M02.04.03 The oculomotor nerve

The oculomotor nerve (CN III) is crucial for both motor and parasympathetic innervation to several structures within the bony orbit. This article explores the anatomy, functions, and clinical significance of the oculomotor nerve.

Overview

• Motor Function:
  • Innervates most of the extraocular muscles, including the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, and inferior oblique muscles.
• Parasympathetic Function:
  • Supplies the sphincter pupillae and ciliary muscles of the eye, playing a role in pupil constriction and accommodation.
• Sympathetic Function:
  • Though not directly involved, sympathetic fibers travel with the oculomotor nerve to innervate the superior tarsal muscle, assisting in eyelid elevation.

Anatomical Course

• Origin:
  • The oculomotor nerve originates from the oculomotor nucleus located in the midbrain, ventral to the cerebral aqueduct.
• Pathway:
  • The nerve emerges from the anterior aspect of the midbrain, passing between the posterior cerebral artery and the superior cerebellar artery.
  • It pierces the dura mater and enters the lateral aspect of the cavernous sinus, where it receives sympathetic fibers from the internal carotid plexus.
  • The nerve exits the cranial cavity via the superior orbital fissure, dividing it into superior and inferior branches.

Branches and Their Functions

• Superior Branch:
  • Motor Innervation: Superior rectus and levator palpebrae superioris muscles.
  • Sympathetic Innervation: Superior tarsal muscle, aiding in eyelid elevation.
• Inferior Branch:
  • Motor Innervation: Inferior rectus, medial rectus, and inferior oblique muscles.
  • Parasympathetic Innervation: Pre-ganglionic fibers to the ciliary ganglion, which innervates the sphincter pupillae and ciliary muscles.

Motor Functions

The oculomotor nerve innervates multiple extraocular muscles responsible for moving the eyeball and upper eyelid.

Superior Branch:

• Superior Rectus: Elevates the eyeball.
• Levator Palpebrae Superioris: Raises the upper eyelid.
• Sympathetic Fibers: Innervates the superior tarsal muscle, helping to keep the eyelid elevated.

Inferior Branch:

• Inferior Rectus: Depresses the eyeball.
• Medial Rectus: Adducts the eyeball.
• Inferior Oblique: Elevates, abducts, and laterally rotates the eyeball.

Parasympathetic Functions

The oculomotor nerve provides parasympathetic innervation to two critical structures in the eye:

• Sphincter Pupillae: Constricts the pupil to reduce light entry.
• Ciliary Muscles: Contracts to make the lens more spherical, adapting for short-range vision.

The pre-ganglionic parasympathetic fibers travel in the inferior branch and synapse in the ciliary ganglion. Post-ganglionic fibers are carried to the eye via the short ciliary nerves.

Clinical Relevance: Oculomotor Nerve Palsy

Oculomotor nerve palsy occurs due to damage to CN III, leading to distinct clinical features:

• Ptosis: Drooping of the upper eyelid due to paralysis of the levator palpebrae superioris.
• Eye Position: A “down and out” position of the eye due to unopposed action of the lateral rectus and superior oblique muscles.
• Impaired Eye Movement: Inability to elevate, depress, or adduct the eye.
• Dilated Pupil: Due to the unopposed action of the dilator pupillae muscle.

Common Causes:

• Raised intracranial pressure.
• Posterior communicating artery aneurysm.
• Cavernous sinus infection or trauma.
• Other causes: diabetes, multiple sclerosis, myasthenia gravis, giant cell arteritis.

Points to Remember

• The oculomotor nerve is responsible for most movements of the eyeball and upper eyelid.
• It provides parasympathetic innervation for pupil constriction and lens accommodation.
• Oculomotor nerve palsy results in characteristic clinical signs like ptosis, a “down and out” eye position, and pupil dilation.

Bibliography

• TeachMeSeries Ltd (2024). Oculomotor Nerve Anatomy and Functions.
• Additional references on cranial nerve anatomy and clinical relevance.