The third cranial nerve is the oculomotor nerve, which is largely responsible for most of the eye's movements. Specifically, it controls upper eyelid elevation; upward, downward and sideways eye movement; eye rotation; lens thickening; and pupil dilation. Damage or paralysis to the third cranial nerve results in the loss of one or a combination of these movements.
The third cranial nerve extends from the anterior mesencephalon, or midbrain. It bears two nuclei: the oculomotor nucleus found in the superior colliculi and the Edinger-Westphal nucleus located posterior to the oculomotor nucleus. Each nucleus innervates a separate set of muscles involved in eye movement.
The oculomotor nucleus innervates a large majority of the muscles involved in eye movement. It controls the levator palpebrae superioris, which is the muscle that moves the upper eyelid upwards. It also controls several muscles that move the eyeball itself: the superior, medial and inferior rectus muscles and the inferior oblique muscle.
When innervated by the third cranial nerve, the superior rectus muscle rotates the eyeball upward and helps in intorsion, or rotation towards the body's midline. The medial rectus muscle, on the other hand, is predominantly responsible for intorsion. The inferior rectus muscle, when innervated by the third cranial nerve, controls the downward rotation of the eyeball as well as extorsion and abduction, both of which move the eye away from the body's midline. The inferior oblique muscle assists in extorsion, abduction and upward rotation of the eyeball.
The other nucleus of the third cranial nerve, the Edinger-Westphal nucleus, controls two minor movements within the eyeball. It innervates the sphincter papillae, which controls pupil dilation. It also is responsible for the accommodation reflex, in which eye movement, lens shape and pupil size simultaneously adjust to allow the eye to focus on a nearby object.
The only two orbital eye muscles not innervated by the third cranial nerve are the lateral rectus muscle and the superior oblique muscle. These two muscles are controlled by other paired nerves: the abducens nerve and the trochlear nerve, respectively. The lateral rectus muscle rotates the eye away from the body's midline; and the superior oblique muscle assists in the inward, outward and downward rotation of the eye.
Damage to the third cranial nerve can result in a form of paralysis known as oculomotor nerve palsy. Depending on the severity of the damage, one eye might be unable to move from its current position or both eyes might be unable to move from their current position. Complete damage to the nerve will result in the eyeball being permanently rotated downward, dilation of the affected pupil and a droopy eyelid.