There are three groups of the muscles of the eye:

• Intraocular: ciliary muscle (changes lens shape) and pupillary dilators/constrictors
• Extraocular muscles of eye movement: 4 recti and 2 oblique muscles
• Extraocular elevator of the upper eyelid: levator palpebrae superioris

The superior division of the oculomotor nerve (CN III) innervates levator palpebrae superioris and the superior and medial recti while the inferior division innervates the inferior rectus and oblique. Meanwhile, the superior oblique is supplied by the trochlear nerve (CN IV) and the lateral rectus by the abducent nerve (CN VI).

LR6SO4 (CN VI innervates lateral rectus, CN IV innervates superior oblique m.)

Mnemonic (innervation of eye muscles)

Recti and oblique muscles

The 6 small skeletal muscles that move the eyeball include 4 four straight muscles (the recti) and two oblique muscles.

The recti originate from a tendinous ring at the back of the bony orbit, each travelling forwards and inserting to the eyeball behind the junction of the cornea and sclera (just anterior to the equator of the eyeball). The medial rectus adducts, lateral rectus abducts, superior rectus elevates and the inferior rectus depresses the eyeball. Since the apex of the bony orbit (where the semitendinous ring lies) points medially, the superior and inferior recti also mediate adduction.

The superior oblique muscle arises superomedial to the optic foramen from the sphenoid bone and loops through a superomedial cartilaginous pulley (the trochlea) to bend back and insert onto the sclera of the posterolateral surface of the eyeball. The inferior oblique muscle arises from the orbital maxillary bone medial to the infraorbital margin and inserts deep to the lateral rectus. Due to their attachments posterior to the equator of the eyeball, the superior oblique mediates depression, intortion and abduction of the eyeball while the inferior oblique mediates elevation, extortion and abduction.

CLINICAL CORNER

Movements of the eye generally are tested by having an individual trace the letter “H” with their eye. This is because the orbital and visual axes do not coincide, so different muscles contribute more to certain actions:
• Superior oblique: Depresses the eye when looking medially
• Inferior oblique: Elevates the eye when looking medially
• Superior rectus: Elevates the eye when looking laterally
• Inferior rectus: Depresses the eye when looking laterally
• Medial rectus: Adduction when pupil is on horizontal plane
• Lateral rectus: Abduction when pupil is on horizontal plane

Levator palpebrae superioris

The levator palpebrae superioris arises from the lesser wing of the sphenoid at the apex of the orbit, in contact with the origins of the superior rectus and superior oblique. It inserts by aponeurosis into the upper tarsal plate and the skin crease in the upper lid. The upper edge of the tarsal plate is reached by Müller’s muscle which is supplied by sympathetic fibres.

CLINICAL CORNER

Paralyses of levator palpebrae superioris causes drooping of the upper eyelid (ptosis). Keeping the eye open requires both sympathetic supply to the smooth muscle fibres and CN III supply to the striated muscle of the muscle. Hence, complete ptosis indicates a oculomotor nerve lesion, whereas partial ptosis may indicate disruption to the sympathetic supply (e.g. Horner’s syndrome).