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 fourth
cranial nerve, which is the Trochlear nerve. And we’ll do that by first making a quick scheme
of the trochlear pathway to get an overview of it. Then we’ll cover the course of the trochlear
nerve and go detailed into its pathway and which structures the oculomotor nerve goes through, and
while doing so we’ll talk through the function of the muscle the trochlear nerve innervates
the superior oblique muscle. Then at the end, we’ll talk a little bit about the
clinical relevance, and pathologies related to the fourth cranial nerve pathway.
So, the trochlear nerve is the fourth cranial nerve, and it gets its name from the Latin word
pulley, “trochleae.” Now a pulley is a device that lifts an object, right? In each eye, the
superior oblique muscle functions as the trochlea, or a pulley. The trochlear nerve innervates the
superior oblique muscle to lift the eyes so you can look down. So, the trochlear nerve innervates
the superior oblique muscle to move the eye in a down-and-out position, and intort the eye.
Let’s see how it innervates it. The nerve starts from a nucleus called the
nucleus of the trochlear nerve, located in the midbrain at level of the inferior colliculus.
Form the nucleus of the trochlear nerve, the motor neuron will leave from the posterior surface of
the midbrain, turn anteriorly to enter and run on the lateral wall of the cavernous sinus. It then
enters the orbit via the superior orbital fissure, to innervate the superior oblique. The trochlear
nerve is exclusively a somatomotor nerve and innervates only one muscle, the superior oblique
Alright so this is the general overview of the trochlear nerve. Now what we’re going to
do, is cover this in a little more detail, starting from the beginning. At the midbrain.
Now, If you take a look at this side view of the brain, we can see the spinal cord
here, the medulla, cerebellum, Pons, Mesencephalon and the diencephalon. And when
we remove the cerebellum, and focus only on the brainstem from the posterior side, as you
see here. You’ll see the mesencephalon, Pons and the medulla. So again the mesencephalon which
is the midbrain is what we’re interested in now. From the posterior view we can see the Cerebral
Peduncles, as well as the tectal plate. The tectal plate consists of the superior colliculi. involved
in incorporating environmental stimuli and coordinating gaze shifts involving eye and head
movements. We can see the brachium of superior colliculus, which is a connecting arm between the
superior colliculus and lateral geniculate body. And we got the inferior colliculi, which takes
in sound information, and sends them further up to the medial geniculate bodies through
the brachium of the inferior colliculi. Alright so that was the external view.
Now what I wanna do, is take this model, and cut it right about here, at the
level of the inferior colliculi. Then we’re going to remove the upper part and
look at it from this perspective, we’ll see this. Here we see the inferior colliculi, the
Cerebral peduncles, the Interpeduncular space, and the aqueduct of the midbrain, which connects
the fourth ventricle to the third ventricle. Now. Within the midbrain, we can find the inferior
colliculi. So keep in mind this is at the level of the inferior colliculi. And there are
a bunch of other structures here like the substantia nigra and the periaqueductal gray
matter. But what’s important now for us to know is that on thesides here we got two nuclei
called the nucleu of the trochlear nerve. When the 4th cranial nerve exits out of the
midbrain, it’s going to cross. So in other words the nucleus of the 4th cranial nerve on the left
side of the midbrain will go to the right eye. The nucleus on the right side of the midbrain
will go to the left eye. And it’ll exit on the posterior surface, and move alongside the midbrain
very tightly and then come out anteriorly. So this is the nucleus. It’s gonna be found in the same
place that you’re gonna see within the oculomotor nerve, just a little bit lower. The ocuylomotor
nerve nuclei are at the level of the superior colliculi, the trochlear nerve nuclei are at level
of the inferior colliculi. So that’s that one. As we look at this figure, you’ll see
that the trochlear nerve exits on the posterior surface of the midbrain, at the
level of the inferior ciolliculi. Turns around the midbrain to continue on anteriorly.
Keep focusing your eyes on the trochlear nerve, and let’s add some other structures.
When the trochlear nerve leaves the posterior side of the midbrain and turns anteriorly, it’ll
pierce the dura mater and run on the lateral wall of the cavernous sinus below the oculomotor nerve,
to then run through the superior orbital fissure to innervate the superior trochlear muscle.
And here you kinda get a different angle where you see the trochlear nerve going through
the superior orbital fissure to innervate the superior trochlear nerve. It’s a
very straight forward nerve, this one. And I wanna remind you again. we got 7 extraocular
muscles. Lateral recus is supplied by the 6th nerve the abducent nerve, superior oblique
is supplied by the 4th nerve the trochlear nerve. Everything else, the superior rectus,
medial and inferior rectus. Inferior oblique as well as the levator palpebrae superiors
are all supplied by the oculomotor nerve. Alright, now. Now that we know that, we need
to know what the actual superior oblique is doing. So we know where it originates,
we know the course of it, we know what it innervates. Now what does it do and then
we’ll talk about the clinical correlation. Let’s first undrtsnad its origin and insertion.
One thing I want you guys to keep in mind is that the superior oblique muscle originates from
the body of sphenoid bone. So in contrast to the other extraocular muscles, superior oblique and
inferior oblique do not originate from the common tendinous ring. Instead superior oblique
originates medially to the tendinous ring. From its sphenoid attachment, the superior oblique
muscle runs anteriorly, and near its insertion, the muscle tendon hooks around a cartilaginous
pulley, called the trochlea of superior oblique. From the trochlea, the tendon takes a sharp
posterolateral turn before inserting onto the posterior-superior-lateral
surface of the eye on the sclera. Now before getting into the actions of this
muscle, it's important to highlight that in reality, almost all movements of the eyeball
involve actions of at least three muscles. The movements of the eyeballs need to be
controlled, precise and well-coordinated to get a clear picture of the outside world.
Now, when this muscle contracts, it actually pulls the tendinous connection this way. If it
pulls this way it pulls the eyeball downwards in that action causing depression of the
eyeball. And also because its pulling on the superior lateral surface, its also going to
abduct and internally rotate the eyeball too. That’s the function of this muscle. Now. The
trochlear nerve is a very delicate nerve that is relatively easily damaged. Damage can be
congenital or occur due to other causes like trauma. Because the superior oblique helps to move
the eye downwards, when the nerve is damaged the eye tends to deviate upwards since there is no
opposing force coming from the superior oblique. Not only that, because the superior oblique
causes internally rotation of the eyeball, if the main intorting muscle is affected then
what’s the eye normally going to do? Extort. So an isolated damage to the superior oblique
muscle causes the eye to naturally shift upwards externally rotate a little. This usually
result in double-vision, diplopia. Since both eyes aren’t coordinated anymore. Some
patients will adapt to this condition and get a head tilt as a compensatory mechanism to
better align the eyes and reduce the diplopia. Now, an Isolated injury of this nerve is
fairly rare and it's usually combined with injuries of other cranial nerves. But damage to
the trochlear nerve can be either congenital or acquired. Congenital defects causes malformation
of the nucleus or the nerve. Acquired damage can be due to trauma or a midbrain stroke. Remember
fibers that emerge from the nucleus decussate before going out from the brainstem, resulting
in contralateral symptoms when the nucleus is affected. But when the nerve is damaged, it
causes ipsilateral symptoms on the same side. Alright let’s recap once again. The nucleus of
the trochlear nerve is located in the midbrain at level of the inferior colliculus.
Form the nucleus of the trochlear nerve, the fibers will leave from the posterior surface
of the midbrain, turn anteriorly to enter, and run on the lateral wall of the cavernous
sinus. It then enters the orbit via the superior orbital fissure, to innervate the superior
oblique. The trochlear nerve is exclusively a somatomotor nerve and innervates
only one muscle, the superior oblique So that was everything I had for the fourth
cranial nerve. The next video is going to be about the fifth cranial nerve, the Vagus nerve.
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