Phototransduction Cascade Lecture Notes

Introduction

• Topic: Phototransduction cascade in the retina
• Goal: Understand how light rays are converted into electrical signals, producing vision

Retinal Layers

Overview

• Retina made up of three main cell layers
• Layers:
  • Outer pigmented layer
  • Photoreceptor layer
  • Bipolar and ganglion cells layer

Outer Pigmented Layer

• Components: Pigmented epithelial cells containing melanin (pigment preventing light scattering).
• Functions:
  • Absorbs light rays to prevent scattering and reflection.
  • Provides nutrients and oxygen to the retina through diffusion from choroidal blood vessels.
  • Acts as a barrier, controlling the substances that enter the retina from the blood.
  • Phagocytoses old photoreceptor discs.

Photoreceptor Layer

• Types: Rods and cones
• Functions of Photoreceptors:
  • Rods:
    • Function in dim light (scotopic vision)
    • Detect shades of gray
    • Poor visual acuity and color detection
    • Contain pigment rhodopsin (made up of retinal and opsin)
  • Cones:
    • Function in bright light (photopic vision)
    • Detect colors (blue, red, green)
    • Good visual acuity and edge detection
    • Contain photopsin

Phototransduction Process

Light Ray Conversion to Electrical Signal

• Light rays hit rhodopsin in rods (similar mechanism in cones)
• Conversion process:
  • Rhodopsin = Retinal + Opsin
  • Light converts 11-cis-retinal to all-trans-retinal.
  • Opsin separates and activates transducin protein.
  • Transducin activates phosphodiesterase, breaking down cyclic GMP.
  • Decrease in cyclic GMP closes Na+ and Ca2+ channels.
  • Hyperpolarization of the photoreceptor cell (becomes more negative).
  • Reduced glutamate release from photoreceptors.
  • Bipolar cells become depolarized, releasing more glutamate onto ganglion cells.
  • Ganglion cells generate action potentials transmitted via the optic nerve to the brain.

Absence of Light

• All-trans-retinal converts back to 11-cis-retinal.
• Cyclic GMP levels increase, opening Na+ and Ca2+ channels.
• Depolarization of photoreceptor cell.
• Increased glutamate release inhibits bipolar cells, reducing signals to ganglion cells.
• Fewer action potentials in the optic nerve.

Horizontal and Amacrine Cells

• Horizontal Cells:
  • Release GABA inhibiting photoreceptors
  • Help with adaptation from dark to light conditions
• Amacrine Cells:
  • Release dopamine, acetylcholine, GABA, and glycine
  • Modulate the activity between bipolar and ganglion cells
  • Ensure precision in visual processing

Future Topics

• Pupillary light reflex
• Dark and light adaptation mechanisms
• Visual pathway details

Note: Further videos will explore these concepts in more detail.