Extraocular muscles · 16 of 24
What does the medial rectus do?
The medial rectus is the largest and strongest of the extraocular muscles. It runs from the annulus of Zinn along the medial orbital wall to insert about five and a half millimetres behind the nasal limbus, and it does one thing: it turns the eye in. It works with the lateral rectus of the other eye to move both eyes to one side, and with the other medial rectus to converge on something near.
- Action
- Adduction only
- Nerve
- Inferior division of the oculomotor nerve (CN III)
- Yoked with
- The opposite lateral rectus, through the medial longitudinal fasciculus
- Also used in
- Convergence, with the near reflex
Horizontal gaze
Looking to the right needs the right lateral rectus and the left medial rectus to contract together, and the two are wired together in the brainstem: the sixth nerve nucleus on the right sends fibres across to the left third nerve nucleus through the medial longitudinal fasciculus. Damage that bundle and the left eye cannot adduct on right gaze although it still converges: an internuclear ophthalmoplegia, the classic brainstem sign of multiple sclerosis in the young and of a small stroke in the old.
When it fails or overacts
A complete third nerve palsy takes the medial rectus and the eye sits abducted, unable to look in. An internuclear ophthalmoplegia spares the muscle and the nerve but cuts the signal, so adduction fails on side gaze and works on convergence, with nystagmus in the other, abducting eye. In childhood esotropia the medial recti overact: a convergent squint, often accommodative in a long-sighted child, and the treatment starts with glasses.
Examine it
Paired with the opposite lateral rectus in horizontal gaze through the medial longitudinal fasciculus.
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