The oculomotor nerve arises from two nuclei within the brainstem.
- The motor nucleus
- The accessory oculomotor nucleus which contains the cell bodies of pre-ganglionic parasympathetic fibres destined for the ciliary ganglion.
The oculomotor nerve arises from the midbrain, and passes anteriorly to enter the cavernous sinus. It runs along the lateral wall, the most superior of the nerves in this part, and divides just prior to passing through the superior orbital fissure into a superior and inferior branch. As it passes through the cavernous sinus it acquires sympathetic fibres from the internal carotid plexus.
- The superior branch supplies the superior rectus and levator palpebrae superioris muscles; sympathetic fibres running with this branch reach the superior tarsal muscle
- The inferior branch supplies the inferior rectus, medial rectus and the inferior oblique muscles as well as contributing the parasympathetic root to the ciliary ganglion.
The oculomotor nerve is motor to all of the extraocular muscles except the superior oblique and lateral rectus. It also provides autonomic input to the ciliary ganglion, which in turn controls pupillary dilatation and intraocular vessels. It has no effect on the lacrimal gland which is controlled by parasympathetic fibres originating with the facial nerve (VII).
- A third nerve palsy typically causes ptosis of the superior eyelid due to loss of function of levator palpebrae superiorus. Interruption of parasympathetic supply leads to pupillary dilatation due to uncontrolled sympathetic activity. This is in contrast to Horner's syndrome which is due to interruption of sympathetic supply; this leads to ptosis due to loss of function of the superior tarsal muscle and pupil constriction due to unopposed parasympathetic input.