Abducent Nerve (Cranial Nerve VI)

Jul 18, 2024

Lecture Notes: Abducent Nerve (CN VI) - Taim Talks Med

Overview of Cranial Nerves

  • Cranial nerves: 12 pairs that exit the brain and brainstem.
  • Today’s focus: Sixth cranial nerve (abducent nerve).

Abducent Nerve (CN VI)

  • Type: Purely motor nerve.
  • Innervates: Lateral rectus muscle (responsible for abduction of the eye).

Pathway of Abducent Nerve

  1. Origin: Nucleus in the Pons.
  2. Journey:
    • Travels through medullopontine sulcus (junction between medulla and pons).
    • Pierces dura mater.
    • Travels through cavernous sinus.
    • Goes through the superior orbital fissure.
    • Passes through the common tendinous ring.
  3. End Point: Innervates lateral rectus muscle.

Function

  • Action: When the lateral rectus muscle contracts, it abducts the eyeball (moves it outward).
  • Example: Right eye abduction means lateral rectus muscle of the right eye is activated, moving the eye outward.

Neuroanatomy Details

  • Brainstem Anatomy: Medulla Oblongata, Cerebellum, Pons, Mesencephalon, Diencephalon.
    • Removal of cerebellum reveals brainstem’s posterior side (Rhomboid Fossa).
    • Rhomboid Fossa: Location of several cranial nerve nuclei.
    • Structures in Rhomboid Fossa: Median sulcus, Medial eminence, Medullary stria, Facial colliculus.
  • Facial Colliculus: Formed by fibers of the facial nerve looping around the abducent nerve nucleus.
    • Cross-section: Shows the relationship between abducent and facial nerve nuclei.
    • Pathway continuity: Abducent fibers go straight out between Pons and Medulla.

Course of Abducent Nerve

  • Leaves through medullopontine sulcus.
  • Pierces dura mater.
  • Enters cavernous sinus (with internal carotid artery, oculomotor nerve (CN III), trochlear nerve (CN IV), ophthalmic branch of trigeminal nerve (CN V1)).
  • Exits cavernous sinus to enter orbit via superior orbital fissure.
  • Passes through common tendinous ring to innervate the lateral rectus muscle.

Hering’s Law of Equal Innervation

  • Purpose: Ensures balanced eye movement, preventing double vision.
  • Mechanism: When one eye’s lateral rectus muscle activates, the other eye’s abducent nucleus is inhibited.
  • Neural Coordination: Uses the Medial Longitudinal Fasciculus to align movements of eye centers on opposite brainstem sides.

Clinical Relevance

  • Compression Risk: The abducens nerve can be easily compressed (e.g., by intracranial pressure).
  • Effects of Compression: Paralysis of the lateral rectus muscle causes medial deviation of the affected eye.
    • Symptoms: Fully adducted eye at rest, inability to abduct the eye.

Conclusion

  • Next topic: Seventh cranial nerve (facial nerve).
  • Reminder: Like, comment, subscribe, and check the link in the description for more support options.

Peace!