Superior rectus muscle
Superior rectus is one of the extrinsic muscles of the eye. Being located outside the eyeball but within the orbit, it belongs to a group called the extraocular muscles. This group of muscles serves to move the eyes within the orbit. It includes the superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique and inferior oblique muscles. Notice how these muscles can be divided into four recti and two oblique muscles.
The four recti muscles all arise from a connective tissue ring called the common tendinous ring (annulus of Zin). This is located at the apex of orbit, surrounding the optic canal. Respectively, the recti muscles insert onto the superior, inferior, medial and lateral sides of the eyeball. Based on their global attachments, these muscles serve to move the eyes in the four cardinal directions, with superior rectus producing elevation, adduction and internal rotation of the eyeball.
|Origin||Common tendinous ring (Annulus of Zinn)|
|Insertion||Anterior half of eyeball (posterior to corneoscleral junction)|
|Action||Elevates, adducts, internally rotates eyeball|
|Innervation||Oculomotor nerve (CN III)|
|Blood supply||Ophthalmic artery, supraorbital branch of ophthalmic artery|
This article will discuss the anatomy and function of the superior rectus muscle.
Origin and insertion
Superior rectus spans from the common tendinous ring (annulus of Zinn) to the superior aspect of the eyeball, just posterior to the corneal limbus (corneoscleral junction). The largest part of superior rectus originates from the superior portion of the common tendinous ring, superolaterally to the optic canal. While a smaller portion arises from the dura mater that surrounds the optic nerve (CN II).
The muscle then courses anterolaterally, crossing over the eyeball’s equator to reach and insert onto the anterior half of the sclera superiorly. The line of insertion is oblique, with medial fibers attaching more anterior than the lateral.
The originating fibers of superior rectus are located inferolaterally to superior oblique and inferomedially to the levator palpebrae superioris muscle. Since levator palpebrae superioris eventually takes a more medial course, it covers the majority of the superior rectus’ superior surface.
The most distal part of the muscle runs superficially to the tendon of superior oblique muscle. The superior branch of oculomotor nerve (CN III) courses over the proximal third of the muscle’s inferior surface before it pierces the muscle belly to provide innervation. The lateral and medial sides of the muscle aren’t related to any particular structure, but rather are cushioned within the periorbital fat tissue.
At the site of its insertion, superior rectus penetrates the fascial sheath of the eyeball (Tenon’s capsule), which in turn reflects back on the muscle’s tendon and creates a thin fascial sleeve. This sleeve has an expansion called the check ligament that connects superior rectus with levator palpebrae superioris muscle and ensures that the two muscles act simultaneously.
This muscle is innervated by the superior branch of the oculomotor nerve (CN III). Note that these fibers of oculomotorius originate from the nucleus of oculomotor nerve which is a somatic motor nucleus, thus providing voluntary control over this muscle.
Contraction of superior rectus causes the eye to simultaneously move in several planes; elevation in the transverse plane, adduction in the vertical plane and internal rotation (intorsion) in the anteroposterior plane. These contrasting movements are achieved because the muscle fibers course obliquely; at a 23 degrees angle with the median plane of the eye.
However, when shifting the gaze upwards, the eyeball can elevate without the movements of adduction and internal rotation. That’s because superior rectus usually contracts together with the inferior oblique muscle, whose fibers and pull are oriented in a way to oppose the adduction and internal rotation produced by the superior rectus and decrease them to a minimum. Thus, the end result is elevation of the eyeball.
Being connected with the levator palpebrae superioris via check ligament, superior rectus contraction is also associated with the elevation of the upper eyelid.