Extraocular Muscles

00:02 Now let's take a look at the muscles that move the eyeball, we call these the extraocular muscles.

00:10 We have four muscles they're oriented and more or less a straight position hence the term rectus.

00:16 So we have four recti muscles.

00:20 We also have two muscles oriented and more oblique orientation.

00:24 And we call those the oblique muscles.

00:28 Very closely associated in the superior aspect is a muscle that acts on the upper eyelid called the levator palpebrae superioris.

00:40 Here we see the LPS or levator palpebrae superioris sitting superior to all of these other muscles.

00:49 If we remove it, we can actually see some of our rectus muscles.

00:55 And we see that these rectus muscles attached to a tendinous ring near the orbital apex.

01:02 We have a lateral rectus and a medial rectus, as well as an inferior rectus and a superior rectus.

01:12 One thing to note here about the shape of the orbit is that the inferior and superior rectus don't point directly straight ahead, they actually point somewhat laterally as they move forward.

01:27 Now let's look at the oblique muscles.

01:31 These don't attach to the tendinous ring.

01:35 The superior oblique attaches near it on the sphenoid bone.

01:39 But it actually comes up through this little pulley system called the trochlea before taking a turn and attaching to the eyeball.

01:49 The inferior oblique attaches to the floor of the orbit and also takes an oblique approach to the attachment at the back of the eyeball.

02:02 Let's look at the innervation.

02:04 We have the superior rectus, medial rectus, inferior oblique and inferior rectus as well as that eyelid muscle, the levator palpebrae superioris.

02:16 All being innervated by cranial III, which is also called the ocular motor nerve given its role in these extra ocular muscles.

02:27 And his passing through the superior orbital fissure to innervate these structures.

02:34 Here we see the greater wing of the sphenoid and the lesser wing of the sphenoid.

02:39 And between those two is where we find that superior orbital fissure.

02:43 One of the other structures that pass through that is the trochlear nerve, or cranial nerve IV.

02:49 And that's what innervates, the superior oblique.

02:52 And this name helps us to remember that because we said it passes through this pulley like structure called a trochlea, hence the name trochlear nerve.

03:02 Here we see the lateral rectus.

03:05 And this is innervated by cranial nerve VI, or the abducens nerve.

03:10 And that name also helps us remember this because the lateral rectus being on the lateral side will cause the eye to abduct, hence its name.

03:22 So let's talk about the movements of the eyeball for a second.

03:26 The lateral rectus, as we just said, based off of the orientation of its fibers will abduct the eye or move it laterally.

03:35 Similarly, the medial rectus will move the eye medially.

03:41 The superior rectus won't move it directly upward, it will move it up and out.

03:46 Again, keeping in mind that orientation of the superior and inferior rectus because as they move forward, they actually start to point out laterally.

03:57 Similarly, the inferior rectus won't just move the eyeball downward, it'll actually move it down and out.

04:04 Conversely, the inferior oblique given the orientation of its fibers is going to move the eye upward and inward.

04:13 Whereas the superior oblique is going to move it downward and inward.

04:21 So let's look at the eyes working together starting with the eyes looking straight ahead in the primary position.

04:30 In order to elevate or look directly up, the extra ocular muscles are going to work in tandem, that means the superior rectus and inferior oblique will both be activated.

04:43 Similarly for depression or looking down, both the inferior rectus and superior oblique will be acting.

04:51 For dextroversion or looking to the right, the right eyeball will move via the lateral rectus and the left will move via the medial rectus.

05:03 For looking up into the right, the right eye only needs to use the superior rectus while the left only needs to use the inferior oblique.

05:13 And similarly, by looking downward and to the right, only the inferior rectus is needed on the right eye and only the superior oblique for the left.

05:24 In terms of laevoversion or looking to the left, it would be just the opposite.

05:28 In this case, the left eye would use the lateral rectus and the right I would use the medial rectus.

05:34 And similarly for looking up into the left or left eye would use superior rectus and the right inferior oblique.

05:43 And looking down into the left, same thing.

05:46 Or left eye would need to use inferior rectus and our right superior oblique.