Inferior rectus muscle
Inferior rectus muscle is one of the 4 straight muscles of the orbit responsible for the movement of the eye in the cardinal directions. These 4 muscles, along with the superior and inferior oblique muscles, belong to the group of the extraocular muscles. The inferior rectus muscle originates from the common tendinous ring, and goes on to attach at the lower anterior surface of the eyeball.
The primary action of the inferior rectus is depression of the eyeball. It acts in a coordinated manner with other extraocular muscles to control the movements of the eyeball and adjust the direction of the gaze.
This article will teach you all you need to know about the anatomy and functions of the inferior rectus muscle.
|Origin||Common tendinous ring (Annulus of Zinn)|
|Insertion||Anterior half of eyeball (posterior to corneoscleral junction)|
|Action||Depresses, adducts, externally rotates eyeball|
|Innervation||Oculomotor nerve (CN III)|
|Blood supply||Ophthalmic artery, infraorbital artery|
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.
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.
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.
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.
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.
Damage to the oculomotor nerve may cause a partial or complete paralysis of the extraocular muscles it supplies, as well as the levator palpebrae and sphincter pupillae muscles. This condition is called the external oculomotor palsy. A complete palsy involves all the muscles innervated by the oculomotor nerve, whereas partial oculomotor palsy involves specific muscles.
Which muscle is affected depends on which division of the oculomotor nerve is damaged. A palsy of the inferior rectus muscle can occur if the inferior branch of the oculomotor nerve is damaged. The most common cause is microangiopathy due to diabetes mellitus. The symptoms are reflected by the functions of the inferior rectus muscle, and include an inability to direct the gaze inferiorly and a double vision that worsens with abduction.
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