Understanding the Optic Nerve Pathway

Oct 12, 2024

Anatomy Lecture Notes: Optic Nerve and Visual Pathway

Introduction

  • Focus: Second cranial nerve, the optic nerve.
  • Vision is a crucial special sense in humans.

Visual Pathway Components

  • Retina
    • Receives visual impulses via rods and cones.
    • Transmits impulses to bipolar then ganglionic cells.
  • Optic Nerve
    • Formed by axons of ganglionic cells, begins at the optic disc.
    • Part of CNS (diencephalon), myelinated by oligodendrocytes.
    • Unique as it is covered by dura mater and exits orbit via optic canal.
  • Optic Chiasma and Tract
    • Partial decussation occurs at optic chiasma.
    • Fibers from nasal half cross, temporal halves do not.
    • Post-decussation, fibers form the optic tract.

Optic Tract

  • Contains contralateral nasal hemiretina fibers and ipsilateral temporal hemiretina fibers.
  • Projects into:
    • Ipsilateral lateral geniculate body
    • Pre-tactile area
    • Superior colliculi

Lateral Geniculate Body

  • Relay station for the optic tract; a six-layered nucleus.
    • Layers 1, 4, 6 receive crossed fibers.
    • Layers 2, 3, 5 receive uncrossed fibers.
  • Receives input from ipsilateral temporal and contralateral nasal hemiretina.
  • Projects via optic radiation to the primary visual cortex (Brodmann's area 17).

Pre-Tactile Area & Superior Colliculi

  • Pre-Tactile Area: in midbrain, in front of the tectum.
  • Superior Colliculi: two elevations in the dorsal midbrain.

Optic Radiation & Visual Cortex

  • Optic radiation arises from the geniculocalcarine tract.
  • Visual Cortex: located on the banks of the calcarine fissure in the occipital lobe.
    • Brodmann's area 17: Vision integration.
    • Areas 18 and 19: Visual association areas.

Schematic of Visual Pathway

  1. Axons of ganglionic cells in the retina → Optic nerve → Optic chiasma → Optic tracts.
  2. Information relayed to:
    • Pre-tactile area
    • Lateral geniculate body
    • Superior colliculi
  3. Lateral geniculate body → Optic radiation → Visual cortex (Areas 17, 18, 19).

Common Lesions and Visual Pathway Disorders

  • Central Optic Chiasma Lesions
    • Cause bitemporal hemianopia (loss of peripheral vision).
    • Often due to pituitary tumor compression.
  • Lateral Optic Chiasma Lesions
    • Cause binasal hemianopia (loss of nasal field vision).
  • Optic Nerve Lesions
    • Cause blindness in the affected eye (e.g., retinobulbar neuritis).
  • Optic Tract Lesions
    • Cause homonymous hemianopia (loss of same visual field halves, right or left).