When we talk about the extraocular muscles, it’s also very important to understand their
innervation. As we work through the innervation of the extraocular muscles, we will
summarize their innervation with a useful mnemonic. But for now, we have three cranial
nerves that will innervate the muscles that attach to the eyeball. The three cranial nerves
that innervate these extraocular muscles will be oculomotor, abducent, and the trochlear
nerve. So we’re looking at cranial nerves III for oculomotor, IV for trochlear,
and then VI for the abducent. The oculomotor bears the brunt of this innervation
as we’ll see. Here is your superior rectus. Oculomotor is its innervation. Here is your
inferior rectus, again oculomotor. The rectus that’s on the lateral side, the lateral rectus
shown here in bright red, this is where we have a change in innervation to the abducent
or cranial nerve VI. Medial rectus is here, oculomotor again. Inferior oblique, oculomotor
and then the superior oblique is going to be innervated by the trochlear nerve,
cranial nerve IV. Here we’re looking at the mnemonic for the lateral rectus.
This is going to be LR6, lateral rectus, LR and then 6 for cranial nerve VI which is the
abducent nerve. The superior oblique is going to be innervated by your trochlear nerve,
cranial nerve IV. So the mnemonic here is SO for superior oblique and then 4 for cranial
nerve IV. Then the rest of these are very, very easy because all the rest of them
are going to be innervated by cranial nerve III. This would be the oculomotor nerve.
Now, let’s take a look again at the levator palpebrae superioris. First, its innervation
is like the majority of the extraocular muscles. It’s going to be innervated by cranial nerve III,
oculomotor as well. Though many of the fibers of the levator palpebrae superioris
are skeletal muscles fibers, it’s important for you to realize that there are some
smooth muscle fibers that pass from its inferior surface and attach into the eyelid itself.
These muscle fibers are very important in maintaining eyelid elevation, keeping that upper
eyelid in an open position. As we are all looking at the smooth muscle fibers at this particular
point, they will not be innervated by the somatic fibers of oculomotor III. Instead,
there will be sympathetic fibers that will innervate these particular smooth muscles.
This has clinical relevance in the case of a lesion of sympathetics or if there’s a lesion
of cranial nerve III, the nerve that is conveying these sympathetic nerve fibers.
If there is a lesion of these sympathetic components, this can result for example
in Horner's syndrome. One of the symptoms or signs if you will of Horner’s is a drooping eyelid.
This is known as ptosis. So the upper eyelid will be in a depressed state.