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Introduction
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What’s up, Taim Talks Med here.
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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, and
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in this segment, we’ll talk detailed about the third cranial nerve, which is the Oculomotor
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nerve.
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And we’ll do that by first making a quick scheme of the oculomotor pathway to get an
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overview of it.
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Then we’ll cover the very of Midbrain anatomy because this is where the oculomotor nerve
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originates from.
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After that, we’re going to cover the course of the oculomotor nerve and go detailed into
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it’s pathway and which structures the oculomotor nerve goes through and while doing so we’ll
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talk through the function of all the muscles that the oculomotor nerve innervates.
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And at the end, we’ll talk a little bit about the clinical relevance around pathologies
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related with the third cranial nerve pathway.
Optic Nerve Scheme
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So, the oculomotor nerve is the third cranial nerve, and the name itself is a clear indication
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of the function of the nerve.
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Oculo means that it’s related to the eye, motor means producing movement.
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Simply from the name then, it is easy to know that the oculomotor nerve will innervate muscles
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that move the eye itself or components of the eye.
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So, the oculomotor nerve allows movement of the eye muscles, constriction of the pupil,
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and the position of the upper eyelid.
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Let’s see how.
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There are two nuclei for the oculomotor nerve that are both located in the midbrain, at
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the level of the superior colliculi.
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Those are called the nucleus of the oculomotor nerve and the accessory oculomotor nucleus,
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which is known as Edinger–Westphal nucleus.
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Those two nuclei are gonna give off fibers that are going to meet, and run together as
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something called the oculomotor complex, which is a combination of somatic and parasympatethic
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neurons.
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This complex, the oculomotor nerve, is going to emerge from the midbrain, in a sulcus called
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oculomotor sulcus, located in the interpeduncular fossa.
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It penetrates the dura mater to run laterally in the wall of the cavernous sinus.
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It enters the orbit through the superior orbital fissure, into a ring called the common tendinous
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ring, and then it splits into a superior branch and an inferior branch.
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The superior branch will innervate the levator palpebrae muscle, which is the upper eyelid
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muscle, and the superior rectus muscle.
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The inferior branch will innervate the Medial rectus, Inferior rectus and the inferior oblique.
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The superior and the inferior branch are the somatomotor fibers of the oculomotor nerve
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complex, meaning they innervate the extraocular muscles.
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The remaining fibers are parasympathetic fibers coming from the Edinger Westphal nucleus.
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The parasympathetic fibers run together with the inferior branch.
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From the inferior branch, a nerve will go out towards the ciliary ganglion.
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The ciliary ganglion is located posterior to the eyeball, and it gives off small ciliary
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nerves which innervate the ciliary muscle, which changes the shape of the lens to focus
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the eye on near objects, it’s called accommodation.
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And it innervates the sphincter pupillae, which constricts the pupil, causes miosis.
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Alright so this is the general overview of the oculomotor nerve.
Midbrain Anatomy
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Now what we’re going to do, is start from the beginning, where the oculomotor nerve
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actually originates from.
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And that is from the midbrain, so let’s go through the anatomy of the midbrain again.
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If you take a look at this sideview of the brain, we can see the spinal cord here, medulla,
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cerebellum, Pons, Mesencephalon and the diencephalon.
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But if we remove the cerebellum, and focus only on the brainstem from the posterior side,
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as you see here.
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You’ll see the mesencephalon, Pons and the medulla.
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So again the mesencephalon which is the midbrain is what we’re interested in now.
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From the posterior view we can see the Cerebral Peduncles, as well as the tectal plate.
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The tectal plate consists of the superior colliculi.
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involved in incorporating environmental stimuli and coordinating gaze shifts involving eye
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and head movements.
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We can see the brachium of superior colliculus, which is a connecting arm between the superior
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colliculus and lateral geniculate body.
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And we got the inferior colliculi, which takes in sound information, and sends them further
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up to the medial geniculate bodies through the brachium of the inferior colliculi.
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From the anterior perspective of the Midbrain, we can see the Cerebral peduncles, and between
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them we can see the interpeduncular fossa.
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Whithin this region, we can find the posterior perforated substance, and we can see the oculomotor
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sulcus of the Mesencephalon.
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From here, that is where our oculomotor nerve will go out from.
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So the oculomotor nerve goes out from the anterior side of the midbrain, from a sulcus
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called the sulcus of the oculmotor nerve.
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Alright so that was the external view.
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Now what I wanna do, is take this model, and cut it right about here, at the level of the
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superior colliculi.
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Then we’re going to remove the upper part and look at it from this perspective, we’ll
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see this.
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Here we see the Superior colliculi, the Cerebral peduncles, the Interpeduncular space, and
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the aqueduct of the midbrain, which connects thfe fourth ventricle to the third ventricle.
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Now.
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Within the midbrain, we can find the substantia nigra, we can find the superior colluculi.
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We can see the periaqueductal gray matter, the reticular formation, the red nuclei which
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take in impulses from the brain and the cerebellum, and give off rubrospinal tracts for muscle
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movement coordination.
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At this level, we can also find the nucleus of the oculomotor nerve which is what we’re
Oculomotor Nerve Course
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interested in, because this is where the oculomotor nerve starts.
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The oculomotor nerve will travel towards the anterior side, and leave through the sulcus
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of the oculomotor nerve on the anterior side of the midbrain.
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The oculomotor nerve is a nerve that consists of somatic fibers and preganglionic parasympathetic
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fibers.
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The somatic fibers are the fibers you see here, coming from the nucleus of the oculomotor
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nerve, which sipplies the extrinsic muscles of the eyeball.
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It’s somatic, so it moved the skeletal muscles voluntarily.
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The parasympathetic portion of the oculomotor nerve comes from the accessory nucleus of
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the oculomotor nerve, which is also called Edinger Westphal nucleus.
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They give off parasympathetic fibres that go together with the oculomotor nerve, forming
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the oculomotor nerve complex.
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Now.
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Here we see the oculomotor nerve leaving the anterior brainstem.
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The anterior midbrain, right?
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When it leaves the anterior surface of the midbrain, the oculomotor nerve is then going
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to penetrate the dura mater and run through the lateral wall of cavernous sinus.
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Now what do do I mean by that?
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That it runs through the lateral wall of the cavernous sinus?
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The cavernous sinus is a part of the dural sinuses, right?
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Carrying venous blood.
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But there are several structures that pass through the cavernous sinus to enter the orbit.
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And they’re usually sub-classified by whether they travel through the sinus itself, or through
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its lateral wall.
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And so the oculomotor nerve is one of the other structures that travel through the lateral
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wall of the cavernous sinus.
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After nerve leaves the cavernous sinus, it enters the orbital cavity through the superior
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orbital fissure, as well as within a fibrous ring called the common tendinous ring.
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Now.
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If we zoom in, you can see that once the oculomotor nerve has entered the orbital cavity, it splits
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into two divisions.
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Right?
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So let’s go ahead and now look at this model from an anterior perspective.
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So here we can see the orbital cavity, right?
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Imagine for a second that the eyeball is right here.
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Around the eyeball, there’re gonna be muscles we call extraocular muscles.
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And there are seven extraocular muscles in total.
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There are a total of four rectus muscles, two oblique muscles, and the standalone levator
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palpebrae superioris.
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The four rectus muscles are the medial rectus, lateral rectus, superior rectus, and inferior
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rectus.
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The oblique muscles are the superior and inferior obliques.
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The rectus and the oblique muscles are all involved in the different gaze position of
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the eyeball.
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The levator palpebrae superioris is primarily responsible for eyelid elevation.
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Now.
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As the oculomotor nerve goes past the common tendinous ring and enters the orbital cavity,
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it’s going to divide into two branches.
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A superior branch, and an inferior branch.
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The superior branch innervates the superior rectus and levator palpebrae superioris
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The inferior branch innervates the inferior and medial rectus and inferior oblique, as
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well as a parasympathetic branch to make the lens globular, for close vision.
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Alright.
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Let’s do that one more time, just a little bit more detailed.
Branches of the Oculomotor Nerve
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So here again we see the superior rectus, medial rectus, inferior rectus.
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We see the superior oblique, inferior oblique.
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And we see the .. Levator palpebrae superioris.
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Now.
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The oculomotor nerve is gonna go like this.
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Then the superior branch passes medially over the optic nerve, and then supplies the superior
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rectus and levator palpebrae superioris.
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Superior rectus elevates the eye, causing the cornea to move superiorly.
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While the levator palpebrae superioris raise the upper eyelid and maintains the upper eyelid
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position.
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The inferior branch of the oculomotor nerve is larger than the superior branch, and it
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divides into three branches.
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One passes beneath the optic nerve to the medial rectus.
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Another, to the inferior rectus.
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The third and longest runs forward to the inferior oblique.
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The medial rectus is an adductor muscle, it works together with the lateral rectus to
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allow the eyes to move from side to side.
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Inferior rectus depresses the eye, causing the cornea and pupil to move inferiorly.
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The inferior oblique is interesting.
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The inferior oblique is actualy coming from the medial side or the nasal side, and coming
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underneath the eyeball and attaches to the inferiorlateral portion of the eyeball.
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And then when it contract it does something really interesting it pulls from that portion
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and it pulls upwards.
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And when it pulls it pulls the eyeball upwards and rotates it outwards.
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So it elevates the eyeball and it rotates the eyeball laterally, so it does what’s
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called superior and lateral rotation.
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The superior oblique we’ll talk about it with he trochlear nerve.
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A nice little acronym someone once told was like this.
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LR6, SO4, and ATR3.
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LR3 means lateral rectus, 6th cranial nevre.
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Superior Oblique 4th cranial nevre.
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And All The Rest, 3rd cranial nerve.
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Ok so that’s that view.
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So, the third cranial nerve has a superior branch and an inferior branch.
Parasympathetic Fibers of Oculomotor Nerve
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Now.
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Together with the inferior branch, there’re gonna be parasympathetic fibres, coming from
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Edinger Westphal nuclei in the midbrain, at the level of the superior colliculus.
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Those fibers are called preganglionic parasympathetic fibers.
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They’re pre-ganglionic so they go to a ganglia.
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What is a ganglia?
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A ganglia is a group of cell bodies located within the peripheral nervous system.
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So those cell bodies there are postganglionic parasympathetic motor neurons.
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From here they’re going to go out as postganglionic fibers within nerves called short ciliary
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nerves.
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And they’re going to supply the Iris, specifically the sphincter pupillae and causes the pupil
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to constrict.
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So it causes pupillary constriction.
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And they’re going to supply the ciliaris muscle and cause the ciliaris muscle to contract
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which alters the shape of the lens particurarly making the lens globular.
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And when you make the lens globular that’s actually gonna be for near vision, for close
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vision.
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It’s called the accommodation reflex.
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Alright, so that’s these.
Clinical Relevance
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Something else that I wanna talk about, with respect to the actual pathway of the oculomotor
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nerve is.
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Throughout its course, all the way from the nuclei of the midbrain to the orbital cavity,
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it do have a number of vulnarble points.
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If there’s a stroke or bleeding whithin the midbrain itself, that can damage the oculomotor
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nuclei causing oculomotor nerve defect.
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If there’s an increased pressure within the cavernous sinus, that can cause compression
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of the oculomotor nerve damaging it.
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Another thing that can happen is an aneurism of the posterior communicating artery up there
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can compress the oculomotor nerve aswell.
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You know, if there’s meningitis or paranasal sinus infection, that can lead to the infection
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spreading over to the oculomotor nerve giving oculomotor nerve infections.
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Trauma to the orbit or the superior orbital fissure can also compress these nerves damaging
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it too.
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Or an uncontrolled diabetes or a chronic hypertension can cause damage to the blood vessels arouns
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the oculomor nerve aswell.
Oculomotor Nerve Palsy
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So how would that manifest?
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How would we see if a person has an oculomotor nerve palsy?
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Lets’s see.
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Superior rectus and inferior oblique elevates the eyeball.
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Medial rectus causes medially rotation.
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As well as superior rectus and inferior rectus causes a little bit of medial rotation aswell.
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Even tho the inferior rectus causes primarily the eye to rotate downwards it do help with
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medial rotation to a certain extent.
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Now.
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If I can’t bring my eyeball upwards, where does it go?
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Downward.
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If I can’t bring my eyeball medially it’s probably gonna go..
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Outwards.
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So.
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Let’s look at this video for a second.
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This is a person undergoing a neurological examination, and notice how the left eye is
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stuck laterally and inferiorly, because the oculomotor nerve is damaged.
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He’s not able to elevate nor medially rotate the eyeball anymore because the medial, inferior
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and superior rectus are damaged, as well as the inferior oblique.
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BUT the lateral movement and inferior movement are spared, because the lateral rectus is
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innervated by the 6th cranial nerve, the abducent nerve.
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And the superior oblique is innervated by the 4th cranial nevre, the trochlear nerve.
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When the two eyes aren’t able to coordinate the movement what happens?
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You get double vision.
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Notice also how the left eyelid is not as elevated as the right eyelid, it’s called
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ptosis this condition.
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Ptosis also happens with oculomotor nerve damage since the levator palpebrae superioris
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is not functioning anymore.
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So we get down and outwards gaze, as well as ptosis with oculomotor nerve damage.
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What else do we get?
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Remember the parasympathetic response constricts the pupil.
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IF it can’t constrict the pupil what happens?
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It dilates.
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So their pupils are gonna be fixed and dilated, and it may not constrict in response to light
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anymore.
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The pupil is often affected when the cause is compression of the 3rd cranial nerve.
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But when the pupil is not affected, the cause is often inadequate blood flow to the nerve
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seen in microangiopathies often in diabetics or hypertension.
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This is because when the vasa nervosum is affected it causes ischemia to the central
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part of the nerve, which is somatomotor involvement only, while the peripheral parts of the nerve
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is autonomic fibers.
Recap
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Alright let’s recap once again.
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The oculomotor nerve starts with two nuclei located in the midbrain at the level of the
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superior colliculus.
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Those are called the nucleus of the oculomotor nerve and the accessory oculomotor nucleus,
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which is known as Edinger–Westphal nucleus.
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Those two nuclei are gonna give off fibers that are going to meet, and run together as
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something called the oculomotor complex, which is a combination of somatic and parasympatethic
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neurons.
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This complex, the oculomotor nerve, is going to emerge from the midbrain, in a sulcus called
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oculomotor sulcus, located in the interpeduncular fossa.
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It penetrates the dura mater to run laterally in the wall of the cavernous sinus.
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It then goes through the superior orbital fissure, into a ring called the common tendinous
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ring, and then it enters the oribal cavity and splits into a superior branch and an inferior
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branch.
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The superior branch will innervate the levator palpebrae muscle, which is the upper eyelid
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muscle, and the superior rectus muscle.
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The inferior branch will innervate the Medial rectus, Inferior rectus and the inferior oblique.
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The parasympathetic fibers coming from the accessory oculomotor nucleus run together
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with the inferior branch.
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From the inferior branch, a nerve will go out towards the ciliary ganglion.
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The ciliary ganglion is located posterior to the eyeball, and it gives off small ciliary
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nerves which innervate the ciliaris muscle and cause the ciliaris muscle to contract
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which alters the shape of the lens particurarly making the lens globular to focus the eye
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on near objects, it’s called accommodation.
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And it innervates the sphincter pupillae, which constricts the pupil, causes miosis.
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So that was everything I had for the third cranial nerve.
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The next video is going to be about the fourth cranial nerve, the trochlear Nerve.
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Alright guys so that pretty much covers the oculomotor nerve.
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Thank you so much for watching another one of my videos.
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Peace.
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