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Nerve supply of eye muscle
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Here is a comprehensive overview of the nerve supply of the eye muscles.
Nerve Supply of the Eye Muscles
The extraocular muscles are controlled by three cranial nerves: CN III (oculomotor), CN IV (trochlear), and CN VI (abducens). A simple mnemonic is LR₆SO₄R₃ — Lateral Rectus (CN VI), Superior Oblique (CN IV), the Rest (CN III).
Quick Reference Table
| Muscle | Primary Action | Nerve |
|---|---|---|
| Lateral rectus | Abduction (lateral) | CN VI — Abducens |
| Superior oblique | Depression + intorsion | CN IV — Trochlear |
| Medial rectus | Adduction (medial) | CN III — Oculomotor |
| Superior rectus | Elevation + intorsion | CN III — Oculomotor |
| Inferior rectus | Depression + extorsion | CN III — Oculomotor |
| Inferior oblique | Elevation + extorsion | CN III — Oculomotor |
| Levator palpebrae superioris | Elevates upper eyelid | CN III — Oculomotor |
CN III — Oculomotor Nerve
Supplies all extraocular muscles except lateral rectus and superior oblique. After entering the orbit through the superior orbital fissure, it divides into two branches:
- Superior division: Superior rectus + levator palpebrae superioris
- Inferior division: Medial rectus, inferior rectus, inferior oblique
The inferior division also carries preganglionic parasympathetic fibers → ciliary ganglion → pupillary constrictor (sphincter pupillae) + ciliary muscle (accommodation).
Nucleus: Midbrain at the level of the superior colliculus.
Note: Superior rectus axons cross the midline and join the contralateral CN III.
CN IV — Trochlear Nerve
- Supplies only the superior oblique muscle
- The superior oblique passes through the trochlea (bony pulley) — hence the name
- Nucleus: Contralateral midbrain at the level of the inferior colliculus
- Only cranial nerve to exit the dorsal brainstem; has the longest intracranial course
Actions of superior oblique:
- Eye adducted → depression (main clinical action)
- Eye abducted → intorsion
CN VI — Abducens Nerve
- Supplies only the lateral rectus muscle (abduction)
- Nucleus: Pons, in the floor of the fourth ventricle
Additional (Non-Extraocular) Muscle Innervation
| Muscle | Nerve | Function |
|---|---|---|
| Sphincter pupillae | CN III (parasympathetic) | Pupil constriction |
| Ciliary muscle | CN III (parasympathetic) | Accommodation (lens thickening) |
| Dilator pupillae | Sympathetic (superior cervical ganglion) | Pupil dilation |
| Müller's muscle | Sympathetic | Accessory lid elevator |
Oblique Muscle Actions Illustrated
Key Clinical Points
| Nerve Palsy | Finding |
|---|---|
| CN III palsy | "Down and out" eye, ptosis, dilated fixed pupil (mydriasis) |
| CN IV palsy | Vertical diplopia; patient tilts head away from affected side to compensate |
| CN VI palsy | Convergent squint; inability to abduct the eye (esotropia) |
Sources: Neuroanatomy through Clinical Cases 3rd Ed.; Kanski's Clinical Ophthalmology 10th Ed.; Principles of Neural Science (Kandel), 6th Ed.
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