Transcript for:
Understanding the Trigeminal Nerve Functions

What's up? Time Talks Med here. Let's continue our cranial nerve series.

Cranial nerves are 12 pair of nerves that exit the brain and the brainstem. And in this segment, we will talk detailed about the fifth cranial nerve, which is the trigeminal nerve. And we will do that by first making a quick scheme of the trigeminal pathway to get an overview of it. Then we will cover the nerve in a little more detail by first talking about the distribution of the trigeminal nerve nuclei within the brainstem.

Then we will talk detailed about... each branch of the trigeminal nerve, which is the ophthalmic nerve, maxillary nerve, and the mandibular nerve. You will often see them labeled as V1, V2, and V3. The V refers to the Roman number 5, as the trigeminal nerve is the 5th cranial nerve. So these numbers refer to the divisions of the nerve according to the region of the face they innervate.

When we're done, we'll quickly go through some fun clinical correlations that I think the general population should be aware about when it comes to this nerve. Alright, awesome! Let's start by making a rough and simplified scheme. Now, this nerve is primarily responsible for sensory innervation of the face and motor control of muscles involved in mastication, or chewing, right?

It starts with three sensory nuclei, called mesencephalic, principal and the spinal nuclei of the trigeminal nerve, and one motor nucleus, all located within the brainstem. The sensory nuclei all have a sensory route that goes towards them, coming from the ganglion, called the trigeminal ganglion. Now, the way the sensory distribution of the trigeminal nerve is formed is by three divisions.

We got the ophthalmic, for the forehead and eyes, maxillary, for the region of the middle face, and the mandibular, for the lower face and the jaw. The motor nucleus will give off motor fibers which will join the mandibular division to innervate primarily the muscles of mastication. Now, I will go through this scheme pretty fast, and then we will talk about it in a little more detail.

And then I'll show this scheme again at the end of this video as a recap so that you hopefully will understand a bit more about this nerve. And don't get overwhelmed by the amount, we will try to break it down as simple as we can. So, Ophthalmic Nerve. The ophthalmic nerve will go through the superior orbital fissure, but before it does that, it gives off the Tentorial Nerve.

providing sensory innervation to the dura mater. Once the ophthalmic nerve goes through the superior orbital fissure, it receives three fibers. And the three main nerves that come together to form V1 are the nasociliary, frontal, and lacrimal nerves.

The acronym NFL, as in American football, is also used to recall these three branches. The frontal nerve goes directly beneath the roof of the orbit, and extend into two terminal divisions, called the supraorbital nerve and the trochlear nerve. The supraorbital nerve goes up towards the forehead, where it splits into two of its own terminal branches, the lateral branch and the medial branch.

The nasociliary will go through the common tendons ring, and split into the posterior etymolar nerve going through the posterior etymolar foramen and the orbital etymolar canal. where it innervates the mucosa of the ethmodal cells and the sphenoid sinus. There's the anterior ethmodal nerves, which enter through the anterior ethmodal foramen and the foramina of the cribriform plate, to innervate the roof of the nasal cavity. We got the long and short ciliary nerves for the eyes.

And we got the infratrochlear nerve for the upper eyelid and the conjunctiva. And we got the lacrimal nerve, which is the thinnest branch of the ophthalmic nerve, going to the lacrimal gland and the upper eyelid. The maxillary nerve will go through the foramen rotundum, but before it enters the foramen, it will give off the meningeal branch that detects stimuli from the dura of the middle cranial fossa. Once it enters the pterygopalatine fossa, it gives off 2-3 nerves to the pterygopalatine ganglion. Then what happens is that it gives off three branches, the posterior superior alveolar nerve, the infraorbital nerve, and the zygomatic nerve.

The zygomatic and the infraorbital nerve will enter the orbital cavity through the inferior orbital fissure. Now, the zygomatic nerve, as it runs through here, it gives off two branches, the zygomatico-temporal branch and the zygomatico-facial branch, which both run through their own respective foramina. The infraorbital nerve will run on the floor of the orbital cavity, and on its way, it's going to give off the middle superior alveolar nerve. and the anterior superior alveolar nerve. Then, the nerve goes through the intraorbital groove and gives off three branches.

It gives off the inferior palpabular nerve, we got the nasal branches, and the superior labial branches, for the upper lips. The third branch here is the posterior superior alveolar nerve. So that's the big part of the maxillary nerve.

But there are other branches that we need to talk about in respect to the pterygopalatine ganglion. One branch is going to the pharyngeal area, called the pharyngeal nerve. There's the naso-palatine nerve. And we got the lesser palatin for the soft palate, and the greater palatin for the hard palate.

Now there are some other couple of branches here too, but I'm only going to focus on the most important ones, to not make this too overwhelming. One important connection here, before we go further, is a connection between the zygomatic and the lacrimal nerve. It's a communicating branch. that provides parasympathetic innervation to the lacrimal gland. Alright, let's do V3, the mandibular.

Notice how motor fibers from the motor nuclei pass below the trigeminal ganglion, without synapsing with it, and then it joins the sensory root of this nerve. This nerve will leave the neurocranium through the foramen ovale, and just as it leaves, it gives off the meningeal branch. which goes back into the neurocranium through the foramina spinosum. Now keep in mind, there might be some variations here, but we'll keep it at that. The V3 division is then going to give off the auricotemporal nerve.

The auricotemporal nerve is composed of two roots that make a small loop encircling the middle meningeal artery, to supply the ear and the temporal region. There is a ganglion here, called the otic ganglion. There are going to be postganglionic parasympathetic secretomotor nerve fibers from the aortic ganglion that join and hitchhike along the auriculophoral nerve to innervate the parotid gland.

Then we got the lingual nerve, which is a sensory nerve for the anterior 2 third of the tongue. Then we got the inferior alveolar nerve. It's going to split into the nerve of the myelohyoid muscle, which innervates the myelohyoid and anterior belly of the digastric muscle, respectively. which is responsible for elevating the hyoid and the complex movements of the jaw, like speaking, swallowing, chewing, and breathing. The inferior alveolar nerve continues its course to enter the mandibular foramen and reach the mandibular canal.

And within the mandibular canal, it gives off branches to supply the teeth of the mandible, hence the name inferior alveolar. The nerve then continues as the mental nerve, which is considered as the terminal branch of the inferior alveolar nerve. The mental nerve then passes the mental foramen of the mandible to emerge on the face and innervate the lower lip. It gives off the buccal nerve, which innervates the skin of the cheeks and the buccal mucosa.

And then we got all these muscle branches responsible for different functions, primarily mastication. Alright, so again, this is just a scheme. A superficial way that hopefully will make sense once we talk quickly through each nerve.

Awesome! So let's go quickly through the course in a little more detail. So here we see the spinal cord, medulla, cerebellum, pons, mesencephalon, and the diencephalon. If we remove the cerebellum and focus only on the posterior side of the brainstem, we will see this.

We can see the medulla, pons, and mesencephalon. Now, I will only show the left side, but keep in mind that everything you see on the left side, you will also find on the right side. Remember, there are three sensory nuclei, and one motor nucleus for the trigeminal nerve.

The first sensory is the mesencephalic nucleus, which receives proprioceptive information from the masticatory muscles, involved in processing information about the position of the jaw and teeth. It is functionally responsible for preventing excessive biting. that may damage the teeth.

Then we got the principal nucleus of the trigeminal nerve, which primarily is the main sensory nucleus that receives facial sensation of touch. We got the spinal nucleus of the trigeminal nerve, which receives information about things like pain and temperature from the ipsilateral face. And then lastly, we got the motor nucleus, providing motor innervation for the muscles of mastication, the tensor tympani, tensor veli palatini, myelohyoid, and the anterior belly of the digastric. So those are the nuclei of the trigeminal nerve.

Now the sensory root of the trigeminal nerve will leave the anterior lateral surface of the pons, as you see here. If you look from this angle, you will see the sensory root of the trigeminal nerve. And right next to it, underneath it, you will see the motor root of the trigeminal nerve.

And as you see here, the sensory root comes from the trigeminal ganglion, while the motor root skips the trigeminal ganglion completely. So this is what it looks like from the lateral view. We got the sensory and the motor root. And the sensory root forms the trigeminal ganglion.

Now, On the other side of the trigeminal ganglion, we got some divisions. There are some sensory divisions coming from the facial area. And these are the ophthalmic nerve, maxillary nerve, and the mandibular nerve.

Let's now go through them one by one, starting with the ophthalmic nerve. Now, the ophthalmic nerve is the most superior branch of the trigeminal nerve, and it's purely a sensory nerve. It innervates the upper part of the face, And that includes structures associated with the eyes, the nasal cavity, the scalp, and the dura mater of the anterior cranial fossa. So let's see how it does that.

The ophthalmic nerve is going to pass through the superior orbital fissure and enter the orbital cavity. But before it does that, it gives off a tentorial nerve providing sensory innervation to the dura mater. Once the ophthalmic nerve goes through the superior orbital fissure, It receives three fibers, frontal nerve, nasociliary, and the lacrimal nerve. The frontal nerve is the thickest branch of the ophthalmic nerve. It courses forward, directly beneath the roof of the orbit.

And inside the orbital cavity, the nerve divides into two terminal branches, and these are the supraorbital nerve and the suprachocolor nerve. The supraorbital nerve passes through the supraorbital notch before it goes up to the forehead. and splits into the lateral branch and the medial branch to supply the skin of the lower forehead, which they provide with sensory innervation. The suprachocolor nerve goes upwards and medially on the forehead, exiting the skull through the suprachocolor foramen, and provides sensory innervation of the skin of the forehead and scap near the midline, just above the nose. Then we got the lacrimal nerve.

This nerve goes along the roof of the orbit, and travel towards the lacrimal gland. So it primarily innervates the lacrimal gland, and also some sensory innervation to the upper eyelid as well. Then we got the nasociliary nerve. This nerve runs from the lateral part of the orbit to its medial wall.

On its way, it gives off a communicating branch that enters the ciliary ganglion. So it carries sensory fibers for the eyeball, which pass through the ciliary ganglion. and then continue as short ciliary nerves to reach the eyeball. It also gives off long ciliary nerves that penetrate the sclera to innervate the sclera and the choroidia.

It also gives off the posterior etmodal nerve that leaves the orbit through the posterior etmodal foramen and enters the anterior cranial fossa. It then descends to the roof of the nasal cavity through the cribriform plate, where it innervates the mucosa of the etmodal. etmodal cells and the sphenoid sinus.

The nasociliary nerve also gives off the anterior etmodal nerve that passes through the anterior etmodal foramen, where it reaches the anterior cranial fossa. It then goes through the foramina of the cribriform plate to reach the anterior part of the roof of the nasal cavity, where it innervates the mucosa of that part. And lastly, we got the infratrochlear nerve, which extends forward to innervate the skin of the medial portion of the upper eyelid and the conjunctiva. Alright, so that was mainly all the branches that I wanna talk about regarding the ophthalmic nerve. Now, let's talk about the 2, the maxillary branch of the trigeminal nerve.

The maxillary nerve is purely a somatosensory nerve as well, that carries information from the midface, that is, all the structures from the lower eyelid to the upper teeth and lips. While the maxillary nerve is in the cranium, it gives off a meningeal branch that carries the sensory impulses from the dura mater in the middle cranial fossa. The nerve then goes through the foramen rotundum, as you see here. It goes through this foramen to reach a space called the pterygopalatine fossa. Once the maxillary nerve is within the pterygopalatine fossa, it gives off branches that go to the pterygopalatine ganglion.

It also gives off the superior posterior alveolar branches. And then it will give off two terminal branches that goes through the inferior orbital fissure, as you see here. And these are the infraorbital nerve and the zygomatic nerve.

Alright, let's go through these nerves, starting with the infraorbital nerve. The infraorbital nerve is the strongest branch of the maxillary nerve, and it crosses through the inferior orbital fissure. After it crosses through the inferior orbital fissure, it goes over the inferior wall of the orbit. The infraorbital nerve first goes through the infraorbital sulcus and then to the infraorbital canal.

And at the end of the canal, it goes out through the infraorbital foramen. Before it exits the infraorbital canal, it gives off the anterior superior alveolar branches and the middle superior alveolar branches, which participate in making the plexus, the innervated teeth of the upper jaw. together with the posterior superior alveolar branches. We'll talk about it in a minute. But now the nerve is going to exit the infraorbital canal through the infraorbital foramen.

It's then going to divide into the inferior palpabular branch that provide innervation for the lower eyelid, external nasal branches that innervate the skin that covers the side of the nose, the internal nasal branches which provide sensory innervation to the nasal septum, and the superior labial branches that innervates the upper lip. Alright, so that was the infraorbital nerve. The posterior superior alveolar branches, as we said, leaves the maxillary nerve in the pterygopalatine fossa. They then cross through the alveolar foramina on the maxillary tuberosity and enter the alveolar canals, and this is where they form a plexus.

the innervated teeth of the upper jaw, together with the anterior superior alveolar branches, and the middle superior alveolar branches. Alright, so that was these nerves. Next, let's talk about the zygomatic nerve. Remember, it comes from the maxillary nerve in the pterygopalatine fossa, and then goes through the inferior orbital fissure, and enters the orbit.

While inside the orbit, the nerve goes along the inferior orbital wall. and branches off to give the zygomatico-facial branch that reaches the upper lateral part of the cheek via the zygomatico-facial foramen to innervate the skin of the zygomatic region. And the zygomatico-temporal branch that reaches the temporal fossa by passing through the zygomatico-temporal foramen, innervating the skin of the temporal region. Another important thing to mention about this nerve is that And on the lateral wall of the orbit, the zygomatic nerve makes an anastomosis with the lacrimal nerve. And thanks to this anastomosis, parasympathetic fibers from the pterygopalatine ganglion reach the lacrimal gland, to make it squeeze some tears.

Alright, now, the next thing that we need to talk about now is the pterygopalatine ganglion, which receives sensory fibers from the maxillary nerve. We're not going to talk detailed about this one. but it does receive sensory fibers from the facial nerve as well.

The pterygopalatine ganglion then gives off certain fibers, like the pharyngeal branches that go off to innervate the mucous membrane of the nasopharynx. It gives off the nasopalatine that provides sensory innervation to the nasal septum and gingivae. There's the greater palatine branches, for primarily the heart palate, and the lesser palatine branches, primarily sensory innervation of the soft palate.

along with the tonsils and the uvula. There are some other ones as well, but I'll keep it at these ones for the sake of simplicity. Alright, so we did the ophthalmic division, we did the maxillary division, let's now do the mandibular division.

The mandibular division contains both sensory and motor fibers. It provides somatosensory innervation to the skin of the lower part of the face, so the lower lip, part of the cheeks, the chin. teeth at the lower jaw, and part of the oral mucosa. The motor fibers innervate 8 muscles in total.

4 masticatory muscles, the anterior belly of the digastic, myelohyoid tensor veliparotene, and the tensor tympani. Alright, let's break this nerve down, starting with the somatosensory nerves. The nerve travels through the foramen ovale to reach the infratemporal fossa, as you see here. Now just as the nerve leaves the skull, It gives off the meningeal branch, which re-enters the neurocranium by going back through the foramen spinosum, as you see here, to innervate the dura mater of the middle cranial fossa. Then we got the auriculotemporal nerve.

The way this works is that from the mandibular nerve, two roots are going to go out and encircle the middle meningeal artery. When these two roots meet, they form the auriculotemporal nerve. This nerve innervates parts of the auricle and the skin of the temporal region.

Then we got the lingual nerve. The lingual nerve is the thickest nerve giving sensory innervation to the anterior 2 third of the tongue. Then a bit more superiorly, we can see the buccal nerve, which innervates the skin of the cheeks and the buccal mucosa.

Then back here, we can see a small ganglion called the otic ganglion. The otic ganglion, now I'm not going to spend too much time on this one, but the otic ganglion is one of the parasympathetic ganglions we have in the head. that provides functions to multiple salivary glands. And what this one does is that it gives off a postganglionic parasympathetic secretomotor nerve fibers that join and hitchhike along the auricotemporal nerve to innervate the parotid gland to produce saliva. Then we got the inferior alveolar nerve.

This one is going to enter the mandibular foramen and pass through the mandibular canal. Within the mandibular canal, It gives off branches that supply the teeth of the mandible, hence the name inferior alveolar. The nerve then continues as the mental nerve, which is considered as the terminal branch of the inferior alveolar nerve.

The mental nerve then passes the mental foramen of the mandible to emerge on the face and innervate the lower lip. Before entering the mandibular foramen, it gives rise to the nerve of the myelohyoid muscle. which provide motor innervation to the myelohyoid and the anterior belly of the digastric muscle.

Then we got the deep temporal nerves, we got one anterior and one posterior branch. Both of them innervate the temporalis muscle. We got the medial pterygoid nerves, innervating the medial pterygoid muscle. The nerve then penetrates the medial pterygoid and reaches the tensor tympani and tensor veli palatini, which it also innervates. And we got the nerve to the lateral pterygoid, which innervates the lateral pterygoid muscle.

Then we got the masseteric nerve, which innervates the temporum and the blur joint, as well as the masseter muscle. So that was everything I had for the trigeminal nerve. One last thing that I want to mention about this nerve that's kind of interesting, you probably heard about the virus herpes simplex type 1, right? Herpes virus is a neurotropic virus that infects the peripheral and central nervous system.

After primary infection in the epithelial cells, HSV-1 spreads retrogradely to the peripheral nervous system, where it establishes a latent infection in the trigeminal ganglia, and it usually stays there for the rest of one's person's life in the trigeminal ganglia. And when the person passes a period with decreased immunity, they will get a secondary infection of this virus again. So the trigeminal ganglion acts like a house for the herpes simplex type 1. Alright, so we've gone through all the most important branches of the trigeminal nerve.

Here you have the overview, once again. It does look harder than it is. The trigeminal nerve is large, but quite comprehensible. So that was everything I had for the fifth cranial nerve. The next video is going to be about the sixth cranial nerve, the abducent nerve.

Thank you so much for watching another one of my videos. If you enjoyed, learned something from it, please remember to like, comment your favorite moment, subscribe, turn on those notifications. If you're looking for other ways to support, go ahead and check out the link in the description box. Have fun y'all, peace.