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Neurophysiology of Visual Processing

Course Information

  • Course Title: Neurophysiology (PHPH20009)

  • Topic: Visual Processing: How light is transduced

  • Professor: Clea Warburton

  • Email: e.c.warburton@bristol.ac.uk

Intended Learning Outcomes

By the end of this lecture, students should be able to:

  • Describe the process of transduction in photoreceptors and light adaptation.

  • Describe the colour visual system in terms of detection and analysis of colour.

Photoreceptor Cells

  • Transduction Process:

    • Photoreceptor cells absorb photons of light via photopigments.

    • Two main types of photoreceptors:

      • Rods: Contain photopigment rhodopsin, sensitive to low light.

      • Cones: Have several photopigments with differing absorption maxima for varied wavelengths of light.

Mechanism of Light Transduction

  • Prototypical Photoreceptor Structure:

    • Outer segment: Contains Na+ channels gated by cGMP (cyclic guanosine monophosphate).

    • Function of cGMP:

      • Binds to the cytoplasmic side of the Na+ channel, facilitating Na+ influx.

      • In the dark, high levels of cGMP keep Na+ channels open, leading to depolarization and glutamate release.

    • Inner segment: Contains non-gated K+ channels.

    • Steady concentrations of Na+ and K+ are maintained by Na+/K+ pumps.

Phototransduction in Darkness and Light

In the Dark

  • Condition:

    • cGMP-gated Na+ channels are open, allowing Na+ influx.

    • Cell depolarizes, leading to the release of glutamate.

    • Photopigment Composition: Rhodopsin consists of opsin and retinal.

In the Light

  • Phototransduction Process:

    • Rods absorb light, causing a conformational change (bleaching) in rhodopsin.

    • Activation Mechanism:

      • Bleaching activates G-protein transducin.

      • Transducin activates cGMP phosphodiesterase (PDE), reducing cGMP concentration.

      • Closure of cGMP-gated Na+ channels occurs, hyperpolarizing the cell and decreasing glutamate release.

Adaptation to Light and Dark

Light Adaptation

  • Process:

    • Adjusts sensitivity in bright conditions, influenced by Ca2+ levels.

    • Light exposure causes a decrease in Ca2+, which allows for some cGMP synthesis, enabling a slight dark current to continue.

Dark Adaptation

  • Process:

    • Increase in photoreceptor sensitivity in the dark (takes 20-25 minutes to reach maximum sensitivity).

    • Involves regeneration of bleached pigment (rhodopsin).

    • Increased levels of pigment in cones within first 5 minutes, followed by greater increases in rhodopsin in rods.

Colour Vision

Mechanism of Colour Detection

  • Role of Cones:

    • Colour detection is dependent on the presence of multiple types of cones.

    • Each cone type is colour blind and responds to specific wavelengths, leading to a combined color perception.

Two-Cone System

  • Types of Cones:

    • S Cones: Detect short wavelengths (blue).

    • L Cones: Detect longer wavelengths (green/yellow).

    • Result in dichromatic vision in many mammals (e.g., dogs, cats).

Trichromatic Vision

  • Addition of a Third Cone:

    • Enhances colour detection by providing sensitivity to red wavelengths.

    • Significance:

      • Trichometric vision enables humans to easily identify and distinguish objects based on colour.

Summary of Key Concepts

  • Transduction Mechanism: Photoreceptors are responsible for light transduction, heavily relying on cGMP and ion channel regulation.

  • Adaptation Processes: Light adaptation involves rapid adjustments, whereas dark adaptation takes longer and involves pigment regeneration.

  • Colour Vision: Three classes of cone receptors aid in colour detection and offer significant advantages in distinguishing objects.