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The abducens nerve is purely a motor nerve supplying the lateral rectus muscle, which is involved in the abduction of the eye.
Introduction
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What’s up, Taim Talks Med here. Let’s continue our Cranial nerve series.
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Cranial nerves are twelve pairs of nerves that exit the brain and the brainstem,
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and in this segment, we’ll talk detailed about the sixth cranial nerve, which is the abducent nerve.
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And we’ll do that by first making a quick scheme of the abducent nerve pathway.
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Then we’ll cover the nerve in a little more detail, by nucleus and the course
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of this nerve. Then we’ll talk a little bit about how the eyes are coordinated,
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through something called Hering’s law, and then end with a little bit of a clinical relevance.
Abducens Nerve Scheme
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Alright, so the abducent nerve is a purely motor nerve, supplying the lateral rectus
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muscle involved in abduction of the eye. Alright so here’s the scheme. We got the
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nucleus of the abducent nerve in Pons. And we got the nerve. The nerve will travel through
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the medullopontine sulcus, or the junction between the medulla and pons. It’ll pierce the dura mater
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and travel through the cavernous sinus, then it’ll go through the superior orbital fissure,
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and then through the common tendinous ring, where it’ll innervate the lateral rectus muscle.
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So the way this works is imagine looking to the right. Signals from the right abducent nucleus
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travel through the abducent nerve, activating the lateral rectus muscle of the right eye. This
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causes the right eye to move outward—a movement known as abduction. Naturally you don’t want
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the left abducent nerve to work at the same time, otherwise you’ll end up looking like this guy. So
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here’s the marvel: to maintain balanced eye movement and prevent double vision,
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we have Hering’s Law of Equal Innervation. We’ll talk a little bit more about this later. But
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that’s the general outline of this nerve. Let’s dive a little deeper into it’s neuroanatomy.
Abducens Nerve Nucleus
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So here we see the brainstem, we see the Medulla Oblongata, Cerebellum, Pons, Mesencephalon,
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and the Diencephalon. Now if we remove the cerebellum, and focus on the brainstem from
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the posterior side, we’ll see this. So we still see the mesencephalon, Pons and the medulla here.
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On the posterior side of the brainstem, we can see something called the Rhomboid Fossa. And
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the rhomboid fossa is a key location where several cranial nerve nuclei are situated.
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And because it houses so many nerves and nuclei, they form external structures you can see here.
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So let’s quickly go through some external structures we see here. In the middle, we
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see the median sulcus that divides the brainstem into two symmetrical halves. On either side, we
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see the medial eminence. We can see the medullary stria which divides Pons from Medulla oblongata.
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And just above the medullary striae, we can find the facial colliculus. And this is what I want us
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to focus on here. Because the facial colliculus is a grossly elevated area on the posterior side
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of Pons that is formed because of this. Here’s a cross section of the distal part of Pons. Here
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we see the abducent nerve nucleus, and the facial nucleus. When fibers from the facial nerve leaves
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the facial nerve nucleus, they loop around the abducent nerve nucleus like this, before it leaves
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on the lateral side of the abducent nerve. And this loop it makes, forms the facial colliculus.
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And so here, just for the visuals of it. We got the 6th cranial nerve, and the 7th cranial
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nerve nucleus. Fibers from the facial nerve, will loop around the abducent nerve nucleus like this,
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then leave the brainstem on the anterior side, between Pons and medulla oblongata.
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And it’ll form the facial colliculus, the elevation on the backside of Pons.
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Fibers from the abducent nerve however, it’s such an easy and nice nerve. It’s just going
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to go straight out on the anterior side between Pons and Medulla, medially to the facial nerve.
Abducens Nerve Course
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Alright. So as the adbucent nerve leaves through the medullopontine sulcus at the junction between
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the pons and the pyramid of the medulla, the nerve then continues and pierces the dura mater as you
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see here. It then enter the cavernous sinus, together with the internal carotid artery, the
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oculomotor nerve (CN III), trochlear nerve (CN IV) and the ophthalmic branch of the trigeminal nerve
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(CN V1) in their course. The abducens nerve then exits the cavernous sinus to enter the orbit via
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the superior orbital fissure and then pass through the common tendinous ring. And when it enters the
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orbital cavity, it goes on the lateral side, as you see here, to innervate the abducent nerve.
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So again, the abducens nerve is a purely motor nerve, responsible for providing general
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somatic efferent/motor innervation to just one muscle, the lateral rectus muscle of the eye.
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And when the lateral rectus muscle contracts, it leads to abduction
Lateral Rectus Muscle Function
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of the eyeball in the horizontal plane. So it’s worth highlighting that the lateral
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rectus muscle of the left eye would abduct the eye to the left, while the muscle on the right eye
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would move it to the right. So always directing the gaze laterally along the horizontal plane.
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Now, naturally, you don’t want these two to contract at the same time. Imagine looking to
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the right. Signals from the right abducent nucleus travel through the abducent nerve,
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activating the lateral rectus muscle of the right eye. This causes the right eye
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to move outward—a movement known as abduction. But here’s the marvel: to maintain balanced eye
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movement and prevent double vision, we have Hering’s Law of Equal Innervation.
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When the right eye’s lateral rectus muscle activates,
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the left abducent nucleus temporarily quiets down. This inhibition prevents the left eye
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from moving outward simultaneously, ensuring our eyes move in harmony.
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To achieve this coordination, the brain uses the Medial Longitudinal Fasciculus,
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which is a neural highway connecting eye movement centres on opposite sides of the brainstem,
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making sure they align their movements accordingly so you don’t get diplopia, double vision.
Clinical Relevance
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Now, understanding the abducens nerve’s role has clinical implications.
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All extraocular muscles of the eye work in a synergistic manner to move the eyeball.
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The abducens nerve however can get easily compressed due to a lesion or rise in intracranial
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pressure, particularly along its course when it stretches while sharply curving at the
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petrous part of the temporal bone. Compression of the abducens nerve would cause paralysis of
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the lateral rectus muscle and lead to a medial deviation of the affected eye. As a result, the
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patient will have a fully adducted eye at rest and will demonstrate an inability to abduct their eye.
Recap
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So that was everything I had for the sixth cranial nerve. The next video is going to be
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about the seventh cranial nerve, the facial nerve. Thank you so much for watching another one of my
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videos. If you enjoyed, learned something from it, please remember to like, comment your favourite
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go ahead and check out the link in the description box. Have fun ya’ll. Peace.
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