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Medial rectus muscle

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Medial rectus
Eyemuscles.png
Rectus muscles:
2 = superior, 3 = inferior, 4 = medial, 5 = lateral
Oblique muscles: 6 = superior, 8 = inferior
Other muscle: 9 = levator palpebrae superioris
Other structures: 1 = Common tendinous ring, 7 = Trochlea, 10 = Superior tarsus, 11 = Sclera, 12 = Optic nerve
Gray785.png
Figure showing the mode of innervation of the Recti medialis and lateralis of the eye.
Details
Origin common tendinous ring at the orbital apex
Insertion 5.5 mm medial to the limbus
Nerve inferior division of the oculomotor nerve
Actions adducts the eyeball (makes it move inwards)
Identifiers
Latin musculus rectus medialis bulbi
TA98 A15.2.07.012
TA2 2044
FMA 49037
Anatomical terms of muscle

The medial rectus muscle is a muscle in the orbit near the eye. It is one of the extraocular muscles. It originates from the common tendinous ring, and inserts into the anteromedial surface of the eye. It is supplied by the inferior division of the oculomotor nerve (III). It rotates the eye medially (adduction).

Structure

The medial rectus muscle shares an origin with several other extrinsic eye muscles, the common tendinous ring. It inserts into the anteromedial surface of the eye. This insertion has a width of around 11 mm.

Nerve supply

The medial rectus muscle is supplied by the inferior division of the oculomotor nerve (III). A branch of it enters the muscle around two fifths along its length. It usually divides into 2 smaller branches, occasionally 3. These further subdivide, becoming smaller down the length of the muscle until they become imperceptible to standard staining around 17 mm from the insertion of the muscle.

Relations

The insertion of the medial rectus muscle is around 7.5 mm from the insertion of the superior rectus muscle, and around 6 mm from the inferior rectus muscle. It is shorter but stronger than the other orbital recti muscles. It rarely changes position significantly when it contracts, unlike the other extraocular muscles.

Function

The medial rectus muscle rotates the eye medially (adduction). It works using a pulley system as it curves around the anterior surface of the eye.

Clinical significance

Strabismus

Strabismus (lazy eye) may be caused by a medial rectus muscle that is located too high in the orbit of the skull.

Esotropia (convergent strabismus) may also be caused by sixth nerve palsy, which causes weakness or paralysis of the lateral rectus muscle. Sometimes, botulinum toxin may be injected into the medial rectus muscle. Whilst this reduces the ability to abduct and adduct the eye for tracking, it corrects the esotropia and so generally improves vision.

Compression

The medial rectus muscle lies directly adjacent to the orbit of the skull. This leaves it vulnerable to being compressed (incarcerated) during skull fractures, which can prevent movement of the eye. This usually resolves when skull fractures are fixed.

Surgical damage

The medial rectus muscle may be damaged during eye surgery or skull surgery, such as functional endoscopic sinus surgery. The damage can be minor, such as bruising, or severe, such as cutting through the muscle partially or completely, and nerve injury.

Additional images

See also

External links


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