Logan Steele
The human eye has six muscles that control eye movement, which are split into two primary groups: the recti muscles and the oblique muscles. The superior rectus muscle is the primary muscle responsible for elevating the eye, allowing you to look up. The superior oblique muscle, on the other hand, is responsible for turning the eye inward. The inferior oblique muscle elevates the eye when looking towards the nose. These muscles work in pairs to control and balance eye movements, ensuring synchronised vision and enabling capabilities such as depth perception and three-dimensional (3D) vision.
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Superior rectus
The superior rectus is an extrinsic muscle of the eye, located outside the eyeball but within the orbit. It is one of the four rectus muscles, along with the inferior rectus, medial rectus, and lateral rectus. The superior rectus is responsible for elevating the eye, particularly when it is in a fully abducted position. It is also involved in adduction and internal rotation of the eye.
The superior rectus originates from the annulus of Zinn, also known as the common tendinous ring, and inserts into the anterosuperior surface of the eye. The insertion point is located approximately 7.5 mm from the medial rectus muscle and 7.1 mm from the lateral rectus muscle. The muscle receives its nerve supply from the superior division of the oculomotor nerve (CN III), which provides voluntary control over its movement.
The superior rectus muscle works in conjunction with other eye muscles to enable a range of eye movements. It is particularly important in elevating the eye vertically and rotating it internally. This muscle is also associated with several medical conditions, including weakness, paralysis, and overactivity, which can impact the ability to elevate the eye during normal vision.
Variations in the structure of the superior rectus muscle are rare. However, in some rare cases, it may be congenitally absent, which can be caused by Apert syndrome. This condition results in a reduced ability to elevate the eye. Additionally, the superior rectus muscle is related to other extraocular muscles, especially the medial rectus muscle and the lateral rectus muscle.
The superior rectus muscle plays a crucial role in eye movement and coordination, working in tandem with other muscles to enable us to see and perceive our surroundings accurately.
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Superior oblique
The superior oblique muscle, or "obliquus oculi superior", is a fusiform muscle that originates in the upper medial side of the orbit (beside the nose). It is one of six extraocular muscles, which completely surround the eyeball, facilitating its movement in various directions. The superior oblique is the longest muscle in this group, spanning from the body of the sphenoid bone to the superolateral aspect of the eyeball.
The primary action of the superior oblique muscle is intorsion (internal rotation), the secondary action is depression (primarily in the adducted position), and the tertiary action is abduction (lateral rotation). The superior oblique muscle works like a pulley, starting at the upper back of the eye and extending forward, then threading through a small bony opening on the upper-inner side of the eye socket called the trochlea of superior oblique. This cartilaginous loop is found attached to the nasal part of the frontal bone. The unique pulley action of the superior oblique along the trochlea is due to the specialised development of the muscle. Embryologic studies show that at an early stage of development, the muscle itself, the insertion tendon, and the cartilaginous pulley travel together in a straight line. At around 12 weeks, the tendon then begins to turn around the trochlear cartilage, creating the pulley action.
The superior oblique is one of two oblique extraocular muscles, which are unique in that they do not originate from the common tendinous ring, but instead externally, superomedially to the ring. The superior oblique is also the only extraocular muscle that receives its innervation through the trochlear nerve (CN IV). The innervation of the superior oblique muscle is distinct from the remaining extraocular muscles in that its nerve supply is from the trochlear nerve (CN IV). Most of the remaining extraocular muscles are innervated by the oculomotor nerve (CN III), except for the lateral rectus, which receives innervation via the abducens nerve (CN VI).
The depressing action of the superior oblique (making the eye look down towards the mouth) is most effective when the eye is in an adducted position. This is because, as the eye is abducted (looks laterally), the contribution made by the superior oblique to depression decreases, as the inferior rectus muscle causes this movement more directly and powerfully.
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Inferior rectus
The inferior rectus is a narrow, strap-shaped muscle of the orbit near the eye. It is one of the four rectus muscles, which also include the superior rectus, the medial rectus, and the lateral rectus. The inferior rectus is also one of the seven extraocular muscles, which also include two oblique muscles: the superior and inferior obliques. The seventh extraocular muscle is the levator palpebrae superioris.
The inferior rectus muscle originates from the common tendinous ring, also called the annulus of Zinn, which is found in the posterior pole of the orbit encircling the margins of the optic canal. It inserts into the anteroinferior surface of the eye, with an insertion width of around 10.5 mm. The insertion is approximately 6 mm from the insertion of the medial rectus muscle and 8 mm from the insertion of the lateral rectus muscle. The inferior rectus passes along the floor of the orbit, superior to the infraorbital canal, and inserts onto the anteromedial part of the sclera. In this region, the fascial sheath of the inferior rectus muscle is thickened and blended with the sheath of the inferior oblique muscle.
The inferior rectus muscle is supplied by the inferior division of the oculomotor nerve (CN III), which provides it with general somatic efferent fibres. It receives its arterial blood supply from the ophthalmic artery and the infraorbital artery, which arise from the internal carotid and maxillary arteries, respectively. The inferior rectus muscle is drained by the corresponding veins: the inferior muscular branch of the ophthalmic vein and sometimes a branch of the infraorbital vein.
The primary action of the inferior rectus muscle is to depress the eye, causing the cornea and pupil to move inferiorly. It is the only muscle capable of depressing the pupil when it is in a fully abducted position. In addition to depression, the inferior rectus muscle also helps adduct and extort the eye. These three actions are exhibited simultaneously due to the muscle's oblique course and attachment to the lower anteromedial half of the sclera. The inferior rectus works in coordination with other extraocular muscles to control the movements of the eyeball and adjust the direction of gaze.
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Medial rectus
The medial rectus is one of the six extraocular muscles located in the orbit. It is the shortest but strongest of the four recti muscles. These muscles are responsible for moving the eyes in the four cardinal directions. The medial rectus muscle moves the eye from side to side (eye adduction) with the lateral rectus, an abductor.
The medial rectus muscle is innervated by the inferior branch of the oculomotor nerve (CN III). The medial rectus muscle originates from the common tendinous ring, inserting into the anteromedial surface of the eye. The insertion of the medial rectus muscle is around 7.5 mm from the insertion of the superior rectus muscle and around 6 mm from the inferior rectus muscle. The medial rectus muscle rarely changes position significantly when it contracts, unlike the other extraocular muscles.
The medial rectus muscle is supplied by the inferior division of the oculomotor nerve (CN III). A branch of it enters the muscle around two-fifths along its length. It usually divides into two smaller branches, occasionally three. These further subdivide, becoming smaller down the length of the muscle until they become imperceptible to standard staining around 17 mm from the insertion of the muscle.
The medial rectus muscle lies directly adjacent to the orbit of the skull, making it vulnerable to being compressed during skull fractures, which can prevent movement of the eye. The medial rectus muscle may also be damaged during eye surgery or skull surgery, such as functional endoscopic sinus surgery.
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Lateral rectus
The lateral rectus is one of the four rectus muscles in each eye, along with the superior, inferior, and medial rectus muscles. These muscles are responsible for the movement of the eye. Each eye has six muscles that control its movement, and together, they enable the eye to move in every direction.
The lateral rectus muscle is a flat, strap-shaped muscle that is wider in its anterior part. It originates in the bottom of the orbital cavity, specifically in the lateral part of the common tendinous ring, also known as the annulus of Zinn. From there, it runs anteriorly and across the lateral part of the orbit to insert at the lateral side of the eyeball. The muscle inserts about 6.9mm to 8mm from the limbus of the cornea.
The lateral rectus muscle is the only muscle supplied by the abducens nerve (CN VI), which enters its medial surface and provides general somatic efferent fibers. The ophthalmic artery, a branch of the internal carotid artery, supplies the lateral rectus muscle either directly or through its lacrimal branch. The lacrimal artery runs along the superior border of the lateral rectus muscle to supply the lacrimal gland.
The primary action of the lateral rectus muscle is the abduction of the eyeball, allowing the eye to turn laterally in the orbit. It works in coordination with the medial rectus muscle of the other eye, which needs to relax when the lateral rectus muscle is active. This movement is also assisted by the superior and inferior oblique muscles. Abnormalities in the lateral rectus muscle can lead to disorders such as strabismus and Duane Syndrome, which result in misalignment of the eyes and impaired eye movement.
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Frequently asked questions
The superior rectus is the muscle that helps elevate the eye, allowing us to look up.
The superior rectus is a rectus muscle, also known as a "straight" muscle due to its direct path from origin to attachment.
The inferior oblique muscle elevates the eye when looking towards the nose, as opposed to looking away.
