Nerve supply

The medial rectus muscle is supplied by the inferior division of the oculomotor nerve (III).^[2] A branch of it enters the muscle around two fifths along its length.^[2] It usually divides into 2 smaller branches, occasionally 3.^[2] 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.^[2]

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.^[1] It is shorter but stronger than the other orbital recti muscles.^[3] It rarely changes position significantly when it contracts, unlike the other extraocular muscles.^[4]

Strabismus

Strabismus (lazy eye) may be caused by a medial rectus muscle that is located too high in the orbit of the skull.^[4]

Esotropia (convergent strabismus) may also be caused by sixth nerve palsy, which causes weakness or paralysis of the lateral rectus muscle.^[6] Sometimes, botulinum toxin may be injected into the medial rectus muscle.^[6] Whilst this reduces the ability to abduct and adduct the eye for tracking, it corrects the esotropia and so generally improves vision.^[6]

Compression

The medial rectus muscle lies directly adjacent to the orbit of the skull.^[7] This leaves it vulnerable to being compressed (incarcerated) during skull fractures, which can prevent movement of the eye.^[7][8] This usually resolves when skull fractures are fixed.^[7]

Surgical damage

The medial rectus muscle may be damaged during eye surgery or skull surgery, such as functional endoscopic sinus surgery.^[9] The damage can be minor, such as bruising, or severe, such as cutting through the muscle partially or completely, and nerve injury.^[9]