Inferior rectus muscle

Origin and insertion

The inferior rectus muscle is a narrow, strap-shaped muscle of the orbit that extends over the floor of the orbit. Alike most of the extraocular muscles, the inferior rectus muscle originates from the common tendinous ring, also called the annulus of Zinn, that is found in the posterior pole of the orbit encircling the margins of the optic canal.

As the muscle courses anterolaterally across the floor of the orbit, its middle part is thickened and then gradually thinned into a tendon. The tendon ends by inserting obliquely on the anteromedial surface of the eyeball, below the limbus of the cornea.

Relations

The inferior rectus muscle originates from the inferior part of the common tendinous ring, adjacent to the attachments of the medial, lateral and superior recti muscles. The common tendinous ring encircles the superior, medial and inferior margins of the optic canal and part of the superior orbital fissure. This is why the straight muscles that attach to it form a cone around the structures that pass through these openings. These structures include the optic sheath containing the optic nerve (CN II) and ophthalmic artery, the superior and inferior divisions of oculomotor nerve (CN III), the nasociliary nerve (CN V1) and abducens nerve (CN VI).

The inferior rectus muscle passes along the floor of the orbit superior to the infraorbital canal, which houses the infraorbital artery. As it inserts onto the anteromedial part of the sclera, the inferior rectus muscle is covered by the inferior oblique muscle. In this region, the fascial sheath of the inferior rectus muscle is thickened and blended with the sheath of the inferior oblique muscle. These fused sheaths give off an anterior expansion, called the inferior check ligament, which attaches itself on the tarsal plate of the inferior eyelid. 

The blended sheaths of the inferior rectus and oblique muscles then blend with the medial and lateral check ligaments, which are triangular sheet expansions of the medial and lateral recti muscles. Together, they form the suspensory ligament of the eyeball, a hammock-like sling that provides support to the eyeball.

Innervation

The inferior rectus is supplied by the inferior division of oculomotor nerve (CN III), which enters its superior surface and provides it with general somatic efferent fibers.

Blood supply

The inferior rectus muscle receives its arterial blood supply from ophthalmic artery and infraorbital artery, which arise from the internal carotid and maxillary artery, respectively.

Functions

Because of its oblique course and attachment on the lower anteromedial half of the sclera, the contraction of the inferior rectus produces three simultaneous actions:

• Depresses the anterior pole of the eyeball
• Adducts the eyeball medially
• Externally rotates the eyeball

In theory, these actions are exhibited with the isolated contraction of the inferior rectus muscle. However, in reality at least three extraocular muscles are involved in every eye movement, which means they work in a coordinated manner. For example, the inferior rectus muscle works both in synergy and opposition with the superior oblique muscle. They work in synergy to depress the pupil, but at the same time they oppose each other because the inferior rectus adducts and extorts the eyeball, whereas the superior oblique abducts and intorts the eyeball. This way these actions are neutralized and the net action is only depression of the eyeball. 

In addition, the aforementioned inferior check ligament that attaches to the tarsal plate of the lower eyelid also has a role in enabling the inferior gaze. By contracting, the inferior rectus muscle pulls on this ligament and depresses the eyelid.