0:00

I want to talk to you about the trochlear nerve,

0:03

the fourth cranial nerve.

0:05

The true origin of the nerve is in the midbrain,

0:09

at the level of the inferior colliculus,

0:12

just above the frenulum of the superior

0:14

medullary velum.

0:15

Now, one tip-off to isolating it is to find the

0:19

aqueduct of Sylvius, go anterior and then about 5 mm over.

0:25

So about 7-8 mm anterior, about 5 mm over.

0:28

And this dark line,

0:29

which represents the posterior longitudinal

0:32

fasciculus, right behind it and medial to it,

0:36

is going to be the trochlear nucleus.

0:37

So, I'm going to make a little dot for the trochlear nucleus.

0:40

Here's the paired one on the contralateral side.

0:43

Don't see the posterior longitudinal fasciculus

0:46

quite as well as we do on the patient's left side.

0:49

You're right.

0:50

And then the trochlear nerve comes out

0:53

and it decussates in the back,

0:56

in the intercollicular region of the

0:59

inferior colliculus just above it,

1:01

sags down a little bit and crisscrosses right

1:04

above the frenulum of the superior

1:07

medullary velum.

1:08

These little dots right here represent

1:10

the trochlear nerve.

1:11

It's a little hard to see in

1:13

the coronal projection,

1:13

and I'm not yet going to try and isolate it.

1:16

Now, after it gets out and crisscrosses,

1:19

I'm going to take my dots away.

1:21

So, let's take them away.

1:23

There are the trochlear nerves.

1:25

They've already decussated.

1:26

And you can see a little bit of the

1:28

trochlear nerve on the left,

1:29

a little volume averaging on the right.

1:31

Now, I'm going to come back again and take its course,

1:35

the so-called interpeduncular course.

1:38

So it's come out, it's decussated in the back,

1:41

and now it's going to come around in its

1:43

interpeduncular course, around the midbrain.

1:46

And then we'll go on the right side this time,

1:48

an equal opportunity sided display.

1:51

We're going to come through the cavernous sinus

1:53

and into the superior orbital fissure,

1:56

on our way to the superior oblique muscle and its pulley,

2:01

which it innervates.

2:03

Now, in the sagittal projection,

2:05

we've isolated the trochlear nerve right there.

2:08

Let's try and isolate it in

2:10

the coronal projection.

2:11

It's a little difficult because it

2:13

is absolutely the thinnest nerve.

2:15

And right there, this dot right here,

2:18

is the right-sided trochlear nerve.

2:20

Now, for you residents and fellows and medical students

2:23

out there, it is the only one to exit posteriorly,

2:26

the only cranial nerve.

2:28

It is also the only cranial nerve

2:31

to decussate intracranially.

2:33

It enters the posterior superior orbital fissure

2:37

outside the tendinous ring.

2:39

And as stated,

2:41

it supplies the superior oblique muscle of the orbit.

2:45

Let's move on, shall we?

2:47

Pomeranz out.