Visual Neuroscience

Photoreceptors & Transduction - Light's First Spark

• Rods: ~120 million; scotopic (night) vision; rhodopsin; slow response; high sensitivity; achromatic.
• Cones: ~6 million; photopic (day) vision; photopsins (color); fovea; fast response; lower sensitivity.

Phototransduction:

• Dark State (Depolarization):
  • High cGMP → Na⁺/Ca²⁺ channels OPEN (dark current).
  • Cell depolarized (~-40mV).
  • Continuous glutamate release.
• Light State (Hyperpolarization):
  • Light → 11-cis retinal to all-trans retinal (in rhodopsin/photopsin).
  • Activates Transducin (G-protein).
  • Transducin activates Phosphodiesterase (PDE).
  • PDE ↓cGMP → Na⁺/Ca²⁺ channels CLOSE.
  • Cell hyperpolarizes (~-70mV).
  • ↓Glutamate release.

⭐ In the dark, photoreceptors are depolarized and continuously release glutamate. Light causes hyperpolarization and reduces glutamate release.

Retinal Processing - Retina's Router Room

• Signals from photoreceptors processed by retinal interneurons. 📌 People Buy Hats And Gloves (Photoreceptor → Bipolar → Horizontal/Amacrine → Ganglion).
• Bipolar Cells (BCs): First-order neurons.
  • ON-center (depolarize to light in center) & OFF-center (hyperpolarize to light in center).
• Horizontal Cells (HCs): Mediate lateral inhibition; shape receptive field surrounds.
• Amacrine Cells (ACs): Diverse; modulate BC-GC synapse (e.g., motion, direction).
• Ganglion Cells (GCs): Output to brain; axons form optic nerve.
  • Types: M (magnocellular: motion), P (parvocellular: detail, color).
  • Center-surround receptive fields enhance contrast.

⭐ Center-surround receptive fields of bipolar and ganglion cells are crucial for edge detection and contrast enhancement.

Visual Pathway & Cortex - Brain's Big Screen

Signal flow from eye to brain:

• Optic Chiasm: Nasal retinal fibers decussate; temporal fibers remain ipsilateral.

  ⭐ A lesion at the optic chiasm typically causes bitemporal hemianopia.

• LGN (Lateral Geniculate Nucleus): Thalamic relay. 📌 "LGN = Light Goes Next".
  • Magnocellular layers (1,2): Motion, spatial analysis ("Where" pathway).
  • Parvocellular layers (3-6): Color, fine detail ("What" pathway).
• Optic Radiations (Geniculocalcarine Tract):
  • Meyer's Loop (Temporal lobe): Info from inferior retina (superior visual field). Lesion → Superior quadrantanopia ("Pie in the sky").
  • Baum's Loop (Parietal lobe): Info from superior retina (inferior visual field). Lesion → Inferior quadrantanopia ("Pie on the floor").
• Primary Visual Cortex (V1): Brodmann area 17; occipital lobe (calcarine sulcus). Retinotopic map; macula has large cortical representation.

Reflexes & Adaptation - Eye's Auto-Focus

• Pupillary Light Reflex (PLR): Regulates light entry.
  • Pathway: CN II → Pretectal Nucleus → Edinger-Westphal Nucleus → CN III → Sphincter Pupillae (miosis).
  • Direct (same eye) & Consensual (opposite eye).

  ⭐ Relative Afferent Pupillary Defect (RAPD) or Marcus Gunn pupil indicates a lesion in the afferent pathway (optic nerve or severe retinal disease) anterior to the optic chiasm.

• Near Reflex Triad: For focusing on near objects. 📌 Mnemonic: CAM
  • Convergence: Eyes adduct (medial recti, CN III).
  • Accommodation: Lens thickens (ciliary muscle, CN III).
  • Miosis: Pupil constricts (sphincter pupillae, CN III).
• Adaptation: Adjusts to varying light levels.
  • Light Adaptation: Fast; pupil constricts, ↓ retinal sensitivity.
  • Dark Adaptation: Slow (~30-40 mins); pupil dilates, rhodopsin regenerates, ↑ retinal sensitivity (rods primarily).

High‑Yield Points - ⚡ Biggest Takeaways

• Visual pathway: Retina → Optic Nerve → Chiasm (nasal decussation) → Tract → LGN → Radiations → Visual Cortex (Area 17).
• LGN: 6 layers; P-cells (form, color), M-cells (motion, depth).
• Visual cortex: V1 (striate); V4 (color), V5/MT (motion). Dorsal stream ("where"), Ventral stream ("what").
• Pupillary light reflex: Afferent CN II, Efferent CN III.
• Optic chiasm lesion: Bitemporal hemianopia. Post-chiasmal: Contralateral homonymous hemianopia.
• Meyer's loop lesion: Contralateral superior quadrantanopia ("pie in the sky").
• Retinotopic organization maintained throughout primary visual pathway.