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Oculomotor Nerve (Cranial Nerve III)

The oculomotor nerve, also known as cranial nerve III, is responsible for most of the eye's movements, including the constriction of the pupil 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 and passes between the posterior cerebral artery and the superior cerebellar artery.
  • Enters the orbit through the superior orbital fissure.

Functions of the Oculomotor Nerve

• Motor Functions:
  • Innervates four of the six extraocular muscles: superior rectus, medial rectus, inferior rectus, and inferior oblique.
  • Controls the levator palpebrae superioris muscle, which elevates the upper eyelid.
• Parasympathetic Functions:
  • Innervates the sphincter pupillae muscle, causing pupil constriction.
  • Innervates the ciliary muscles, allowing for accommodation of the lens for near vision.

Clinical Relevance

• Oculomotor Nerve Palsy:
  • Results from damage to the oculomotor nerve.
  • Symptoms: Ptosis (drooping eyelid), mydriasis (dilated pupil), and eye deviated "down and out" due to unopposed action of the lateral rectus and superior oblique muscles.
  • Causes: Aneurysms, diabetes, head trauma, or brainstem infarction.
  • Treatment: Depends on the underlying cause; may include surgical intervention, medication, or observation.
• Horner's Syndrome:
  • Can result from lesions affecting the sympathetic pathways that run alongside the oculomotor nerve.
  • Symptoms: Ptosis, miosis (constricted pupil), and anhidrosis (lack of sweating) on the affected side of the face.
  • Causes: Tumours, stroke, or spinal cord injury.
  • Treatment: Addressing the underlying condition; symptomatic treatment may include medications to alleviate ptosis.
• Oculomotor Nerve Testing:
  • Assessing eye movement and pupil responses to light and accommodation.
  • Clinical tests include checking for ptosis, pupil size and reactivity, and eye movement in all directions.

Summary

The oculomotor nerve is essential for eye movements, pupil constriction, and maintaining an open eyelid. It arises from the midbrain and innervates several muscles involved in these functions. Damage to the oculomotor nerve can lead to conditions such as oculomotor nerve palsy and Horner's syndrome, which require appropriate diagnosis and treatment.