The medial rectus muscle is a

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

in the

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

near the eye. It is one of the

extraocular muscles The extraocular muscles (extrinsic ocular muscles), are the seven extrinsic muscles of the human eye. Six of the extraocular muscles, the four recti muscles, and the superior and inferior oblique muscles, control movement of the eye and the othe ...

. It originates from the

common tendinous ring The common tendinous ring, also known as the annulus of Zinn, or annular tendon, is a ring of fibrous tissue surrounding the optic nerve at its entrance at the apex of the orbit. It is the common origin of the four recti muscles of the group of e ...

, and inserts into the anteromedial surface of the eye. It is supplied by the inferior division of the oculomotor nerve (III). It rotates the eye medially (adduction).

Structure

The medial rectus muscle shares an origin with several other extrinsic eye muscles, the

. It inserts into the anteromedial surface of the eye. This insertion has a width of around 11 mm.

Nerve supply

The medial rectus muscle is supplied by the inferior division of the oculomotor nerve (III). A branch of it enters the muscle around two fifths along its length. It usually divides into 2 smaller branches, occasionally 3. 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.

Relations

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 inferior rectus muscle is a muscle in the orbit near the eye. It is one of the four recti muscles in the group of extraocular muscles. It originates from the common tendinous ring, and inserts into the anteroinferior surface of the eye. It ...

. It is shorter but stronger than the other orbital

recti The rectus abdominis muscle, ( la, straight abdominal) also known as the "abdominal muscle" or simply the "abs", is a paired straight muscle. It is a paired muscle, separated by a midline band of connective tissue called the linea alba. It ex ...

muscles. It rarely changes position significantly when it contracts, unlike the other extraocular muscles.

Function

The medial rectus muscle rotates the eye medially (adduction). It works using a pulley system as it curves around the anterior surface of the eye.

Clinical significance

Strabismus

Strabismus Strabismus is a vision disorder in which the eyes do not properly align with each other when looking at an object. The eye that is focused on an object can alternate. The condition may be present occasionally or constantly. If present during a ...

(lazy eye) may be caused by a medial rectus muscle that is located too high in the

of the

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

Esotropia Esotropia is a form of strabismus in which one or both eyes turns inward. The condition can be constantly present, or occur intermittently, and can give the affected individual a "cross-eyed" appearance. It is the opposite of exotropia and usual ...

(convergent strabismus) may also be caused by

sixth nerve palsy Sixth nerve palsy, or abducens nerve palsy, is a disorder associated with dysfunction of cranial nerve VI (the abducens nerve), which is responsible for causing contraction of the lateral rectus muscle to abduct (i.e., turn out) the eye. The ina ...

, which causes

weakness Weakness is a symptom of a number of different conditions. The causes are many and can be divided into conditions that have true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, i ...

or paralysis of the

lateral rectus muscle Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

. Sometimes, botulinum toxin may be injected into the medial rectus muscle. Whilst this reduces the ability to abduct and adduct the eye for tracking, it corrects the esotropia and so generally improves vision.

Compression

The medial rectus muscle lies directly adjacent to the

of the

. This leaves it vulnerable to being compressed (incarcerated) during skull fractures, which can prevent movement of the eye. This usually resolves when skull fractures are fixed.

Surgical damage

The medial rectus muscle may be damaged during eye surgery or skull surgery, such as functional endoscopic sinus surgery. The damage can be minor, such as

bruising A bruise, also known as a contusion, is a type of hematoma of tissue, the most common cause being capillaries damaged by trauma, causing localized bleeding that extravasates into the surrounding interstitial tissues. Most bruises occur clos ...

, or severe, such as cutting through the muscle partially or completely, and

nerve injury Nerve injury is an injury to nervous tissue. There is no single classification system that can describe all the many variations of nerve injuries. In 1941, Seddon introduced a classification of nerve injuries based on three main types of nerve f ...

Additional images

File:Eye_movements_adductors.jpg, Eye movement of medial rectus muscle, superior view. File:Gray869.png, Horizontal section of the eyeball. File:Gray890.png, Dissection showing origins of right ocular muscles, and nerves entering by the superior orbital fissure. File:ThreeNeuronArc.png , Vestibulo-ocular reflex File:Slide7uu.JPG, Medial rectus muscle File:Slide14uu.JPG, Medial rectus muscle File:Slide3aba.JPG, Extrinsic eye muscle. Nerves of orbita. Deep dissection. File:Slide1abaa.JPG, Extrinsic eye muscle. Nerves of orbita. Deep dissection. File:Slide2abaa.JPG, Extrinsic eye muscle. Nerves of orbita. Deep dissection. File:Slide3abab.JPG, Extrinsic eye muscle. Nerves of orbita. Deep dissection. File:Slide4abab.JPG, Extrinsic eye muscle. Nerves of orbita. Deep dissection. File:Slide5abab.JPG, Extrinsic eye muscle. Nerves of orbita. Deep dissection.

References

External links

* * ()
Diagram at howstuffworks.com
{{Authority control Muscles of the head and neck Human eye anatomy de:Augenmuskeln#Musculus rectus medialis