Extraocular Muscles

 

👁️ Extraocular Muscles: Overview

🔢	Muscle	Function	Innervation
1️⃣	Superior rectus	Elevation, adduction, intorsion	CN III (Oculomotor)
2️⃣	Inferior rectus	Depression, adduction, extorsion	CN III (Oculomotor)
3️⃣	Medial rectus	Adduction (inward)	CN III (Oculomotor)
4️⃣	Lateral rectus	Abduction (outward)	CN VI (Abducens)
5️⃣	Superior oblique	Intorsion, depression, abduction	CN IV (Trochlear)
6️⃣	Inferior oblique	Extorsion, elevation, abduction	CN III (Oculomotor)

👉 Plus one accessory: | 🆓 | Levator palpebrae superioris | Elevates upper eyelid | CN III (Oculomotor) + Sympathetic fibers |

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📍 Origin & Insertion

Most extraocular muscles originate from the apex of the orbit at the common tendinous ring (annulus of Zinn) and insert onto the sclera of the eyeball.

Muscle	Origin	Insertion
Rectus muscles	Common tendinous ring	Anterior sclera
Superior oblique	Sphenoid bone (above ring) → trochlea	Posterolateral sclera
Inferior oblique	Anterior orbital floor (maxilla)	Posterolateral sclera
Levator palpebrae	Lesser wing of sphenoid	Skin of upper eyelid

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🎯 Primary Actions of Each Muscle

Muscle	Primary Action	Secondary Actions
Superior rectus	Elevation	Intorsion, adduction
Inferior rectus	Depression	Extorsion, adduction
Medial rectus	Adduction	–
Lateral rectus	Abduction	–
Superior oblique	Intorsion	Depression, abduction
Inferior oblique	Extorsion	Elevation, abduction

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🔑 Mnemonic for Innervation:

LR6 SO4, all the rest are 3

• Lateral Rectus – CN VI

• Superior Oblique – CN IV

• All others (including levator palpebrae) – CN III

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🧾 Clinical Relevance:

• CN III Palsy: Eye is "down and out" + ptosis, dilated pupil

• CN IV Palsy: Vertical diplopia, head tilt away from lesion

• CN VI Palsy: Medial deviation (esotropia), can't abduct

• Strabismus: Misalignment of eyes due to EOM imbalance

• Nystagmus: Involuntary rhythmic eye movements (can involve EOMs)