Anatomy of Visual Pathways

Retina & Optic Nerve - Eye's First Signals

• Retina: Converts light to neural signals; innermost eye layer.
  • Photoreceptors:
    • Rods: Scotopic (night, dim light) vision, peripheral retina, rhodopsin. ~120 million.
    • Cones: Photopic (daylight, color) vision, central retina (fovea), iodopsin. ~6 million. 📌 Cones = Color, Central.
  • Fovea Centralis: Highest cone density, site of sharpest visual acuity. Located in macula lutea.
  • Ganglion Cell Layer (GCL): Axons converge to form the optic nerve.
• Optic Nerve (CN II): Transmits visual information from retina to brain.
  • Composed of ~1.2 million GCL axons.
  • Optic Disc (Papilla): Physiological blind spot (no photoreceptors); where optic nerve exits and central retinal artery/vein enter/exit.
  • Myelination: Begins posterior to lamina cribrosa (by oligodendrocytes).

⭐ The central retinal artery and vein pierce the optic nerve approximately 1.25 cm behind the globe.

Optic Chiasm & Tract - The Great Crossover

• Optic Chiasm: X-shaped, anterior to pituitary.
  • Nasal retinal fibers (medial) decussate (cross).
  • Temporal retinal fibers (lateral) remain ipsilateral (do not cross).
• Optic Tract: Posterior to chiasm.
  • Contains ipsilateral temporal & contralateral nasal fibers.
  • Left Tract: Carries Right visual field information.
  • Right Tract: Carries Left visual field information.
  • Projects to Lateral Geniculate Body (LGB).

⭐ Pituitary adenomas are a common cause of bitemporal hemianopia due to compression of the decussating nasal fibers at the optic chiasm.

LGN & Optic Radiations - Relay & Fan Out

• Lateral Geniculate Nucleus (LGN): Thalamic visual relay.
  • Has 6 layers:
    • Layers 1, 4, 6: Input from contralateral nasal retina.
    • Layers 2, 3, 5: Input from ipsilateral temporal retina.
  • Magnocellular (M) layers (1,2): Motion, depth (Where pathway).
  • Parvocellular (P) layers (3-6): Color, fine detail (What pathway).
• Optic Radiations: LGN axons to primary visual cortex (Brodmann area 17).
  • Superior fibers (via parietal lobe): Carry inferior visual field information.
  • Inferior fibers (Meyer's Loop, via temporal lobe): Carry superior visual field information.
  • 📌 Mnemonic: PITS (Parietal-Inferior field defect, Temporal-Superior field defect).

⭐ Lesions of Meyer's loop (temporal lobe) cause a contralateral superior quadrantanopia ('pie in the sky').

Visual Cortex & Blood Supply - Brain's Vision HQ

• Primary Visual Cortex (V1): Brodmann area 17; located in occipital lobe, primarily on banks of calcarine sulcus.
  • Function: Initial, detailed processing of visual stimuli (form, motion, color).
• Retinotopic Organization: Precise point-to-point projection from retina.
  • Cuneus gyrus (superior bank of calcarine sulcus): Represents contralateral lower visual field.
  • Lingual gyrus (inferior bank of calcarine sulcus): Represents contralateral upper visual field.
  • Macula: Largest cortical representation, located at posterior occipital pole.
• Blood Supply:
  • Mainly Posterior Cerebral Artery (PCA).
  • Occipital pole (macular cortex): Often receives dual supply from PCA & Middle Cerebral Artery (MCA).

  ⭐ Macular sparing in posterior cerebral artery (PCA) occlusion is often attributed to dual blood supply to the occipital pole (macular cortex) from both PCA and middle cerebral artery (MCA).

• Clinical Significance: PCA occlusion typically causes contralateral homonymous hemianopia; macular sparing is common due to dual supply.

High‑Yield Points - ⚡ Biggest Takeaways

• Nasal retinal fibers cross at optic chiasm; temporal fibers do not.
• Optic tracts synapse in the Lateral Geniculate Nucleus (LGN).
• Optic radiations link LGN to primary visual cortex (occipital lobe).
• Meyer's loop (temporal lobe) carries superior visual field info; lesions cause "pie in the sky".
• Baum's loop (parietal lobe) carries inferior visual field info; lesions cause "pie on the floor".
• Macula has large, posterior representation in visual cortex.
• Chiasmal lesions typically cause bitemporal hemianopia.