Muscles

Extraocular muscles

Within the orbit there are six muscles which help control movements of the eye. A large proportion of these muscles comprise only of striated muscle. However, those that have a role in movements of the eyelids can also contain smooth muscle. 

                         Extraocular muscles image courtesy of Webvision. All rights reserved.

The extraocular muscles include the four recti muscles and the two obliques. Another muscle called the levator palpebrae superioris can also be grouped with these as it causes elevation of the upper eyelid.  

  Levator palpebrae superioris

·         This is the most superior of the orbital muscles. It attaches to the lesser wing of the sphenoid where it creates a large, wide tendon. This is then free to insert into the upper eyelid to causes elevation.

 Recti

The tendinous ring which surrounds the optic canal, along with part of the superior orbital fissure, serves as the common origin for this group of muscles. Each muscle then passes forwards to the junction where the cornea and sclera meet. They then insert to their corresponding region on the eyeball, in order to carry out a particular function.

·         The medial rectus is located on the medial side of each eyeball. Its only function is to serve during adduction, for example, to bring the pupil closer to the midline of the body. It is innervated by the inferior division of the occulomotor nerve (Cranial Nerve III).

·         The lateral rectus causes abduction of the eyeball e.g. to pull the pupil away from the midline of the body. This muscle is supplied by the abducens nerve (CN VI). 

·         The superior rectus is involved in elevation of the eyeball. However, it can also contribute to intorsion (inward convergent rotation) and adduction. Innervation to this muscle is supplied by the occulomotor nerve (CN III).

·         The inferior rectus causes adduction, depression, lateral rotation and also extorts the eye.

 Obliques

·         The superior oblique attaches to the lesser wing of the sphenoid and to the common tendinous ring. It forms a tendon after passing between the medial wall and the roof of the orbit. This tendon traverses a loop of fibrous tissue called the trochlea and attaches to the superolateral region of the eyeball, beneath the superior rectus. It causes both abduction and depression and is innervated by the trochlear nerve (CN IV).

 ·         The inferior oblique attaches to the floor of the orbit, passing below the inferior rectus, it inserts into the inferolateral part of the eye behind the equator. It contributes to abduction and elevation of the eyeball. This muscle is innervated by the occulomotor nerve (CN III).