Test your understanding with 10 random multiple-choice questions from the question bank.
Introduction
0:04
What’s up, Taim Talks Med here. Let’s continue our Cranial nerve series.
0:11
Cranial nerves are twelve pairs of nerves that exit the brain and the brainstem, and in this
0:16
segment, we’ll talk detailed about the fourth cranial nerve, which is the Trochlear nerve.
0:21
And we’ll do that by first making a quick scheme of the trochlear pathway to get an overview of it.
0:28
Then we’ll cover the course of the trochlear nerve and go detailed into its pathway and which
0:33
structures the oculomotor nerve goes through, and while doing so we’ll talk through the function
0:39
of the muscle the trochlear nerve innervates the superior oblique muscle. Then at the end,
0:45
we’ll talk a little bit about the clinical relevance, and pathologies
0:49
related to the fourth cranial nerve pathway. So, the trochlear nerve is the fourth cranial
Trochlear Nerve Scheme
0:55
nerve, and it gets its name from the Latin word pulley, “trochleae.” Now a pulley is a device
1:02
that lifts an object, right? In each eye, the superior oblique muscle functions as the trochlea,
1:10
or a pulley. The trochlear nerve innervates the superior oblique muscle to lift the eyes so you
1:17
can look down. So, the trochlear nerve innervates the superior oblique muscle to move the eye in a
1:24
down-and-out position, and intort the eye. Let’s see how it innervates it.
1:29
The nerve starts from a nucleus called the nucleus of the trochlear nerve, located in
1:34
the midbrain at level of the inferior colliculus. Form the nucleus of the trochlear nerve, the motor
1:40
neuron will leave from the posterior surface of the midbrain, turn anteriorly to enter and run on
1:48
the lateral wall of the cavernous sinus. It then enters the orbit via the superior orbital fissure,
1:54
to innervate the superior oblique. The trochlear nerve is exclusively a somatomotor nerve and
2:04
innervates only one muscle, the superior oblique Alright so this is the general overview of the
Course of the Trochlear Nerve
2:10
trochlear nerve. Now what we’re going to do, is cover this in a little more detail,
2:14
starting from the beginning. At the midbrain. Now, If you take a look at this side view of
2:20
the brain, we can see the spinal cord here, the medulla, cerebellum, Pons,
2:24
Mesencephalon and the diencephalon. And when we remove the cerebellum, and focus only on
2:29
the brainstem from the posterior side, as you see here. You’ll see the mesencephalon, Pons
2:35
and the medulla. So again the mesencephalon which is the midbrain is what we’re interested in now.
2:41
From the posterior view we can see the Cerebral Peduncles, as well as the tectal plate. The tectal
2:48
plate consists of the superior colliculi. involved in incorporating environmental stimuli and
2:54
coordinating gaze shifts involving eye and head movements. We can see the brachium of superior
3:00
colliculus, which is a connecting arm between the superior colliculus and lateral geniculate body.
3:06
And we got the inferior colliculi, which takes in sound information, and sends them further up
3:12
to the medial geniculate bodies through the brachium of the inferior colliculi.
3:16
Alright so that was the external view. Now what I wanna do, is take this model,
3:21
and cut it right about here, at the level of the inferior colliculi.
3:26
Then we’re going to remove the upper part and look at it from this perspective, we’ll see this.
3:32
Here we see the inferior colliculi, the Cerebral peduncles, the Interpeduncular space,
3:37
and the aqueduct of the midbrain, which connects the fourth ventricle to the third ventricle. Now.
3:44
Within the midbrain, we can find the inferior colliculi. So keep in mind this is at the level
3:50
of the inferior colliculi. And there are a bunch of other structures here like the
3:54
substantia nigra and the periaqueductal gray matter. But what’s important now for us to know
4:00
is that on thesides here we got two nuclei called the nucleu of the trochlear nerve.
4:06
When the 4th cranial nerve exits out of the midbrain, it’s going to cross. So in other words
4:12
the nucleus of the 4th cranial nerve on the left side of the midbrain will go to the right eye.
4:18
The nucleus on the right side of the midbrain will go to the left eye. And it’ll exit on the
4:24
posterior surface, and move alongside the midbrain very tightly and then come out anteriorly. So this
4:32
is the nucleus. It’s gonna be found in the same place that you’re gonna see within the oculomotor
4:37
nerve, just a little bit lower. The ocuylomotor nerve nuclei are at the level of the superior
4:43
colliculi, the trochlear nerve nuclei are at level of the inferior colliculi. So that’s that one.
4:51
As we look at this figure, you’ll see that the trochlear nerve exits on the
4:56
posterior surface of the midbrain, at the level of the inferior ciolliculi. Turns
5:02
around the midbrain to continue on anteriorly. Keep focusing your eyes on the trochlear nerve,
5:08
and let’s add some other structures. When the trochlear nerve leaves the posterior
5:13
side of the midbrain and turns anteriorly, it’ll pierce the dura mater and run on the lateral wall
5:20
of the cavernous sinus below the oculomotor nerve, to then run through the superior orbital fissure
5:27
to innervate the superior trochlear muscle. And here you kinda get a different angle where
5:32
you see the trochlear nerve going through the superior orbital fissure to innervate
5:36
the superior trochlear nerve. It’s a very straight forward nerve, this one.
5:40
And I wanna remind you again. we got 7 extraocular muscles. Lateral recus is supplied by the 6th
5:48
nerve the abducent nerve, superior oblique is supplied by the 4th nerve the trochlear
5:53
nerve. Everything else, the superior rectus, medial and inferior rectus. Inferior oblique
6:00
as well as the levator palpebrae superiors are all supplied by the oculomotor nerve.
6:06
Alright, now. Now that we know that, we need to know what the actual superior oblique is
Functions of the Superior Oblique
6:11
doing. So we know where it originates, we know the course of it, we know what
6:17
it innervates. Now what does it do and then we’ll talk about the clinical correlation.
6:23
Let’s first undrtsnad its origin and insertion. One thing I want you guys to keep in mind is
6:29
that the superior oblique muscle originates from the body of sphenoid bone. So in contrast to the
6:35
other extraocular muscles, superior oblique and inferior oblique do not originate from the common
6:42
tendinous ring. Instead superior oblique originates medially to the tendinous ring.
6:48
From its sphenoid attachment, the superior oblique muscle runs anteriorly, and near its insertion,
6:54
the muscle tendon hooks around a cartilaginous pulley, called the trochlea of superior oblique.
7:01
From the trochlea, the tendon takes a sharp posterolateral turn before inserting onto
7:08
the posterior-superior-lateral surface of the eye on the sclera.
7:12
Now before getting into the actions of this muscle, it’s important to highlight that in
7:18
reality, almost all movements of the eyeball involve actions of at least three muscles.
7:24
The movements of the eyeballs need to be controlled, precise and well-coordinated
7:29
to get a clear picture of the outside world. Now, when this muscle contracts, it actually
7:35
pulls the tendinous connection this way. If it pulls this way it pulls the eyeball downwards
7:41
in that action causing depression of the eyeball. And also because its pulling on
7:46
the superior lateral surface, its also going to abduct and internally rotate the eyeball too.
7:53
That’s the function of this muscle. Now. The trochlear nerve is a very delicate nerve that
Trochlear Nerve Palsy
7:59
is relatively easily damaged. Damage can be congenital or occur due to other causes like
8:06
trauma. Because the superior oblique helps to move the eye downwards, when the nerve is damaged the
8:13
eye tends to deviate upwards since there is no opposing force coming from the superior oblique.
8:20
Not only that, because the superior oblique causes internally rotation of the eyeball,
8:25
if the main intorting muscle is affected then what’s the eye normally going to do? Extort.
8:31
So an isolated damage to the superior oblique muscle causes the eye to naturally shift upwards
8:39
externally rotate a little. This usually result in double-vision, diplopia. Since
8:44
both eyes aren’t coordinated anymore. Some patients will adapt to this condition and
8:50
get a head tilt as a compensatory mechanism to better align the eyes and reduce the diplopia.
8:56
Now, an Isolated injury of this nerve is fairly rare and it’s usually combined with
9:02
injuries of other cranial nerves. But damage to the trochlear nerve can be either congenital or
9:08
acquired. Congenital defects causes malformation of the nucleus or the nerve. Acquired damage can
9:15
be due to trauma or a midbrain stroke. Remember fibers that emerge from the nucleus decussate
9:21
before going out from the brainstem, resulting in contralateral symptoms when the nucleus is
9:26
affected. But when the nerve is damaged, it causes ipsilateral symptoms on the same side.
Recap
9:33
Alright let’s recap once again. The nucleus of the trochlear nerve is located in the midbrain
9:38
at level of the inferior colliculus. Form the nucleus of the trochlear nerve,
9:42
the fibers will leave from the posterior surface of the midbrain, turn anteriorly to enter,
9:48
and run on the lateral wall of the cavernous sinus. It then enters the orbit via the superior
9:54
orbital fissure, to innervate the superior oblique. The trochlear nerve is exclusively
10:00
a somatomotor nerve and innervates only one muscle, the superior oblique
10:06
So that was everything I had for the fourth cranial nerve. The next video is going to be
10:11
about the fifth cranial nerve, the Vagus nerve. Thank you so much for watching another one of my
10:16
videos. If you enjoyed, learned something from it, please remember to like, comment your favourite
10:21
moment, subscribe. Turn on those notifications. If you looking for other ways to support,
10:25
go ahead and check out the link in the description box. Have fun ya’ll. Peace.
©Copyright 2025 for Taimtalksmed. Powered by Abdalrahman Atwah.
