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.

Pupillary constriction (miosis) and pupillary dilation (mydriasis), depend upon parasympathetic and sympathetic innervation respectively, of the muscles in the iris. Sympathetic axons arise in the posterolateral hypothalamus and descend uncrossed through the midbrain, pons, and lateral medulla. These first-order sympathetic axons continue down the cervical spinal cord to synapse at the intermediolateral cell column of the spinal cord at the level of C8 through T2.

The second-order preganglionic neurons exit the spinal canal, enter the paravertebral sympathetic chain and ascend to synapse in the superior cervical sympathetic ganglion at the bifurcation of the common carotid artery. Some of the third-order postganglionic neurons anastomose with the ophthalmic branch of the trigeminal nerve and enter the globe posteriorly. However, most of the sympathetic postganglionic neurons pass through the ciliary ganglion and penetrate the globe via the long ciliary nerves, then form an adrenergic terminal at the pupillodilator muscle.

Preganglionic parasympathetic fibers arise in the accessory oculomotor (Edinger-Westphal) nucleus found in the high midbrain, at the level of the superior colliculus. These axons lie superficially on the third cranial nerve, as it courses through the subarachnoid space, and enter the orbit via the superior orbital fissure to synapse in the ciliary ganglion. The postganglionic parasympathetic fibers enter the posterior portion of the globe via eight to twenty short ciliary nerves, and then terminate at the pupillary sphincter and ciliary body, to mediate pupillary constriction.