CN 3: Oculomotor Nerve (Scheme, Pathway, Clinical Relevance) | Neuroanatomy

Content:
0:00 Introduction
01:00 Optic Nerve Scheme
03:46 Midbrain Anatomy
06:20 Oculomotor Nerve Course
09:55 Branches of the Oculomotor Nerve
12:15 Parasympathetic Fibers of Oculomotor Nerve
13:29 Clinical Relevance
14:33 Oculomotor Nerve Palsy
17:03 Recap

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Oculomotor Pathway Scheme / Overview:
Oculomotor nerve (nervus oculomotorius) allows movement of the eye muscles, constriction of the pupil and the position of the upper eyelid
Starts with nucleus of oculomotor nerve and accessory oculomotor nucleus at the midbrain. They run together and exit through the oculomotor sulcus in the interpeduncular fossa. Then goes through the lateral wall of the cavernous sinus, goes through the superior orbital fissure and common tendinous ring, and then divides into superior branch and the inferior branch (somatomotor fibers). Parasympathetic fibers from the edinger-westphal nucleus run with the inferior branch and then goes to the ciliary ganglion as pre-ganglionic fibers. From here short cilliary nerves exit towards the ciliary muscle and sphincter pupillae.

Midbrain Anatomy:
Posterior view: Cerebral peduncles (pedunculus cerebri) and tectal plate. Tectal plate consists of superior colliculi (colliculi superiores), Brachium of superior colliculus (brachium colliculi superioris), inferior colliculi (colliculi inferiores) and brachium of the inferior colliculus (brachium colliculi inferior)
Anterior view: Cerebral peduncles and inferpeduncular fossa (fossa interpeduncularis), Oculomotor sulcus of the mesencephalon (sulcus nervi oculomotorii)
Internal view: Aqueduct of midbrain, substantia nigra, superior colluculi, periaqueductal grey substance (sibstantia grisea centralis), reticular formation (formatio reticularis), red nucleus (nucleus ruber), nucleus of the oculomotor nerve (nucleus nervi oculomotorii), Edinger-Westphal nucleus/accessory nucleus of oculomotor nerve (nucleus accessorii nervi oculomotorii)

Course of the Oculomotor Nerve:
Starts at the midbrain (level of superior colliculi)
Somatic fibers from the nucleus of the oculomotor nerve
Preganglionic parasympathetic fibers from the Edinger-Westphal nucleus/accessory nucleus of oculomotor nerve
Leaves through the Oculomotor sulcus of the mesencephalon on the anterior surface
It penetrates the dura mater and then pass through the lateral border of the cavernous sinus.
Enters through the superior orbital fissure and common tendinous ring.
Splits into superior branch and inferior branch
Superior branch innervate the superior rectus and levator palpebrae superioris
Inferior branch innervate the medial rectus, inferior rectus and inferior oblique

Extraocular muscles:
Medial Rectus, Lateral Rectus, Inferior Rectus, Superior Rectus.
Superior Oblique and Inferior Oblique
Levator Palpebrae Superioris

Superior Branch:
Superior rectus which elevates the eye
Levator palpebrae superioris which elevated the eyelid

Inferior Branch
Medial Rectus which is an adductor muscle
Inferior Rectus which depresses the eye
Inferior Oblique which originates medially and attaches to the inferolateral wall of the eye causing superior and lateral rotation

Autonomic innervation:
Pre-ganglionic parasympathetic fibers from the edinger-westphal nucleus
Runs with the inferior branch, and goes to the ciliary ganglion
Ciliary ganglia give off post-ganglionic parasympathetic motor neurons as short ciliary nerves.
Goes to pupillary sphincter causing pupillary constriction
Goes to ciliary muscles for the accommodation reflex

Clinical Relevance:
Stroke/bleeding of the midbrain can cause damage
Increased pressure in cavernous sinus
Posterior communicating artery aneurysms
Meningitis
Paranasal sinus infection
Trauma
Uncontrolled diabetes
Chronic hypertension

Manifestation of oculomotor nerve damage
Gaze stuck in downward out position
Video illustrating left sided oculomotor nerve palsy
○ Neurological examination shows how the eye with oculmotor nerve palsy can't rotate medially.
○ This condition causes diplopia (double vision), Ptosis, dilated pupil

Sources:
Singh, I. (2017). Human neuroanatomy (10th ed.).
Helwany M, Bordoni B. Neuroanatomy, Cranial Nerve 1 (Olfactory) [Updated 2022 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-
Kozlowski, T. (2017). Memorix Anatomy: The Complete Study Guide. 2nd ed. Thieme Medical Publishers.
Oculomotor nerve palsy illustration video: https://studmed.uio.no/elaring/fag/ne...