What's up, Timetalksmed 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 sixth cranial nerve, which is the abducent nerve. And we will do that by first making a quick scheme of the abducent nerve pathway.
Then we will cover the nerve in a little more detailed by nucleus and the course of this nerve. Then we will talk a little bit about how the nerves are coordinated. through something called Herring's law. And then end with a little bit of clinical relevance.
Alright, so the abducent nerve is purely motor nerve, supplying the lateral rectus muscle involved in abduction of the eye. Alright, so here's the scheme. We got the nucleus of the abducent nerve in pons, and we got the nerve. The nerve will travel through the medullopontine sulcus, or the junction between the medulla and pons.
It will pierce the dura mater and travel through the cavernous sinus. Then it will go through the superior orbital fissure, and then go through the common tendinous ring, where it will innervate the lateral rectus muscle. So the way this works is, imagine looking to the right.
Signals from the right abducent nucleus travel through the abducent nerve, activating the lateral rectus muscle of the right eye. This causes the right eye to move outwards. a movement known as abduction. Naturally, you don't want the left abducent nerve to work at the same time. So here's the marvel.
To maintain balanced eye movements and prevent double vision, we have Herring's law of equal innervation. We'll talk a little bit more about this later, but that is the general outline of this nerve. Let's dive a little bit deeper into its neuro anatomy. So here we see the brainstem, we see the medulla oblongata, cerebellum, Pons, Mesencephalon, and the Diencephalon.
If we remove the cerebellum and focus on the brainstem from the posterior side, we'll see this. So we still see the mesencephalon, pons, and medulla here. On the posterior side of the brainstem, we can see something called the rhomboid fossa. And the rhomboid fossa is a key location where several cranial nerve nuclei are located. And because it houses so many nerves and nuclei, they form external structures you can see here.
So let's quickly go through some external structures we see here. In the middle, we can see the median sulcus that divides the brainstem into two symmetrical halves. On either side, we can see the medial eminence, we can see the medulla ristraea, which divides the pons and medulla oblongata. And just above the medulla ristraea, we can find the facial colliculus.
And this is what I want us to focus on here, because the facial colliculus is a grossly elevated area on the posterior side of Pons that is formed because of the abducens and the facial nerve. Here's a cross section of the distal part of Pons. Here we see the abducens nerve nucleus and the facial nucleus.
When fibers of the facial nerve leave the facial nerve nucleus, they loop around the abducens nerve like this, before it leaves on the lateral side of the abducens nerve. And this loop it makes forms the facial colliculus. And so here just for the visuals of it, we got the 6th cranial nerve and the 7th cranial nerve.
Fibers from the facial nerve will loop around the abducent nerve nucleus like this, then leave the brainstem on the anterior side between pons and the bedulla oblongata, and it will form the facial colliculus, the elevation on the backside of pons. Fibers from the abducent nerve however, it's such an easy and nice nerve. It's just going to go straight out on the anterior side between the pons and the medulla, medially to the facial nerve. Alright, so as the abducent nerve leaves through the medullopontine sulcus at the junction between the pons and the pyramids of the medulla, the nerve then continues and pierces the dura mater, as you see here. It then enters the cavernous sinus, together with the internal carotid artery, the oculomotor nerve, the cochlear nerve, and the ophthalmic branch of the trigeminal nerve in their course.
The abducent nerve then exits the cavernous sinus to enter the orbit via the superior orbital fissure and then pass through the common tendinous ring. And when it enters the orbital cavity, it goes on the lateral side to innervate the abducent nerve. So again, the abducent nerve is purely a motor nerve, responsible for providing general somatic efferent. or motor innervation to one muscle, which is the lateral rectus muscle of the eye. And when the lateral rectus muscle contracts, it leads to abduction of the eyeball in a horizontal plane.
So it's worth highlighting that the lateral rectus muscle of the left eye would abduct the eye to the left, while the muscle on the right eye would abduct the eye to the right. So always directing the gaze laterally along the horizontal plane. Now naturally, you don't want these two to contract at the same time. Imagine looking to the right, signals from the right abducent nerve travel through the abducent nerve, activating the lateral rectus of the right eye.
This causes the right eye to move outwards, a movement known as abduction. But here's the marvel. To maintain balanced eye movement and prevent double vision, we have Hering's law of equal innervation.
So, when the right eye's lateral rectus muscle activates, the left abducent nucleus temporarily quiets down. This inhibition prevents the left eye from moving outwards simultaneously, ensuring our eyes to move in harmony. And to achieve this coordination, the brain uses the medial longitudinal fasciculus, which is a neural highway connecting the eye movement centers on the opposite side of the brainstem, making sure they align their movement accordingly, so you don't get diplopia or double vision.
Now, understanding the abducent nerve's role has clinical implications. All extraocular muscles of the eye works in a synergistic manner to move the eyeball. The abducent nerve, however, can easily get compressed due to a lesion or rise in intracranial pressure, especially along its course where it stretches while sharply curving at the petrous part of the temporal bone.
And compression of the abducent nerve will cause paralysis of the lateral rectus muscle and lead to a medial deviation of the affected eye, and as a result, the patient will have a fully adducted eye at rest and will demonstrate an inability to abduct their eye. So that was everything I had for the 6th cranial nerve. The next video is going to be about the 7th cranial nerve, the facial nerve. Thank you so much for watching another one of my videos. If you enjoyed, learn 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.
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