Cranial Nerves + The Eye PDF

CN III: Oculomotor Nerve Function: General somatic motor → Innervates most extraocular muscles General visceral motor → Parasympathetic innervation of the sphincter pupillae and ciliary muscles of the eye There are also sympathetic fibres that run with CN III to innervate the superior tars...

CN III: Oculomotor Nerve Function: General somatic motor → Innervates most extraocular muscles General visceral motor → Parasympathetic innervation of the sphincter pupillae and ciliary muscles of the eye There are also sympathetic fibres that run with CN III to innervate the superior tarsal muscle (helps raise the eyelid) Course: Emerges from the anterior surface of the midbrain Pierces the dura mater to enter the cavernous sinus Here it is joined by sympathetic fibres from the internal carotid plexus Exits the cranial cavity via the superior orbital fissure where it then divides into: ○ Superior branch: Superior rectus, levator palpebrae superioris + sympathetic fibres to superior tarsal m. ○ Inferior branch: Inferior rectus, medial rectus, inferior oblique + parasympathetic fibres to ciliary ganglion CN III: Oculomotor Nerve Parasympathetics: Preganglionic parasympathetic fibres travel with the inferior branch to reach the ciliary ganglion in the orbit There, they synapse and the postganglionic fibres travel via the short ciliary nerves to: ○ Sphincter pupillae → constricts pupil ○ Ciliary muscles → contraction causes lens to become more spherical, better for short range vision CN III Palsy Damage to the oculomotor nerve can be caused by: Raised ICP Posterior communicating artery aneurysm Cavernous sinus infection or trauma There are 3 clinical features of oculomotor nerve palsy: 1. ‘Down and out’ position of eye at rest → unopposed activity of lateral rectus and superior oblique 2. Ptosis → paralysis of levator palpebrae superioris and unopposed activity of orbicularis oculi 3. Mydriasis (dilated pupil) → paralysis of sphincter pupillae and unopposed action of the dilator pupillae muscle