Extraocular Muscle Innervation Mnemonic
The simplest way to remember which cranial nerves innervate the six extraocular muscles. Lateral rectus by CN VI (abducens), superior oblique by CN IV (trochlear), all others by CN III (oculomotor).
The Mnemonic
"LR6 SO4 rest 3"
Breakdown
Lateral Rectus = CN VI
Abducens nerve abducts the eye (moves laterally)
Superior Oblique = CN IV
Trochlear nerve - intorsion and depression in adduction
All others = CN III
Oculomotor: SR, IR, MR, IO + levator palpebrae
Clinical Relevance
CN III palsy: ptosis, "down and out" eye, dilated pupil. CN IV palsy: diplopia looking down and in, head tilt. CN VI palsy: can't abduct eye, esotropia. Essential for neurological examination.
Study Tips
- ✓CN numbers match: VI has 6 letters (abducens), IV has 4 letters
- ✓Superior Oblique is the only muscle going through a pulley (trochlea)
- ✓CN III does the heavy lifting - most eye movements
- ✓CN III palsy: eye looks "down and out" (only LR and SO work)
FAQs
Common questions about this mnemonic
With CN III paralyzed, only the lateral rectus (CN VI) and superior oblique (CN IV) function. The unopposed lateral rectus pulls the eye out (abduction), and the superior oblique pulls it down when abducted.
Use the H-pattern: test lateral/medial movements, then look up-and-out, up-and-in, down-and-out, down-and-in. Each position isolates specific muscles. Ask about diplopia in each gaze direction.