Q 1.9 Optic Nerve: Anatomy and Physiology


  • The optic nerve (also known as cranial nerve II or CN II) plays a pivotal role in vision. It serves as the bridge between the retinal layer of the eyes and the visual cortex of the brain.
  • Anatomy:
    • The optic nerve consists of approximately 1 million myelinated axons originating from the ganglion cells of the retina.
    • It extends from the posterior part of the eyeball (about 2 mm medial to the posterior pole) to the suprasellar space in the middle cranial fossa.
    • The optic nerve has several segments:
      1. Intraocular part: Located at the optic disc, where fibers move to the retro-orbital region.
      2. Intraorbital part: Runs from the posterior part of the eyeball to the optic canal, surrounded by all three meningeal layers.
      3. Intracanalicular part: Inside the optic canal of the sphenoid bone.
      4. Intracranial part: Travels superior to the diaphragma sellae and the cavernous sinus, ultimately forming the optic chiasm.
    • Blood Supply: The optic nerve receives blood supply from the ophthalmic branch of the internal carotid artery, posterior ciliary arteries, and the central retinal artery.
    • Fibers:
      • Visual afferent fibers: Transmit visual impulses from the retina to the lateral geniculate body of the thalamus.
      • Pupillary afferent fibers: Regulate the pupillary light reflex.
      • Efferent fibers: Travel to the retina but have an unknown function.
      • Photostatic fibers: Responsible for visual body reflexes.
    • Visual Pathway: Optic nerve ➡ optic tract ➡ lateral geniculate body ➡ optic radiation ➡ visual cortex (Brodmann area 17).

Pathology and Impairment Syndromes

  1. Anterior Ischaemic Optic Neuropathy (AION):
    • AION results from inadequate blood supply to the anterior portion of the optic nerve.
    • It can lead to sudden vision loss, often affecting older individuals.
    • Risk factors include hypertension, diabetes, and arteritis.
  2. Papilledema:
    • Swelling of the optic disc due to increased intracranial pressure.
    • Commonly seen in conditions like intracranial tumors, hydrocephalus, or idiopathic intracranial hypertension.
  3. Optic Neuritis:
    • Inflammation of the optic nerve, often associated with multiple sclerosis.
    • Presents with vision loss, pain, and color desaturation.
  4. Optic Glioma:
    • A tumor arising from glial cells within the optic nerve.
    • Common in children and may cause visual impairment.
  5. Compressive Optic Neuropathy:
    • Pressure on the optic nerve due to tumors, aneurysms, or other lesions.
    • Results in gradual vision loss.


Anatomy of the Optic Pathway

The optic pathway is like a relay race for visual information, passing the baton from the eyes to the brain. Here are the key players:

  1. Retina: The starting point. Photoreceptor cells in the retina capture light and convert it into electrical signals.
  2. Optic Nerve (Cranial Nerve II): The sprinter. It carries these signals from each eye to the brain.
  3. Optic Chiasm: The exchange point. Here, some nerve fibers cross over to the opposite side, creating an X-shaped structure.
  4. Optic Tract: The relay leg. It continues from the chiasm, carrying visual information to different brain regions.
  5. Optic Radiation: The final stretch. These nerve fibers fan out under the temporal and parietal lobes, leading to the visual cortex in the occipital lobe.


Common Optic Pathway Syndromes

  1. Optic Neuritis:

  • Symptoms: Sudden vision loss, eye pain, and color desaturation.
  • Cause: Inflammation of the optic nerve (often associated with multiple sclerosis).
  • Location: Usually affects one eye.
  • Visual Field Defect: Central scotoma (blind spot in the center of vision).

2. Chiasmal Lesions:

  • Symptoms: Bitemporal hemianopia (loss of outer visual fields).
  • Cause: Tumors (e.g., pituitary adenomas) compressing the chiasm.
  • Location: At the optic chiasm.
  • Visual Field Defect: Tunnel vision (loss of peripheral vision).

3. Homonymous Hemianopia:

  • Symptoms: Half of the visual field lost on the same side in both eyes.
  • Cause: Lesions along the optic tract or radiation (e.g., strokes).
  • Location: Post-chiasmatic (beyond the chiasm).
  • Visual Field Defect: Either left or right half of vision missing.

4. Meyer’s Loop Syndrome:

  • Symptoms: Superior quadrantanopia (loss of upper visual field).
  • Cause: Damage to Meyer’s loop (part of the optic radiation).
  • Location: Temporal lobe.
  • Visual Field Defect: Upper outer quadrant missing.

5. Parietal Lobe Lesions:

  • Symptoms: Inferior quadrantanopia (loss of lower visual field).
  • Cause: Damage to the optic radiation near the parietal cortex.
  • Location: Parietal lobe.
  • Visual Field Defect: Lower outer quadrant missing.

Remember, the optic nerve is not just a conduit for visual information—it’s a fascinating structure with intricate functions and clinical implications. As you continue your medical journey, explore its complexities and appreciate its role in our perception of the world!











Verified by Dr. Petya Stefanova