The Muscles Involved in Extraocular Movements and the Neruologic Ennervation of the Eye and Pupil

The eye is an extremely complex organ which is responsible for sight. This article discusses the muscles involved in extraocular movements of the eyes, and the neruologic innervation of the pupil.

There are six extraocular muscles which act to rotate an eye about its vertical, horizontal, and antero-posterior axes: the medial rectus (MR), the lateral rectus (LR), the superior rectus (SR), the inferior rectus (IR), the superior oblique (SO), and the inferior oblique (IO). A given extraocular muscle moves an eye in a specific manner, as follows:

• medial rectus (MR) moves the eye toward the nose
• lateral rectus (LR) moves the eye away from the nose
• superior rectus (SR) primarily moves the eye upward and secondarily rotates the top of the eye toward the nose
• inferior rectus (IR) primarily moves the eye downward and secondarily rotates the top of the eye away from the nose
• superior oblique (SO) primarily rotates the top of the eye toward the nose and secondarily moves the eye downward
• inferior oblique (IO) primarily rotates the top of the eye away from the nose and secondarily moves the eye upward

The primary muscle that moves an eye in a given direction is known as the agonist; A muscle in the same eye that moves the eye in the same direction as the agonist is known as a synergist, while the muscle in the same eye that moves the eye in the opposite direction of the agonist is the antagonist.

Each extraocular muscle is innervated by a specific cranial nerve:

• medial rectus (MR) cranial nerve III
• lateral rectus (LR) cranial nerve VI
• superior rectus (SR) cranial nerve III
• inferior rectus (IR) cranial nerve III
• superior oblique (SO) cranial nerve IV
• inferior oblique (IO) cranial nerve III

All of the extraocular muscles, with the exception of the inferior oblique, form a cone within the bony orbit. The apex of this cone is located in the posterior aspect of the orbit, while the base of the cone is the attachment of the muscles around the midline of the eye. Within the cone runs the optic nerve (cranial nerve II), and within the optic nerve are contained the ophthalmic artery and the ophthalmic vein.