Extraocular Muscles: Actions and Nerves

Master the six extraocular muscles: actions, cranial nerve innervation, the LR6 SO4 rule and position-dependent vertical action for board exams.

Reviewed for accuracyJuly 27, 20263 min read

Anterior view of the right eye showing the six extraocular muscles: superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique curving from the superomedial direction to insert on the superotemporal globe, and inferior oblique curving from the anterior nasal floor to insert on the inferotemporal globe.

Warm-up

How many extraocular muscles move the globe?

Lesson

The Extraocular Muscles in 30 Seconds

Six muscles rotate each globe on three axes and drive every eye movement you assess on exam. Their coordinated action underlies binocular alignment, and any single muscle or nerve lesion produces a predictable, testable pattern of misalignment.

The Six Muscles: Actions and Innervation

Medial rectus: adduction. Innervated by the oculomotor nerve (CN III).
Lateral rectus: abduction. Innervated by the abducens nerve (CN VI).
Superior rectus: in primary position, primarily elevates, secondarily intorts and adducts. Innervated by CN III.
Inferior rectus: in primary position, primarily depresses, secondarily extorts and adducts. Innervated by CN III.
Superior oblique: in primary position, primarily intorts, secondarily depresses and abducts. Innervated by the trochlear nerve (CN IV).
Inferior oblique: in primary position, primarily extorts, secondarily elevates and abducts. Innervated by CN III.

4 Facts Exams Always Ask

LR6 SO4 rule: the lateral rectus is CN VI, the superior oblique is CN IV, and CN III innervates every other extraocular muscle plus the levator.
Vertical action of the rectus and oblique groups depends on horizontal eye position. In abduction the SR and IR are the prime elevator and depressor. In adduction the IO and SO take over those roles.
The annulus of Zinn is the common tendinous ring at the orbital apex where the four rectus muscles originate, surrounding the optic canal.
CN IV is the only cranial nerve that exits the brainstem dorsally and crosses the midline before reaching its target muscle, the contralateral superior oblique.
The superior oblique tendon passes through the trochlea, a fibrocartilaginous pulley in the superomedial orbit, which redirects its line of pull and determines its functional actions.
The inferior oblique is the only extraocular muscle that originates anteriorly, from the medial orbital floor near the lacrimal fossa, rather than from the orbital apex.

The Classic Trap

Candidates routinely mix up which muscle elevates or depresses the adducted versus the abducted eye. They memorize "superior rectus elevates" without qualifying eye position, then miss questions built around adduction, where the inferior oblique becomes the primary elevator and the superior oblique the primary depressor. The fix is to always specify horizontal position first: abduction isolates the rectus muscles for elevation and depression testing, while adduction isolates the oblique muscles.

Clinical Pearl

In a fourth nerve palsy, the unopposed inferior oblique causes hypertropia that worsens on gaze into the field of the paralyzed superior oblique, so patients develop a compensatory head tilt away from the affected side to minimize diplopia. This head tilt is a direct bedside consequence of the SO's intorsion and depression roles taught above, and forms the basis of the Bielschowsky head tilt test used to confirm CN IV palsy.

Check yourself

Checkpoint 1

Which cranial nerve innervates the lateral rectus muscle?

Checkpoint 2

When the eye is adducted, the superior oblique acts primarily to:

Clinical case

A 42-year-old presents with vertical diplopia worse when reading and looking down. On exam, hypertropia of the right eye increases when the right eye is adducted, and the patient tilts their head to the left at rest. Extraocular motility is otherwise full.

The common tendinous ring gives origin to which group of muscles?

Explanation

The annulus of Zinn is the origin for the superior, inferior, medial, and lateral rectus muscles. The superior oblique and inferior oblique arise from different locations outside this ring.

Case discussion

The pattern fits a right superior oblique palsy. The SO is the primary depressor in adduction, so weakness there produces hypertropia that worsens when the eye is adducted and looking down, exactly as taught in the rectus versus oblique position-dependent action rule. The left head tilt is the classic compensatory posture for CN IV palsy described in the Clinical Pearl.

Sources & Reviewer Info

Reviewed by Vasilis Inglezis: Optometrist and Ocular Oncology Imaging Specialist, Ocular Oncology Center, Athens, Greece. Last updated July 27, 2026.

Diagram created with AI image generation, clinically reviewed for accuracy.

References

American Academy of Ophthalmology, Basic and Clinical Science Course, Section 2: Fundamentals and Principles of Ophthalmology (Ocular Anatomy)
American Academy of Ophthalmology, Basic and Clinical Science Course, Section 6: Pediatric Ophthalmology and Strabismus
Von Noorden GK, Campos EC. Binocular Vision and Ocular Motility
Snell RS, Lemp MA. Clinical Anatomy of the Eye
Schmidt-Erfurth U, Kohnen T, eds. Encyclopedia of Ophthalmology
American Academy of Ophthalmology, EyeWiki: Extraocular Muscles

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