Makindo Medical Notes

Related Subjects: |Olfactory Nerve |Optic Nerve |Oculomotor Nerve |Trochlear Nerve |Trigeminal Nerve |Abducent Nerve |Facial Nerve |Vestibulocochlear Nerve |Glossopharyngeal Nerve |Vagus Nerve |Accessory Nerve |Hypoglossal Nerve

The oculomotor nerve (cranial nerve III) is responsible for most eye movements, pupil constriction, and maintaining an open eyelid.

Anatomy of the Oculomotor Nerve

• Origin: Arises from the midbrain at the level of the superior colliculus.
• Course:
  • Exits the midbrain, passing between the posterior cerebral artery and superior cerebellar artery.
  • Enters the orbit via the superior orbital fissure.

Functions

• Motor:
  • Innervates 4 of 6 extraocular muscles: superior rectus, medial rectus, inferior rectus, inferior oblique.
  • Controls levator palpebrae superioris → elevates upper eyelid.
• Parasympathetic:
  • Sphincter pupillae → pupil constriction (light reflex).
  • Ciliary muscles → lens accommodation for near vision.

Clinical Relevance

• Oculomotor Nerve Palsy:
  • Symptoms: ptosis, dilated pupil (mydriasis), eye deviated "down and out".
  • Causes: posterior communicating artery aneurysm, diabetes (microvascular), trauma, brainstem infarction.
  • Treatment: depends on cause (e.g., surgery for aneurysm, control vascular risk factors).
• Testing:
  • Check extraocular movements – look for impaired up, down, and medial gaze.
  • Assess for ptosis and pupil reaction to light and accommodation.

Summary

The oculomotor nerve is crucial for eye movements, pupillary constriction, and eyelid elevation. CN III palsy produces ptosis, dilated pupil, and a "down and out" eye. Horner’s syndrome, though also causing ptosis, involves sympathetic pathway damage and produces a small pupil instead. Posterior communicating artery aneurysm is a classic cause of compressive CN III palsy.