Eyes that remained fixed in one spot would severely limit your ability to take in the world around you. Fortunately, nerves transmit signals from the brain to the muscles attached to the eye to make them move up, down and sideways. Nerves also tell the pupil when to dilate, so a more complete view can be seen in times of stress or danger. Three cranial nerves control eye movement: cranial nerves III, IV and VI; all must work together.
Cranial Nerve III
Cranial nerve III is called the oculomotor nerve. This is an apt name because the nerve transmits signal to four of the six muscles used to move the eye as well as the muscle that moves the upper eyelid, called the levator palpebrae superioris muscle.
Each muscle has a main function as well as secondary and tertiary functions, according to optometric phyisician Ted Montgomery. The inferior rectus primarily moves the eye down but also moves it away from the nose and inward. The superior rectus moves the eye upward, toward the nose and inward. The inferior oblique moves the eye away from the nose, upward and outward. The medial rectus turns the eye toward the nose. The eye muscles work together to make all the intricate movements required for you to see the world above, below and to the side of you.
The oculomotor nerve also controls involuntary pupil and ciliary body movement; pupil movement dilates and constricts the eye, and the ciliary body helps the eye accommodate, or focus on, close-up objects. Damage to the oculomotor nerve can cause the eye to look down and outward; the eyelid may droop, and the pupil will remain dilated all the time.
Cranial Nerve IV
Cranial nerve IV, also called the trochlear nerve, transmits signals to the superior oblique muscle, which turns the eye toward the nose primarily, downward secondarily and outward as its tertiary function. Damage to the trochlear nerve results in double vision, which causes people to tilt their heads toward the shoulder of the unaffected eye, Montgomery explains.
Cranial Nerve VI
Cranial nerve VI is called the abducens nerve. This nerve is attached to the lateral rectus muscle of the eye, which moves the eye laterally so that it looks away from the nose. Damage to the abducens nerve leaves the eye turned slightly inward and causes double vision, which leads to turning the face in the direction of the affected eye to see clearly.
