Oculomotor Nerve – Cranial Nerve III/3

Introduction

• Nerve to the eye muscles
• Supplies only motor fibers. Does not contain sensory fibers.
• Before it synapses in the midbrain, the neurons are UMN. After it synapses in its nucleus in the midbrain, the fibers carried to the muscles are LMN.

Functions in General:

• GVE (general visceral efferent): supplies the constrictor pupillae muscle of the iris and ciliary muscle of the lens.

• GSE (general somatic efferent AKA general somatic motor):  innervates levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique muscles.

(All except Lateral rectus and Superior oblique.)

Mnemonic: SO4 LR6 = 
Superior Oblique supplied by cranial nerve 4 (trochlear n.)
Lateral Rectus supplies by cranial nerve 6 (abducent n.)

Nuclei of Trigeminal Nerve in Brainstem

Figure 1: Oculomotor nuclei in section of midbrain at level of superior colliculus

Figure 2: Level of section of brainstem in Figure 1

Figure 3: Oculomotor nucleus at level of superior colliculus 

Main oculomotor nucleus

• In the anterior part of the gray matter around the cerebral aqueduct;
• Contains groups of nerve cells that supply all extrinsic muscles of the eye (except SO4 LR6).
• Nerve fibers exiting the nucleus pass through the red nucleus to emerge from the interpeduncular fossa as the ocolomotor  nerve.
• It receives corticonuclear fibers from the cerebral hemispheres.
• It receives the tectobulbar tract from the superior colliculus (to connect the Abducent nucleus with the visual cortex).
• Receives fibers from medial longitudinal fasciculus by which it is connected to the nuclei of 4^th (trochlear), 6^th (abducent) and 8^th (vestibulocochlear) cranial nerves.

Accessory parasympathetic nucleus (Edinger Westphal nucleus)

• Situated posterior to the main oculomotor nucleus.
• Receives corticonuclear fibers (from the cerebral cortex) for the accommodation reflex and fibers from the pretectal nucleus for the light reflexes. These fibers are preganglionic.
• Nerve fibers exiting the nucleus pass through the red nucleus to emerge from the interpeduncular fossa as the ocolomotor nerve.

Note: some aspects of this nerve will also be discussed with the Optic nerve in the visual reflex pathway.

Emerges from brainstem

-  Emerges on the anterior surface of the midbrain in the interpeduncular fossa.

Figure 4: Oculomotor nerve emerging from interpeduncular fossa in midbrain

Route

• After emerging in the interpeduncular fossa it continues into the middle cranial fossa.
• It enters the anterior edge of the tentorium cerebelli and then passes into the lateral wall of the cavernous sinus.

Figure 5: Cavernous sinus showing passage of Oculomotor nerve in the lateral wall

Figure 6: Cavernous sinusare in internal skull

Exit/entry from/into skull

Nerve	Exits	Enters
Oculomotor (CN III)	Superior orbital fissure	Orbit

Figure 7: side view of internal cranial cavity showing exit of Oculomotor nerve branches from skull

Figure 8: Superior Orbital Fissure

Branches

Just before it enters the orbit, the oculomotor nerve divides into:

-       Superior branch innervates:

o   Superior rectus muscle

o   Levator palpebrae superioris muscle

-       Inferior branch innervates:

o   Medial rectus muscle

o   Inferior rectus muscle

o   Inferior oblique muscle

o   Ciliary branch to ciliary ganglion

Figure 9: Exit of Oculomotor nerve branches from superior orbital fissure

Figure 10: Superior orbital fissure

Lesions: Oculomotor (3rd Nerve) Palsy

To understand this, you need to understand the extraocular eye muscle movement and their nerve supply. This video briefly explains really the eye movements and muscles with a way to memorize which muscle causes which action.

Lesion	Findings	Reason for findings
Complete Oculomotor lesion	At rest, eye looks downward & laterally (eye can’t look up, down or medially due to paralysis of extraocular muscles supplied by oculomotor nerve.)	Downward: activity of superior oblique Laterally: activity of lateral rectus
	Diplopia	Due to one eye looking normally in all directions while other only looking laterally and downward
	Ptosis (drooping of upper eyelid)	Paralysis of levator palpebrae superioris
	Dilated pupil (loss of light reflex)	Paralysis of sphincter pupillae and unopposed action of dilator pupillae (supplied by sympathetic)
	Loss of accommodation reflex	Paralysis of sphincter pupillae and ciliary muscles of the lens
Internal ophthalmoplegia	Loss of autonomic supply of sphincter pupillae and ciliary muscle
External ophthalmoplegia	Paralysis of extraocular muscles

Figure 11: Ophthalmic Atlas Images (EyeRounds.org, The University of Iowa) / CC BY-NC-ND 3.0

Figure 12: Right Oculomotor nerve palsy

References

Drake, R., Vogl, A., & Mitchell, A. (2010). Chapter 8: Head and neck - orbit. Gray's anatomy for students (2nd ed., pp. 878-902). Philadelphia: USA: Elsevier: Churchill Livingstone.

Snell, R. (2010). The cranial nerve nuclei and their central connections and distribution. Clinical neuroanatomy (7th ed., pp. 331-370). Philadelphia: Lippincott Williams and Wilkins.

Snell, R. (2007). Chapter 18: The eye and the ear. Clinical anatomy by systems (pp. 657-685). Philadelphia: USA: Lippincott Williams & Wilkins.

Figure references

1. Done by me
2. "Posterolateral view of brainstem" from Netter's Atlas of Human Anatomy 5e / retouched from original
3. By Gray696.png: User:mcstrother derivative work: Mcstrother (Gray696.png) [Public domain], via Wikimedia Commons / retouched from original
4. Done by me
5. Drake: Gray's Anatomy for Students 2e
6. By Anatomist90 (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons  / retouched from original
7. Unknown (if you know source please message me so I can mention it)
8. "Lacrimal apparatus" by Drake: Gray's Anatomy for Students 2e
9. "Innervation of orbit and eyeball, anterior view" Drake: Gray's Anatomy for Students 2e
10. "Formation of the lacrimal sac" by Drake: Gray's Anatomy for Students 2e
11. Ophthalmic Atlas Images (EyeRounds.org, The University of Iowa) / CC BY-NC-ND 3.0
12. Unknown (if you know source please message me so I can mention it)