Medicine & USMLE

Oculomotor Nerve (CN III)

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Cranial Nerves
  1. Olfactory Nerve (CN I)
  2. Optic Nerve (CN II)
  3. Oculomotor Nerve (CN III)
  4. Trochlear Nerve (CN IV)
  5. Trigeminal Nerve (CN V)
  6. Abducens Nerve (CN VI)
  7. Facial Nerve (CN VII)
  8. Vestibulocochlear Nerve (CN VIII)
  9. Glossopharyngeal Nerve (CN IX)
  10. Vagus Nerve (CN X)
  11. Accessory Nerve (CN XI)
  12. Hypoglossal Nerve (CN XII)

Summary

The oculomotor nerve, also known as cranial nerve number 3 or the third cranial nerve, is a motor nerve that transmits motor information from the brain to the muscles of the eye. Specifically, the oculomotor nerve mediates eye movements and eyelid elevation, as well as pupillary constriction and accommodation. As such, it is an important efferent component of the pupillary light reflex in one eye at a time, the accommodation reflex, and the vestibulo-ocular reflex. The oculomotor nerve originates from the midbrain and exits the skull at the super orbital fissure.  Finally, it’s worth noting that the oculomotor nerve is often damaged in uncal herniations, which can compress the nerve near its origin at the midbrain.

Key Points

  • Oculomotor Nerve (Cranial Nerve III)
    • Nerve Type
      • Motor
    • Function
      • Central fibers
        • Eye movement
          • Innervates superior, inferior and medial rectus and inferior oblique (most eye movements)
          • Test function via six cardinal positions of gaze test (tracking H-shape)
          • Damage leads to unopposed CN IV and VI action, leading to a down-and-out gaze
        • Eyelid elevation/opening
          • Innervates levator palpebrae
          • Damage leads to eyelid drooping (ptosis)
        • Central fibers more affected by vascular disease
          • Peripheral fibers (closer to blood supply) are relatively spared
      • Peripheral fibers
        • Pupil constriction
          • Parasympathetic transmission innervates sphincter pupillae which constricts pupil, controlling amount of light in eye
          • Involved in pupillary light reflex (motor constriction of pupils in response to light)
        • Accommodation
          • Parasympathetic transmission innervates ciliary muscles, which adjust lens to allow better short range vision
        • Peripheral fibers more affected by external compression
          • Central fibers are relatively spared (since they are located internally)
    • Nerve Entry/Exit
      • Exits from the midbrain
        • Oculomotor nucleus for motor function
          • Located in rostral midbrain near superior colliculus and red nucleus
        • Edinger-Westphal nucleus for parasympathetics
        • Nerve exits at interpeduncular fossa in midbrain
    • Cranial foramen
      • Superior orbital fissure
    • Reflexes
      • Pupillary light reflex (efferent motor)
        • Damage leads to loss of constriction in one eye only (non-consensual reflex loss)
        • CN II is responsible for afferent sensory
      • Accommodation reflex (efferent motor)
        • Responsible for adjusting lens to focus vision at center of gaze
        • Damage leads to lack of visual acuity
      • Vestibulo-ocular reflex (efferent motor)
        • Responsible for adjusting and stabilizing vision in response to vestibular balance
        • Damage leads to vertigo and nausea
    • Other Information
      • Often damaged in transtentorial (uncal) herniations, leading to paralysis of oculomotor muscles