medial rectus
Also called MR, medial rectus muscle, internal rectus
The medial rectus is the horizontal extraocular muscle on the nasal side of the globe, and its only action from primary position is adduction, turning the eye inward.1 It arises from the annulus of Zinn and inserts 5.5 mm behind the nasal edge of the cornea, the closest insertion of the four rectus muscles.1 The two medial recti working together are the muscles of convergence.2
The inferior division of the oculomotor nerve, the third cranial nerve, supplies the medial rectus.2,3 The antagonist of the medial rectus is the lateral rectus of the same eye.
An isolated loss of adduction with convergence still intact points to an internuclear ophthalmoplegia, a lesion of the brainstem pathway that drives the muscle, rather than a medial rectus palsy.
More detail
In a version the yoke of the medial rectus is the lateral rectus of the other eye, while in convergence its partner is the other medial rectus. Weakness produces an exotropia that is largest in gaze toward the nose. A long-standing sixth nerve palsy can tighten the medial rectus into a contracture, which then restricts abduction and gives positive forced ductions.4 The oculomotor nerve enters the muscle 15 mm from the origin.2
Sources (4)
- Kanski, Clinical Ophthalmology (2024), ch. 18
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 3 and ch. 4
- Adler's Physiology of the Eye, ch. 7, Table 7.1
- Yanoff and Duker, Ophthalmology (2023), ch. 9
Reviewed October 2026