Photoreceptors and Vision

Introduction to Photoreceptors

  • Vision as a Sensory System: Humans have highly developed vision, a common trait among primates, focused on visible light.
  • Visible Light Spectrum: Humans perceive light in a range of approximately 400 to 700 nanometers.

Structure of the Eye

  • Eye Anatomy: Light enters and focuses through a lens onto the retina at the back of the eye.
  • Retina Composition:
    • Contains photoreceptor cells (photoreceptors) responsible for light detection.
    • Layers of cells: Ganglion cells, bipolar cells, followed by photoreceptors, indicating a layered structure.
  • Blind Spot: Created by the optic nerve's exit point where no photoreceptors are located.
  • Comparison with Octopus Eyes: Unlike human eyes, octopus eyes possess photoreceptors at the front, avoiding blind spots.

Mechanism of Vision

  • Photoreceptors Types:
    • Rods: Responsible for low-light vision (scotopic vision), only one type of photopigment (rhodopsin), perceives shades of gray.
    • Cones: Responsible for color vision (photopic vision), comes in three types with different photopigments (blue, green, red).
  • Photopigments' Functionality:
    • Absorb specific light wavelengths, each with a peak sensitivity.
    • Brain processes signals from different cones to perceive a wide range of colors using combinatorial integration.

Color Perception

  • Color Mixing:
    • Each cone type responds specifically to certain wavelengths (blue, green, red).
    • Brain combines signals to interpret various colors, even though only three cones are present due to relative receptor activation.
  • Example of Signal Integration: Light at 580 nanometers stimulates red cones more than green cones; perceived as orange.

Evolution of Photoreceptors

  • Phylogenetics of Color Vision:
    • Old World monkeys and higher primates possess three photoreceptors, allowing broader color discrimination than New World monkeys with only two.
  • Adaptations in Other Animals: Cats possess more rods leading to superior night vision at the expense of color discrimination and fine detail.

Mantis Shrimp Vision

  • Unique Photoreceptors: Mantis shrimp have 12 to 16 types of photoreceptors, giving them the ability to see a wide range of colors.
  • Complexities: Despite their extensive range, they may lack the fine discrimination capabilities that humans have.

Phototransduction in Rods

  • Photoreceptor Structure: Rods contain disks with rhodopsin composed of opsin and cis-retinal (vitamin A derivative).
  • Light Activation:
    • Light causes cis-retinal to convert to trans-retinal, activating the phototransduction cascade.
    • Triggers a hyperpolarization effect, which eventually stops neurotransmitter release, allowing bipolar cells to transmit signals.

Signal Processing and Action Potentials

  • Signal Transmission to Bipolar Cells: In darkness, rods release an inhibitory neurotransmitter, leading to hyperpolarization of bipolar cells.
  • Results of Light Exposure:
    • Light exposure alters sodium ion flow, hyperpolarizing rods and stopping inhibitory signal, allowing bipolar cells to fire and signal ganglion cells.

Cones vs. Rods Recovery

  • Bleaching Phenomenon: After intense light exposure, rods and cones become "bleached" and need time to recover.
    • Rods: Much slower recovery time (5 to 7 minutes).
    • Cones: Faster recovery (less than a couple of minutes).

Vision Impairment Post-Light Exposure

  • Temporary Loss of Vision: Significant exposure can lead to more prolonged periods of impaired vision until photopigments are replaced.
  • Unique to Color and Brightness: Each cone may get temporarily "bleached" under specific light conditions, affecting color perception in those regions.

Conclusion

  • Reminder on the Complexity of Vision: Despite human visual systems being advanced, they are plastic. Discussion on these mechanisms can lead to a deeper understanding of vision's complexity during practical application.