The oculomotor nerve is the third of 12 pairs of cranial nerves in the brain. This nerve is responsible for eyeball and eyelid movement. It follows the olfactory and optic nerves in terms of order.
The oculomotor nerve involves two separate components, each of which has a distinct function.
The somatic motor component supplies four extraocular muscles in the eye and the upper eyelid's levator palpebrae superioris with motor (movement) fibers. It controls the muscles that allow for visual tracking and fixation by the eye. Visual tracking is the ability to follow an object as it moves across the field of vision. Fixation is the ability to focus on a stationary object.
The visceral motor component controls parasympathetic innervation (nerves related to involuntary actions) of the ciliary muscles and constrictor papillae, aiding in accommodation and pupillary light reflexes. Accommodation is the ability of the eye to keep an object in focus as the object’s distance from the eye changes. Pupillary light reflexes are automatic changes in dilation (size) of the pupil, which regulate the amount of light that enters the eye, making sure the light is enough to see but not too bright.
The oculomotor nerve can become paralyzed in a condition known as oculomotor nerve palsy. This condition can result from multiple sclerosis or other demyelinating diseases, direct trauma, space-occupying lesions (such as brain cancer), microvascular disease (such as diabetes), or spontaneous subarachnoid hemorrhage (bleeding into the space between two of the membranes that cover the brain). A berry aneurysm is a type of subarachnoid hemorrhage.
The oculomotor nerve is responsible for the majority of eye and eyelid movements, although the trochlear nerve and abducens nerve also contribute to eye movements.