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Photochemistry of Vision

Jul 28, 2024

Photochemistry of Vision

Introduction

  • Topics Covered: Layers of retina, rods and cones, conversion of light to electrical activity, optic nerve, and photochemistry of vision.

Retina Structure

  • Retina: Sensor or sensory aspect of the eye, converts light into electrical activity/action potential.
  • Layers: Arranged in a specific order from outermost to innermost.
    • Outermost Layer: Pigmented layer (melanin pigment), absorbs light to prevent reflection.
    • Innermost Layer: Inner limiting membrane, first layer light falls on.

Layers of Retina

  1. Pigmented Layer
    • Made of melanin pigment, absorbs light, prevents reflection.
    • Contains vitamin A, crucial for vision.
  2. Rods and Cones Layer
    • Outer nuclear layer, contains the nuclei of rods and cones.
  3. Outer Plexiform Layer
    • Connection between rods/cones and inner cells.
  4. Inner Nuclear Layer
    • Contains nuclei of amacrine, bipolar, and horizontal cells.
  5. Inner Plexiform Layer
    • Connection between inner cells and ganglionic cells.
  6. Ganglion Cell Layer
    • Axons form the optic nerve.
  7. Inner Limiting Membrane
    • Light travels through all these layers before reaching rods and cones.

Visual Acuity

  • Fovea: High visual acuity due to direct light fall on photoreceptors.
    • Diameter: 0.3 mm.
    • Fewer rods, more cones.
    • Avascular Zone: Inner layers displaced outwards.

Rods and Cones Structure

  • Segments: Outer (near pigmented layer) and inner (contains mitochondria).
  • Rhodopsin (rods) and color pigments (cones) present in disc-like structures.
  • Mitochondria: Provides ATP for photochemical reactions.
  • Differences: Rods are slender (2-5 µm), Cones are conical (5-8 µm, as small as 1.5 µm in fovea).

Photochemistry of Vision: Rhodopsin

  1. Rhodopsin: A combination of scotopsin and 11-cis retinal.
  2. Activation by Light
    • Rhodopsin splits into all-trans retinal and scotopsin upon light exposure.
    • Intermediate products: Bathorhodopsin, lumirhodopsin, metarhodopsin I, metarhodopsin II.
  3. Metarhodopsin II
    • Activates transducin (G-protein).
    • Transducin activates cyclic GMP phosphodiesterase which breaks down cyclic GMP, closing sodium channels leading to hyperpolarization.

Visual Cycle

  • 11-cis retinal is converted to all-trans retinal by light.
  • Reformation of Rhodopsin: Two processes
    • Inside rods using isomerase enzyme.
    • Outside in the pigmented layer involving vitamin A.
  • Vitamin A Deficiency: Can cause night blindness.

Mechanism of Light to Electrical Signal Conversion

  1. Dark Conditions
    • Sodium-potassium ATPase and cyclic GMP gated sodium channels maintain membrane potential at -40 mV.
  2. Light Exposure
    • Decreases sodium influx, leading to hyperpolarization.
    • Hyperpolarization signals next layer of cells (unlike depolarization in other receptors).

Amplification of Signal

  • G-protein coupled receptors amplify the signal allowing small amounts of light to produce significant electrical energy.

Conclusion

  • Next topic: Color vision and the theory of color vision.

Important Points

  • Light activates rods/cones → electrical activity → optic nerve → brain.
  • Inner layers are critical for light processing; displacement in fovea enhances visual details.
  • Rhodopsin: key molecule in rods for photochemistry.
  • Mechanism involves changes in cyclic GMP concentration affecting sodium channels.

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