'll talk through all of these as we move throughout the tutorial but first we want to talk about the muscles of facial expression.

So let's begin by talking about the innervation to the muscles of facial expression which are all highlighted in green in this image. And the muscles of facial expression are supplied by the branches of the facial nerve. In this image, as we mentioned, shows you a lateral view of the left side of the face.

As you'll notice, the brainstem and the associated intracranial structures are ghosted through the temporal bone to give us a clear and complete view of its pathway and our brainstem is highlighted in blue and given a label just on our right here. As the branchial motor portion of the facial nerve exits the stylomastoid foramen, it courses anteriorly through the parotid gland – I'm going to dot that in for you just here – and the parotid gland is a major salivary gland that lies just anterior to the tragus of the ear. And within the substance of the parotid gland, the facial nerve divides into many branches with considerable variation. This forms a structure called the facial nerve plexus. There are six distinct branches with each branch going to supply the muscles in a particular area of the face.

And what we're going to do is we're going to briefly go through the branches. So, the first one is the temporal branch, and this branch ascends from the parotid gland superiorly towards the temporal bone and I'll just briefly outline the parotid gland for you once more in this image. There's also the zygomatic branch which courses more anterosuperiorly towards the orbit which I've outlined for you in blue, and again if I outline the zygomatic arch for you, you can see how this branch travels closely to the arch. The facial plexus also gives rise to the buccal branch which traverses the face heading towards the cheek. It also gives rise to the marginal mandibular branch which, as you can see on this image, follows the inferior border of the mandible which, of course, is our lower jaw. The cervical branch then extends inferiorly into the neck region and finally, the posterior auricular branch is directed posteriorly behind the ear. And, of course, our imaginary ear is drawn for you in blue.

Let's, of course, begin with the temporal branch, and as we mentioned, this branch emerges from the superior aspect of the parotid gland which I've again outlined for you in blue and it crosses the zygomatic arch to supply some of the muscles highlighted in green as well as a few additional muscles. And these muscles which I'm going to draw for you in dark gray mostly are the frontal belly of the occipitofrontalis which elevates the eyebrows, the superior auricular muscle which is just superior to the ear, the anterior auricular muscle which is anterior to the ear and attaches to the zygomatic bone, and the superior part of the orbicularis oculi which is the muscle that encircles the eye and helps to close it – and that's highlighted for you in dark gray.

So let's move on to talk about the zygomatic branches of the facial nerve, and here in this image, we see our lateral view of the head again with the zygomatic branches of the facial nerve highlighted in green. And this nerve can be a single or a paired branch that makes its way to the musculature just below the eye. And this branch mostly supplies the inferior part of the orbicularis oculi which helps the eyelids close, the levator labii superioris which elevates and everts the upper lip, the levator labii superioris alaeque nasi muscle which depresses the alas, the zygomaticus minor muscle which retracts the upper lip, the zygomaticus major muscle which elevates the labial commissure, and the nasalis which wrinkles the skin over the dorsum of the nose.

Moving on to the buccal branch, the buccal branch of the facial nerve can be singular or paired and traverses the face supplying the muscles below the eyes and above the mouth. As you might guess, many of these muscles overlap with the zygomatic branches, however, more importantly, it supplies the muscles associated with the upper lip and cheek such as the buccinator muscle which is involved with keeping the cheeks taut, the inferior fibers of the levator labii superioris and the upper part of the orbicularis oris which encircles the lips.

The next branch we're going to look at is the marginal mandibular branch which we can see here highlighted in green and there are usually two branches which descend in course along the mandible towards the chin and the lower lip, and these branches supply the risorius which as the name suggests helps the lips to smile, the depressor labii inferioris which depresses the mouth, the mentalis which elevates the skin of the chin, the depressor anguli oris which helps with frowning, and the lower fibers of the orbicularis oris which helps shape the mouth when kissing or pouting.

The next nerve that we're going to look at – the cervical branch of the facial nerve – seen here in green is a fairly simple nerve and it leaves the parotid gland inferiorly to pass anteriorly to supply only one muscle, the platysma, which is involve in depressing the mandible.

So in this last branch that we're going to talk about – the posterior auricular nerve – we can see a magnified section of the branches around the external acoustic meatus. And before the facial nerve enters the parotid gland, it gives off the posterior auricular branch which passes posteriorly behind the ear and towards the occipital bone. And this branch has two components – the auricular part which supplies the posterior auricular muscle and the occipital part which supplies the occipital belly of the occipitofrontalis muscle, and that's highlighted in blue for you on this image.

There are a couple of other minor branches of the facial nerve that we just want to briefly mention and they each supply one muscle and these are the digastric branch which supplies the posterior belly of the digastric muscles and the stylohyoid branch which supplies the stylohyoid muscle which, of course, is highlighted in blue. And these branches arise shortly after the facial nerve leaving the stylomastoid foramen and project to the associated muscles.

So, this concludes the voluntary motor components of the facial nerve and now that we're finished with that, we just want to have a brief chat about some of the sensory components of the facial nerve. And in this image, we can see the external ear showing the different sensory areas and the general sensory areas of the facial nerve are associated with the conchal depression of the ear with the scattering of axons traveling with the auricular branch of the vagus nerve ending in the same region. But the facial nerve looks after the conchal area and the conchal depression is this depression just here highlighted in blue.

So now that we're finished talking about the facial nerve and its pathway and innervation, let's backtrack a bit and talk about the facial nerve nuclei which is the region in the brainstem from which the facial nerve arises. And in this image if the brainstem cut in sagittal section, you can see the nuclei and the tracts in pink and purple and, of course, the facial nerve nucleus which is also considered the main motor nucleus is highlighted in green and located in the lower pons which is this segment just here. And it's the motor nucleus which exerts control over the muscles of facial expression and the other skeletal nerves innervated by the facial nerve.

With regards to the parasympathetic supply of the facial nerve, however, the pathway is slightly different. The parasympathetic supply of the facial nerve innervates the salivary glands on the floor of the mouth, the lacrimal glands and also the minor salivary glands that are present in the mucosa of the nasal and oral cavities, and the nuclei that provide this innervation are the superior salivary nucleus seen here in green which contains the cell bodies of the parasympathetics of the submandibular and sublingual glands while the lacrimal nucleus just next to the superior salivatory nucleus and is essentially the same structure highlighted in green on this image holds cell bodies of the parasympathetic axons that are directed towards the lacrimal gland. You probably already know this but both of the nuclei are located in the lower pons.

There are a couple more nuclei I want to talk about that are involved with the facial nerve and both of them are involved with sensory innervation. The first lot I want to talk about are the special sensory nuclei which in the case of the facial nerve are associated with taste from the anterior two-thirds of the tongue which we mentioned earlier in the tutorial, and these axons travel back in the temporal bone to the geniculate ganglion which houses the cell bodies and continues with the intermediate nerve or the sensory root of the facial nerve to the nucleus of the solitary tract. And this nucleus and its tracts are located within the white matter of the medulla oblongata which I'm highlighting for you in blue.

But, again, we'll just go over this fairly quickly having another look at the course of the facial nerve and, if you remember from our previous slides, we mentioned that the chorda tympani is the nerve that provides the anterior two-thirds of the tongue with its sensations of taste and that it piggybacks off the lingual nerve. If we bring up our image of the facial nerve and start our little route in the anterior two-thirds of the tongue which I have labeled for you in the lower left just here, we can see the lingual nerve which comes off the mandibular nerve and that is now labeled for you in blue, and the lingual nerve then is joined by the chorda tympani which itself is a branch of the facial nerve and you can see the facial nerve curving down from the geniculate ganglion just here. and, of course, you can now see the intermediate nerve or the sensory root of the facial nerve labeled in blue making its way up to the pontomedullary junction where it enters the medulla oblongata which is seen in this image.

And, finally, the last little area that we want to talk about is the small area of the external ear that conducts general sensation via the facial nerve and will synapse on the main sensory nucleus of the trigeminal nerve like most areas of the head and face. And you can see that nucleus highlighted in green.

So now that we're finished talking about the facial nerve, let's talk about some clinical notes. And the most commonly discussed clinical scenario to do with the facial nerve is a condition called Bell's Palsy which is a dysfunction of the facial nerve that usually presents as unilateral facial muscle paralysis which you can see on the right of the dotted line here – so the patient's left side of their face. The affected patient also receives a loss of taste to the anterior two-thirds of the tongue and also has some sound sensitivity. And Bell's Palsy usually occurs when the facial nerve becomes dysfunctional at the site of the facial canal within the petrous portion of the temporal bone, and the cause of Bell's Palsy is unknown but maybe associated with lesions or inflammation within the facial canal.

When relating the anatomy to the symptoms, we can see that weakness or paralysis of the muscles of facial expression can be attributed to damage or disruption of the facial nerve before it divides into several branches within the parotid gland. Loss of taste is due to an impaired involvement with the chorda tympani whereas sound sensitivity can also occur because the nerve to the stapedius is affected reducing the stapedius muscle's ability to stabilize the stapes when conducting sound.

Now, Bell's Palsy can be permanent or temporary depending on its cause, and the temporary causes such as inflammation in the facial canal can be reduced with anti-inflammatories, however, the impact of temporary paralysis of the muscles of facial expression can also be improved with physical therapy and muscular training.

So now that we've come to the end of this tutorial, let's quickly have a look at what we've covered. And, of course, in this tutorial, we began by talking about the most medial part of the facial nerve which is the intracranial portion which can be divided into a motor root and into a sensory root. We also talked about the intratemporal portion which is located within the temporal region and houses the geniculate ganglion which houses the cell bodies for the sensory component of the facial nerve, the greater petrosal nerve which transmits parasympathetic and taste axons, the pterygopalatine ganglion which is located within the pterygopalatine fossa and houses the cell bodies of the postganglionic parasympathetic nerve fibers, and, of course, we also looked at the chorda tympani which transmits taste fibers to the anterior two-thirds of the tongue.

We looked at the submandibular ganglion which hangs off the lingual nerve and houses the synapses between pre- and postganglionic parasympathetic axons before finally looking at the nerve to the stapedius which, of course, supplies the stapedius muscle within the inner ear.

We then looked at the extracranial portion or the branchial motor portion which is essentially the motor segment of the facial nerve, and within that we examined the temporal branch which supplies the muscles of facial expression which include the frontal belly of the orbitofrontalis muscle and the superior auricular muscle as well as the anterior auricular muscle which attaches to the zygomatic bone, and the superior part of the orbicularis oculi which is the muscle that encircles the eye.

The next branch of the branchial motor section was the zygomatic branch which innervates the muscles located inferior to the eye and these include the inferior part of the orbicularis oculi which helps the eyelids close, the levator labii superioris which elevates and everts the upper lip, the levator labii superioris alaeque nasi which depresses the alas, the zygomaticus minor which retracts the upper lip, zygomaticus major which elevates the labial commissure and the nasalis which wrinkles the skin over the dorsum of the nose.

This was followed by the buccal branch which supplies the muscles below the eyes and above the mouth and these include the buccinator and the inferior fibers of the levator labii superioris as well as the upper fibers of the orbicularis oris. And we also looked at the marginal mandibular branch which supplies the risorius, the depressor labii inferioris which depresses the mouth, the mentalis which elevates the skin of the chin, the depressor anguli oris which helps with frowning, and the lower fibers of the orbicularis oris.

We looked at the cervical branch which just supplies the platysma and we also looked at the posterior auricular branch whose auricular parts supplies the posterior auricular muscle and whose occipital part supplies the occipital belly of the occipitofrontalis muscle. The other thing we looked at are the minor branches which included the digastric branch which supplies the posterior belly of the digastric muscles and the stylohyoid branch which, of course, supplies the stylohyoid.

We then moved on to talk about the main motor nucleus of the facial nerve which supplies all the motor branches of the facial nerve as well as the superior salivary nucleus which is the site of origin of presynaptic parasympathetic cell bodies as well as the lacrimal nucleus. We also talked about the solitary nucleus which is associated with neurons conducting taste as well as the main sensory nucleus of the trigeminal nerve. We briefly talked about Bell's Palsy which is a facial nerve dysfunction which symptomatically results in unilateral facial muscle paralysis, loss of taste to the anterior two-thirds of the tongue and sound sensitivity.

That's all we have time for today and thanks again as always for watching.

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