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M01.02.032 The Extraocular Muscles
The extraocular muscles are crucial structures within the orbit that control the movement of the eyeball and the superior eyelid. This article provides a detailed overview of their anatomy, including their attachments, innervation, and functions, with clinical relevance for USMLE Step 1 preparation.
Classification of Extraocular Muscles
The seven extraocular muscles are classified into two functional groups:
1. Muscles Controlling Eye Movement
- Recti Muscles: Superior rectus, inferior rectus, medial rectus, lateral rectus
- Oblique Muscles: Superior oblique, inferior oblique
2. Muscle Controlling Superior Eyelid Movement
- Levator palpebrae superioris (LPS)
Detailed Anatomy of Extraocular Muscles
Levator Palpebrae Superioris (LPS)
| Attachments | Originates from the lesser wing of the sphenoid bone above the optic foramen and attaches to the superior tarsal plate of the upper eyelid. |
|---|---|
| Actions | Elevates the upper eyelid. |
| Innervation | – Oculomotor nerve (CN III) for skeletal muscle fibers. |
Recti Muscles
These muscles originate from the common tendinous ring and attach to the sclera of the eyeball.
| Muscle | Attachments | Actions | Innervation |
|---|---|---|---|
| Superior Rectus | Superior part of the tendinous ring to superior/anterior sclera. | Elevation, adduction, medial rotation. | Oculomotor nerve (CN III) |
| Inferior Rectus | Inferior part of the tendinous ring to inferior/anterior sclera. | Depression, adduction, lateral rotation. | Oculomotor nerve (CN III) |
| Medial Rectus | Medial part of the tendinous ring to anteromedial sclera. | Adduction. | Oculomotor nerve (CN III) |
| Lateral Rectus | Lateral part of the tendinous ring to anterolateral sclera. | Abduction. | Abducens nerve (CN VI) |
Oblique Muscles
Unlike the recti muscles, the oblique muscles take an angular path to the posterior surface of the sclera.
| Muscle | Attachments | Actions | Innervation |
|---|---|---|---|
| Superior Oblique | Body of sphenoid bone, tendon passes through a trochlea, attaches to posterior/superior sclera. | Depression, abduction, medial rotation. | Trochlear nerve (CN IV) |
| Inferior Oblique | Anterior orbital floor to posterior/lateral sclera. | Elevation, abduction, lateral rotation. | Oculomotor nerve (CN III) |
Clinical Relevance
Cranial Nerve Palsies
| Cranial Nerve | Paralyzed Muscle(s) | Effects |
|---|---|---|
| Oculomotor (CN III) | Most extraocular muscles, LPS | “Down and out” eye position due to lateral rectus and superior oblique dominance. Ptosis. |
| Trochlear (CN IV) | Superior oblique | Diplopia; head tilt compensates for muscle weakness. |
| Abducens (CN VI) | Lateral rectus | Medial deviation of the eye (adducted) due to unopposed medial rectus. |
Mnemonic for Innervation:
- LR6 (Lateral Rectus – CN VI)
- SO4 (Superior Oblique – CN IV)
- R3 (Remaining – CN III)
Horner’s Syndrome
Features of Sympathetic Denervation:
- Partial ptosis: Denervation of superior tarsal muscle.
- Miosis: Constriction of the pupil due to loss of dilator pupillae function.
- Anhidrosis: Reduced sweating on the ipsilateral side of the face.
Causes:
- Pancoast tumor, aortic aneurysm, thyroid carcinoma.
Key Points to Remember
- Attachments: Extraocular muscles either originate from the common tendinous ring or other orbital structures.
- Innervation: Understand the cranial nerves involved and use the mnemonic LR6SO4R3.
- Actions: Each muscle contributes uniquely to eyeball or eyelid movement.
- Clinical Context: Cranial nerve palsies and Horner’s syndrome are essential clinical correlates.