The inferior 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 four

recti 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 oth ...

in the group of

extraocular muscle 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 oth ...

s. 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 ex ...

, and inserts into the anteroinferior surface of the eye. It depresses the eye (downwards).

Structure

The inferior rectus muscle originates from the

(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 The corneal limbus (''Latin'': corneal border) is the border between the cornea and the sclera (the white of the eye). It contains stem cells in its palisades of Vogt. It may be affected by cancer or by aniridia (a developmental problem), among ...

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 st ...

. It may also be supplied by a branch of the

infraorbital artery The infraorbital artery is an artery in the head that branches off the maxillary artery, emerging through the infraorbital foramen, just under the orbit of the eye. Course The infraorbital artery appears, from its direction, to be the continuatio ...

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

ophthalmic vein Ophthalmic veins are vein Veins are blood vessels in 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 the pulmonary and umbilical veins, bot ...

, 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 of ...

(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 E ...

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, the ...

. This is similar to the other

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 ot ...

Relations

The insertion of the inferior rectus muscle is around 6 mm from the insertion of the

medial rectus muscle The medial rectus muscle is a muscle in the orbit near the eye. It is one of the extraocular muscles. It originates from the common tendinous ring, and inserts into the anteromedial surface of the eye. It is supplied by the inferior division of t ...

, and around 8 mm from the insertion 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 cons ...

. A parasympathetic branch that supplies the

ciliary muscles The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscleSchachar, Ronald A. (2012). "Anatomy and Physiology." (Chapter 4) . in the eye's middle layer, uvea (vascular layer). It controls accommodation for viewing o ...

of the

pupil The pupil is a black 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 ...

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 An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all co ...

, and helps

extort Extortion is the practice of obtaining benefit through coercion. In most jurisdictions it is likely to constitute a criminal offence; the bulk of this article deals with such cases. Robbery is the simplest and most common form of extortion, al ...

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, or

paralysed Paralysis (also known as plegia) is a loss of motor function in one or more muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory damage. In the United States, roughly 1 in 50 ...

, this can cause

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 ...

. This can lead to elevation of the eye, 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 ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical specialty that uses operative manual and instrumental techniques on a person to investigate or treat a pat ...

on the inferior rectus muscle may damage the parasympathetic branches to the

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