Lecture on Phototransduction Cascade

Overview

• The lecture focuses on the phototransduction cascade, detailing how light rays are converted into electrical signals to produce vision.
• It includes a deep dive into the layers of the retina and their functions.

Layers of the Retina

• Three Main Cell Layers: Outer Pigmented Layer, Photoreceptor Layer, Ganglion Layer
  1. Outer Pigmented Layer
     • Contains melanin to absorb light rays and prevent scattering and reflection.
     • Provides nutrition and oxygen to the retina via diffusion from choroidal blood vessels.
     • Acts as a barrier controlling substances from blood into retinal tissue.
     • Functions:
       • Prevents light scattering/reflection.
       • Provides blood supply.
       • Engages in phagocytosis of cellular debris (e.g., photoreceptor discs).

Photoreceptor Layer

• Two Types: Rods and Cones
  1. Rods
     • Responsible for scotopic vision (dim or dark light, and shades of gray).
     • Pigment: Rhodopsin (Retinal + Opsin).
     • Cannot provide accurate edge detection or color vision.
  2. Cones
     • Responsible for photopic vision (color vision and high visual acuity in bright light).
     • Pigment: Photopsin (varieties sensitive to Blue, Green, and Red wavelengths).
     • Good at edge detection and precise vision.

Transduction Process

• Process in Rods (similar in Cones):

  1. Light rays (photons) hit Rhodopsin in the outer segment of the rod's disks.
  2. Conversion of 11-cis Retinal to All-trans Retinal, freeing Opsin.
  3. Opsin activates Transducin (G-protein).
  4. Transducin activates Phosphodiesterase (PDE).
  5. PDE breaks down Cyclic GMP (cGMP) to GMP.
  6. Lower cGMP levels cause closure of Na^+ and Ca^2+ channels.
  7. Cell hypopolarization occurs, leading to decreased Glutamate release.
  8. Low Glutamate stimulates Bipolar neurons, creating EPSPs leading to APs in Ganglion cells.

• Without Light (Dark Conditions):

  1. All-trans Retinal converts back to 11-cis Retinal, re-binding Opsin.
  2. Transducin and PDE are inactivated.
  3. cGMP levels rise, opening Na^+ and Ca^2+ channels.
  4. Cell depolarization occurs, leading to increased Glutamate release.
  5. High Glutamate inhibits Bipolar neurons, creating IPSPs, reducing APs in Ganglion cells.

Horizontal and Amacrine Cells

• Horizontal Cells
  • Release GABA to inhibit photoreceptors, aiding light adaptation.
  • Help in transitioning from dark to bright light and vice versa (light adaptation).
• Amacrine Cells
  • Release various neurotransmitters (Dopamine, Acetylcholine, GABA, Glycine).
  • Modulate signals between Bipolar and Ganglion cells, ensuring precise visual pathways.

Next Steps

• The next lecture will cover Pupillary Light Reflex, Dark Adaptation, and Light Adaptation.
• Followed by the overall visual pathway.

Conclusion

• Importance of understanding the phototransduction cascade for vision and visual pathway modulation.