Inferior Rectus
Extraocular · Head & Neck
The inferior rectus is the extraocular muscle that principally depresses the eyeball. It arises from the inferior part of the common tendinous ring, the annulus of Zinn, at the orbital apex and passes forward and laterally to insert into the inferior sclera about 6.5 mm behind the corneal limbus. Owing to the divergence between the orbital and visual axes, its primary action is depression, accompanied by secondary adduction and extorsion (lateral rotation). It is innervated by the inferior division of the oculomotor nerve (CN III).
Origin, Insertion, Action & Nerve
Origin
Inferior part of the common tendinous ring (annulus of Zinn) at the orbital apex
Insertion
Inferior sclera, ~6.5 mm posterior to the corneal limbus
Action
Depresses, adducts, and extorts (laterally rotates) the eyeball
Nerve
Inferior division of the oculomotor nerve (CN III)
Attachments explained
The Inferior Rectus is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Inferior part of the common tendinous ring (annulus of Zinn) at the orbital apex
Insertion
Inferior sclera, ~6.5 mm posterior to the corneal limbus
Action & function
When the Inferior Rectus contracts, it produces the following movement: Depresses, adducts, and extorts (laterally rotates) the eyeball.
As part of the Extraocular group of the Head & Neck, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The Inferior Rectus receives its nerve supply from the Inferior division of the oculomotor nerve (CN III).
Because a muscle can only contract when its nerve is intact, injury to the Inferior division of the oculomotor nerve can weaken or paralyse the Inferior Rectus, impairing the movements it normally produces (depresses, adducts, and extorts (laterally rotates) the eyeball). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Inferior Rectus (the OIANS method)
Mirror the inferior rectus against the superior rectus: it sits below the eye and pulls it down, and its secondary actions of adduction and extorsion are the opposite torsion to its superior counterpart. As with the superior rectus, its pulling line lies medial to the visual axis, so depression is purest when the eye is abducted, which is exactly the position used to test it clinically.
Anchor its nerve by noting that the inferior division of the oculomotor nerve supplies the inferior rectus, medial rectus, and inferior oblique, sparing only the superior rectus and levator among the CN III muscles. Holding this grouping in mind lets you predict which movements survive a partial oculomotor lesion and keeps the two divisions of the nerve straight.
Frequently asked questions
Where is the Inferior Rectus located?
The Inferior Rectus is a muscle of the Extraocular group, located in the Head & Neck.
What is the origin of the Inferior Rectus?
Inferior part of the common tendinous ring (annulus of Zinn) at the orbital apex
What is the insertion of the Inferior Rectus?
Inferior sclera, ~6.5 mm posterior to the corneal limbus
What movements does the Inferior Rectus produce?
Depresses, adducts, and extorts (laterally rotates) the eyeball
What nerve supplies the Inferior Rectus?
Inferior division of the oculomotor nerve (CN III)
Is the Inferior Rectus free to study in OIANS?
The Inferior Rectus is always free to browse. Its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory. Quiz and Flashcard practice for the Head & Neck is part of the one-time Lifetime upgrade, though; only the Upper Limb decks are free to test yourself on.
Related muscles
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