Head

Extraocular Muscles

Reading Time:

Overview


We move our eyes constantly, and need to be able to move them in order to follow moving objects, read words written on a page and perform any activity that requires visual pursuit. We have 6 extraocular muscles that move the eyeball and one that moves the upper eyelid. These muscles are innervated by three cranial nerves (oculomotor, trochlear, abducens).


Gross Anatomy


There are six muscles that move the eyeball itself, one muscle that moves the upper eyelid, and one that shuts the eye tight. The muscles that move the eyeball itself are as follows: the medial rectus attaches to the medial surface of the eyeball, adducts the eye and is innervated by the oculomotor nerve. The lateral rectus attaches to the lateral surface of the eyeball, abducts the eye and is innervated by the trochlear nerve. The inferior rectus attaches to the superior surface if the eyeball, elevates the eye and is innervated by the oculomotor nerve. The superior rectus attaches to the superior part of the eyeball, depresses the eye and is innervated by the oculomotor nerve. The superior oblique attaches to the superior part of the eyeball, causes eye intorsion, moves the eye down and out and is innervated by the trochlear nerve. The inferior oblique attaches to the inferior part of the eyeball moves the eye up and in and causes eyeball extorsion, and is innervated by the oculomotor nerve. The two oblique muscles run through sling like pulleys before they insert onto the surface of the eyeball, and act as pure elevators/depressors when the eye is adducted.

 

The levator palpebrae superioris (innervated by the oculomotor nerve) is the muscle that moves the upper eyelid. The orbicularis oculi (innervated by the facial nerve) is the muscle that shuts the eye tight. The facial nerve also innervates the lacrimal glands that produce tears.


Clinical Anatomy


3rd nerve palsy- This presents as a down and out eye (as the lateral rectus and superior rectus are performing their actions only), with a dilated pupil (constrictor pupillae is innervated by the short ciliary branches of the ciliary ganglion, which run on the outside of the oculomotor nerve) and a ptosis (levator palpebrae superioris is denervated).

 

6th nerve palsy- This is abducens nerve palsy. Abducens innervates the lateral rectus muscle, and hence the patient will not be able to abduct the eye beyond the midline.

 

Horner’s syndrome- One third of the innervation to levator palpebrae superioris is sympathetic. Hence, in Horner’s syndrome when the sympathetic innervation is absent, the upper eyelid will partially droop i.e. partial ptosis. The pupil will also be dilated in Horner’s syndrome, as the sympathetic long ciliary nerves of the ciliary ganglion innervate the dilator pupillae muscle.

 

Myaesthenia Gravis- This usually presents as double vision and drooping eyelids, due to anti-acetylcholine receptor antibodies causing muscle weakness. The eye muscles are often impacted first.


Quick Anatomy


Key Facts

Developmental precursor- Somitomeres 1-4 (paraxial mesoderm cranial to the occipital somites).

Muscles- Superior rectus, inferior rectus, medial rectus, lateral rectus, inferior oblique, superior oblique.

Aide-Memoire

SO4 LR6

Superior Oblique innervated by the fourth cranial nerve (trochlear nerve)

Lateral rectus innervated by the 6th cranial nerve (abducens)

The others are supplied by the oculomotor nerve.


Summary


There are 6 extraocular muscles that move the eyeball and one that moves the upper eyelid. These muscles are innervated by three cranial nerves (oculomotor, trochlear, abducens).


References


Lorem ipsum dolor sit amet, sapien platea morbi dolor lacus nunc, nunc ullamcorper. Felis aliquet egestas vitae, nibh ante quis quis dolor sed mauris.