Oculomotor, trochlear, abducens nerve

Oculomotor Nerve (CN III)

somatomotor

nucleus: in the midbrain.

Anatomical Course:

• Originates from the oculomotor nucleus located within the midbrain.
• Emerges from the anterior aspect of the midbrain, passing inferiorly to the posterior cerebral artery and superiorly to the superior cerebellar artery.
• Pierces the dura mater and enters the cavernous sinus, receiving sympathetic branches from the internal carotid plexus within the cavernous sinus.
• Leaves the cranial cavity via the superior orbital fissure.

Division:

Divides into superior and inferior divisions after exiting the cranial cavity.

Superior Division:

Provides motor innervation to:

• Superior rectus
• Levator palpebrae superioris

Sympathetic fibers run with the superior division to innervate the superior tarsal muscle.

Inferior Division:

Provides motor innervation to:

• Inferior rectus
• Medial rectus
• Inferior oblique

Also supplies pre-ganglionic fibers to the ciliary ganglion.

Motor Functions:

Superior Branch:

• Superior Rectus: Elevates the eyeball.
• Levator Palpebrae Superioris: Raises the upper eyelid.
• Sympathetic fibers innervate the superior tarsal muscle, maintaining eyelid elevation.

Inferior Branch:

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

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Parasympathetic Functions:

• Sphincter Pupillae: Constricts the pupil, regulating light entry into the eye.
• Ciliary Muscles: Adjusts the lens for better short-range vision.

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Ciliary Ganglion

Tiny, but mighty, this 1-2 mm nerve bundle hangs out in the orbit, chilling right in front of the superior orbital fissure. Nestled in the fatty tissue, it cozies up between the optic nerve and the rectus lateralis bulbi muscle. Though it does get some sympathetic fiber that just pass through it.

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Damage

• Dropping of the eyelid
• Dilated pupil
• Inability to move the eye in some directions
• Double vision and difficulty to focus

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Trochlear Nerve (CN IV):

nucleus: posterior part of the midbrain just inferior to the oculomotor nucleus.

fibers: only somatomotor

Anatomical Course:

• The trochlear nerve kicks off its journey from the trochlear nucleus in the brain, making its exit from the posterior midbrain.
• It then travels forward and downward through the subarachnoid space, before piercing the dura mater near the posterior clinoid process of the sphenoid bone.
• Next, it glides along the lateral wall of the cavernous sinus before making its grand entrance through the superior orbital fissure.

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Motor Function:

You only have one job nerve!one job!: it innervates just one muscle.

• Superior Oblique: This muscle is responsible for depressing and intorting the eyeball.

Note : this nerve makes the longest root inside the skull's base.

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Abducens Nerve (CN VI):

nucleus: the pons, next to its median plane.

fibers: only somatomotor

Anatomical Course:

• The abducens nerve begins its journey at the abducens nucleus in the pons of the brainstem, making its exit at the junction of the pons and the medulla.
• It then enters the subarachnoid space and pierces the dura mater to travel through an area known as Dorello’s canal.
• At the tip of the temporal bone, it exits Dorello’s space and moves into the cavernous sinus, finally making its way into the orbit via the superior orbital fissure.

Motor Function:

• This nerve is dedicated to one main task: it innervates a single muscle. 1.Lateral Rectus: This muscle is responsible for abducting the eyeball, or moving it laterally.

Note: its intradural course is the longest of cranial nerves.

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references:

• Salvador notes (Final notes)
• "Answers for final exam" notes