The inferior 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 four

recti 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 ...

in the group of extraocular muscles. It originates from the common tendinous ring, and inserts into the anteroinferior surface of the eye. It depresses the eye (downwards).

Structure

The inferior rectus muscle originates from the common tendinous ring (annulus of Zinn). It inserts into the anteroinferior surface of the eye. This insertion has a width of around 10.5 mm. It is around 7 mm from the corneal limbus.

Blood supply

The inferior rectus muscle is supplied by an inferior muscular branch of the

ophthalmic artery The ophthalmic artery (OA) is an artery of the head. It is the first branch of the internal carotid artery distal to the cavernous sinus. Branches of the ophthalmic artery supply all the structures in the orbit around the eye, as well as some ...

. It may also be supplied by a branch of the

infraorbital artery The infraorbital artery is a small artery in the head that arises from the maxillary artery and passes through the inferior orbital fissure to enter the orbit, then passes forward along the floor of the orbit, finally exiting the orbit through the ...

. It is drained by the corresponding veins: the inferior muscular branch of the

ophthalmic vein Ophthalmic veins are vein Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those ...

, and sometimes a branch of the infraorbital vein.

Nerve supply

The inferior rectus muscle 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).

Development

The inferior rectus muscle develops from the embryonic

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...

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 ...

. This is similar to the other

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 ...

Relations

The insertion of the inferior rectus muscle is around 6 mm from the insertion of the medial rectus muscle, and around 8 mm from the insertion 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 ...

. A parasympathetic branch that supplies the ciliary muscles of the

pupil The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black becau ...

passes close to the inferior rectus muscle.

Variation

Very rarely, the inferior rectus muscle may be congenitally absent. This may cause inferior rectus palsy, where the eye cannot be depressed.

Function

The inferior rectus muscle depresses, adducts, and helps extort the eye. It is the only muscle that is capable of depressing the

when it is in a fully abducted position.

Clinical significance

Strabismus

If the inferior rectus muscle is damaged,

weak Weak may refer to: Songs * Weak (AJR song), "Weak" (AJR song), 2016 * Weak (Melanie C song), "Weak" (Melanie C song), 2011 * Weak (SWV song), "Weak" (SWV song), 1993 * Weak (Skunk Anansie song), "Weak" (Skunk Anansie song), 1995 * "Weak", a son ...

, or paralysed, this can cause

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 ...

. This can lead to elevation of the

, as 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 ...

remains stronger. For minor cases, prism glasses can be used to gradually realign the eye. Alternatively for serious cases, it may be surgically corrected by slightly weakening the

(opposite) - this reduces the elevation of the eye, and corrects the strabismus. This procedure may lead to overcorrection of the strabismus, but is otherwise generally successful.

Surgery

Any

surgery Surgery is a medical specialty that uses manual and instrumental techniques to diagnose or treat pathological conditions (e.g., trauma, disease, injury, malignancy), to alter bodily functions (e.g., malabsorption created by bariatric surgery s ...

on the inferior rectus muscle may damage the parasympathetic branches to the ciliary muscles of the

. This may cause problems with control of the pupil. Nearby blood vessels and nerves may also be damaged.

Additional images

File:Slide12uu.JPG, Inferior rectus muscle File:Slide2abaa.JPG, Extrinsic eye muscle. Nerves of orbita. Deep dissection.

References

External links

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