Eye muscle anatomy | Overview
There are six extraocular muscles that move the globe (eyeball). These muscles are named the superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique.
Upgaze, or turning the eye upward, is primarily the work of the superior rectus muscle, with some contribution by the inferior oblique muscle.
Downgaze, or turning the eye downward, is primarily the work of the inferior rectus, with some contribution by the superior oblique.
Abduction, or turning the eye outward toward the ear, is primarily done by the lateral rectus.
Adduction, or turning the eye inward toward the nose, is primarily done by the medial rectus.
The eye is rotated medially by the superior rectus and superior oblique, and is rotated laterally by the inferior rectus and inferior oblique. In addition, the levator palpebrae superioris muscle, which is not seen on the drawing, elevates the eyelid.
The extraocular muscles are innervated by three cranial nerves (CN), CN III (oculomotor nerve), CN IV (trochlear nerve), and CN VI (abducens nerve). The relationship between the cranial nerve nuclei in the brainstem, the cranial nerves, and the muscles that the nerves innervate can be visualized in the schematic below.
CN VI and IV are fairly straightforward. The paired right and left CN VI arise from the pons in the midbrain, and send their axons into the orbits to innervate the right and left lateral rectus muscles, respectively. Therefore, CN VI is responsible for abducting each eye (turning it to look laterally or toward the ear). The paired right and left trochlear nuclei are in the midbrain. Their axons, which make up CN IV, exit the midbrain, cross the midline, and send their axons into the orbits to innervate the left and right superior oblique muscles, respectively. Therefore, CN IV is primarily responsible for turning each eye downward when it is already looking inward toward the nose.
CN III is a bit more complicated, as it innervates all of the remaining extraocular muscles. Therefore, each oculomotor nucleus is actually made up of overlapping subnuclei, and each subnucleus sends its axons to innervate a specific extraocular muscle. The right and left oculomotor nuclei are located in the midbrain. The axons from the right or left nucleus leave the midbrain and come together to form the body of the right or left CN III. As the nerve enters the orbit, it splits into the superior branch of CN III and the inferior branch of CN III. The superior branch of CN III innervates the superior rectus and the levetor palpebrae superioris. The lower branch innervates the medial rectus, inferior rectus, and inferior oblique.
If you would like to review the result of malfunction of specific extraocular muscles and cranial nerves, you are likely to find the UC Davis Health System Neurological Eye simulator fun and educational. The eyes follow the mouse around the screen. A word of caution, however: consider closing all essential documents before clicking on the link. Otherwise, it may freeze your computer.