What’s up, Taim Talks Med here. Let’s continue our Cranial nerve series. Cranial nerves are twelve pairs of nerves that exit the brain and the brainstem, and in this segment, we’ll talk detailed about the sixth cranial nerve, which is the abducent nerve. And we’ll do that by first making a quick scheme of the abducent nerve pathway. Then we’ll cover the nerve in a little more detail, by nucleus and the course of this nerve. Then we’ll talk a little bit about how the eyes are coordinated, through something called Hering’s law, and then end with a little bit of a clinical relevance. Alright, so the abducent nerve is a 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’ll pierce the dura mater and travel through the cavernous sinus, then it’ll go through the superior orbital fissure, and then through the common tendinous ring, where it’ll 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 outward—a movement known as abduction. Naturally you don’t want the left abducent nerve to work at the same time, otherwise you’ll end up looking like this guy. So here's the marvel: to maintain balanced eye movement and prevent double vision, we have Hering's Law of Equal Innervation. We’ll talk a little bit more about this later. But that’s the general outline of this nerve. Let’s dive a little deeper into it’s neuroanatomy. So here we see the brainstem, we see the Medulla Oblongata, Cerebellum, Pons, Mesencephalon, and the Diencephalon. Now 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 the 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 situated. 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 see the median sulcus that divides the brainstem into two symmetrical halves. On either side, we see the medial eminence. We can see the medullary stria which divides Pons from Medulla oblongata. And just above the medullary striae, 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 this. Here’s a cross section of the distal part of Pons. Here we see the abducent nerve nucleus, and the facial nucleus. When fibers from the facial nerve leaves the facial nerve nucleus, they loop around the abducent nerve nucleus like this, before it leaves on the lateral side of the abducent 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 nucleus. 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 medulla oblongata. And it’ll 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 Pons and Medulla, medially to the facial nerve. Alright. So as the adbucent nerve leaves through the medullopontine sulcus at the junction between the pons and the pyramid of the medulla, the nerve then continues and pierces the dura mater as you see here. It then enter the cavernous sinus, together with the internal carotid artery, the oculomotor nerve (CN III), trochlear nerve (CN IV) and the ophthalmic branch of the trigeminal nerve (CN V1) in their course. The abducens 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, as you see here, to innervate the abducent nerve. So again, the abducens nerve is a purely motor nerve, responsible for providing general somatic efferent/motor innervation to just one muscle, the lateral rectus muscle of the eye. And when the lateral rectus muscle contracts, it leads to abduction of the eyeball in the 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 move it 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 nucleus travel through the abducent nerve, activating the lateral rectus muscle of the right eye. This causes the right eye to move outward—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. When the right eye's lateral rectus muscle activates, the left abducent nucleus temporarily quiets down. This inhibition prevents the left eye from moving outward simultaneously, ensuring our eyes move in harmony. To achieve this coordination, the brain uses the Medial Longitudinal Fasciculus, which is a neural highway connecting eye movement centres on opposite sides of the brainstem, making sure they align their movements accordingly so you don’t get diplopia, double vision. Now, understanding the abducens nerve's role has clinical implications. All extraocular muscles of the eye work in a synergistic manner to move the eyeball. The abducens nerve however can get easily compressed due to a lesion or rise in intracranial pressure, particularly along its course when it stretches while sharply curving at the petrous part of the temporal bone. Compression of the abducens nerve would cause paralysis of the lateral rectus muscle and lead to a medial deviation of the affected eye. 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 sixth cranial nerve. The next video is going to be about the seventh cranial nerve, the facial 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 favourite moment, subscribe. Turn on those notifications. If you are looking for other ways to support, go ahead and check out the link in the description box. Have fun ya’ll. Peace.