The medial rectus muscle is a

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

in the

orbit In celestial mechanics, an orbit (also known as orbital revolution) 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 ...

near the

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

. It is one of the

extraocular muscles The extraocular muscles, or extrinsic ocular muscles, are the seven extrinsic muscles of the eye in human eye, humans and other animals. Six of the extraocular muscles, the four recti muscles, and the superior oblique muscle, superior and inferior ...

. It originates from the common tendinous ring, and inserts into the anteromedial surface of the eye. It is supplied by the inferior division of the

oculomotor nerve The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements o ...

(III). It rotates the eye medially (adduction).

Structure

The medial rectus muscle shares an origin with several other extrinsic eye muscles, the common tendinous ring. It inserts into the anteromedial surface of the

. This insertion has a width of around 11 mm.

Nerve supply

The medial rectus muscle is supplied by the inferior division of the

(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 Staining is a technique used to enhance contrast in samples, generally at the Microscope, microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissue (biology), tissues), in cytology (microscopic ...

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 The superior rectus muscle is a muscle in the orbit. It is one of the extraocular muscles. It is innervated by the superior division of the oculomotor nerve (III). In the primary position (looking straight ahead), its primary function is elevatio ...

, and around 6 mm from the inferior rectus muscle. It is shorter but stronger than the other orbital recti muscles. It rarely changes position significantly when it contracts, unlike the other extraocular muscles.

Function

The medial rectus muscle rotates the

medially (adduction). It works using a

pulley Sheave without a rope A pulley is a wheel on an axle or shaft enabling a taut cable or belt passing over the wheel to move and change direction, or transfer power between itself and a shaft. A pulley may have a groove or grooves between flan ...

system as it curves around the anterior surface of the eye.

Clinical significance

Strabismus

Strabismus Strabismus is an eye disorder in which the eyes do not properly align with each other when looking at an object. The eye that is pointed at 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, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

Esotropia Esotropia () is a form of strabismus in which one or both eyes turn 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 us ...

(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 inab ...

, which causes

weakness Weakness is a symptom of many different medical 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, ...

paralysis Paralysis (: paralyses; also known as plegia) is a loss of Motor skill, motor function in one or more Skeletal muscle, muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory d ...

of the

lateral rectus muscle The lateral rectus muscle is a muscle on the lateral side of the eye in the orbit. It is one of six extraocular muscles that control the movements of the eye. The lateral rectus muscle is responsible for lateral movement of the eyeball, specif ...

. Sometimes,

botulinum toxin Botulinum toxin, or botulinum neurotoxin (commonly called botox), is a neurotoxic protein produced by the bacterium ''Clostridium botulinum'' and related species. It prevents the release of the neurotransmitter acetylcholine from axon en ...

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

. 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 Functional endoscopic sinus surgery (FESS) is a procedure that is used to treat sinusitis and other conditions that affect the Paranasal sinuses, sinuses. Sinusitis is an inflammation of the sinuses that can cause symptoms such as congestion, he ...

. 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 clo ...

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

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 The vestibulo-ocular reflex (VOR) is a reflex that acts to stabilize Gaze (physiology), gaze during head movement, with eye movement due to activation of the vestibular system, it is also known as the cervico-ocular reflex. The reflex acts to im ...

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: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

Structure

Nerve supply

Relations

Function

Clinical significance

Strabismus

Compression

Surgical damage

Additional images

See also

References

External links