Medicine & USMLE

Optic Nerve (CN II)

10,607 views
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 optic nerve, also called cranial nerve number two or the 2nd cranial nerve, is a sensory nerve that transmits our sense of sight or vision to the brain. The nerve originates from retinal ganglion cells in the back of our eye, and travels through the optic canal in the skull to enter the brain. As the sensory component of vision, the optic nerve plays major roles in the sensory components of the pupillary light reflex in both eyes, as well as the accommodation reflex.

Key Points

  • Optic Nerve (Cranial Nerve II)
    • Nerve Type
      • Sensory
    • Function
      • Vision/sight
        • Test function via Snellen chart for visual acuity and pupillary light reflex
    • Nerve entry/exit
      • Does not originate from or travel to brainstem nuclei
      • Originates from retinal ganglion cell axons
        • Pathway: retina → optic nerve → optic chiasm (only medial fibers cross)→ optic tract → LGN → optic radiations (dorsal and ventral streams) → primary visual cortex
    • Cranial foramen
      • Optic Canal
    • Reflexes
      • Pupillary light reflex (afferent sensory)
        • Visual fields with low ambient light (luminosity) is transmitted to brain stem, which adjusts motor output via CN III
        • Damage to CN II leads to loss of constriction in both eyes (consensual reflex loss)
      • Accommodation reflex (afferent sensory)
        • Unfocused image (poor contours and contrast) at center of gaze is transmitted to brain stem, which adjusts motor output via CN III
        • Damage leads to loss of visual acuity