transduction of light stimuli

• What do photoreceptor cells do with light stimuli?

  They transduce light stimulus by absorbing photons of light through photopigment.
• What is the photopigment found in rods?

  Rhodopsin
• What is rhodopsin composed of?

  Rhodopsin is a 2-component protein that triggers a G-protein coupled signaling pathway.
• What do cones contain that allows them to detect light?

  Several cone photopigments with different absorption maxima.
• What does different absorption maxima in cones indicate?

  Different cones are sensitive to different wavelengths of light.
• How do Na+ channels in photoreceptors function in the dark?

  cGMP binds to the cytoplasmic side of the channel, allowing Na+ influx.
• What is the membrane potential of photoreceptors in the dark?

  • 30 mV
• What happens to glutamate release by photoreceptors in the dark?

  Glutamate is released due to depolarization from Na+ entering the cell.
• What occurs to rhodopsin when it absorbs light?

  Rhodopsin undergoes a conformational color change known as 'bleaching'.
• What is the role of transducin in phototransduction?

  Transducin is activated by bleaching of rhodopsin and activates cGMP phosphodiesterase (PDE).
• What happens to cGMP levels when light is present?

  cGMP levels decrease, leading to the closure of cGMP-gated Na+ channels.
• What is the result of hyperpolarization in photoreceptors?

  There is a decrease in the release of glutamate at synaptic terminals.
• What role does calcium play in light adaptation?

  Calcium inhibits cGMP synthesis, and reduced Ca2+ levels allow some synthesis of cGMP.
• How does adaptation to light occur in photoreceptors?

  Light stimulus makes ion concentrations more negative, but prolonged light exposure leads to a slight increase in positivity.
• How long does adaptation to dark take?

  It takes 20-25 minutes.
• What happens to the sensitivity of photoreceptors in the dark?

  Sensitivity increases when in the dark.
• What occurs during the first 5 minutes of dark adaptation?

  Cones become more sensitive than rods, but this does not last long.
• What is the primary factor for increased sensitivity in rods during dark adaptation?

  Increased rhodopsin in rods produces a greater increase in sensitivity.
• What is the role of the pupil in dark adaptation?

  The action of the pupil also contributes to increased sensitivity in the dark.
• What is the basis of color detection in the human eye?

  Color detection depends on cones and the presence of more than one type of cone.
• What is the range of wavelengths that humans can see?

  Humans can see between 400 and 700 nm.
• What is a characteristic of each type of cone?

  Each type of cone is color blind and produces a univariant response reflecting the amount of energy absorbed.
• What is dichromatic vision?

  Dichromatic vision involves two types of cones detecting energy and wavelength contrast.
• What is chromatic aberration?

  Chromatic aberration occurs when short wavelength images are out of focus while longer wavelength images are in focus.
• How does the number of S-cones compare to L-cones in the retina?

  There are fewer S-cones than L-cones on the retina.
• What is the significance of the fovea in terms of cone distribution?

  There are no S-cones in the fovea, which allows for good spatial resolution.
• What type of vision do most mammals apart from primates have?

  Most mammals have blue-yellow vision (dichromat vision).
• What advantage does trichromatic vision provide?

  Trichromatic vision allows for the detection of red, which is advantageous for identifying certain foods.
• What is the role of the third cone in vision?

  The addition of a third cone allows for trichromatic vision.
• What are the processes involved in phototransduction?

  1. Photoreceptor cells absorb photons of light.
  2. Rhodopsin undergoes bleaching and activates transducin.
  3. Transducin activates cGMP phosphodiesterase (PDE).
  4. PDE reduces cGMP levels, closing Na+ channels.
  5. Hyperpolarization occurs, decreasing glutamate release.
• What are the differences between light and dark adaptation?

  • Light adaptation:
  • Rapid response to increased light
  • Involves decreased cGMP and hyperpolarization
  • Dark adaptation:
  • Slower response (20-25 mins)
  • Involves regeneration of rhodopsin and increased sensitivity
• What are the types of color vision in mammals?

  • Dichromat vision:
  • Blue-yellow vision (most mammals)
  • Trichromat vision:
  • Blue-green-yellow vision (includes red detection)
• What are the characteristics of the cone types in the human retina?

  • L-cones:
  • More numerous, detect energy and wavelength contrast
  • S-cones:
  • Fewer in number, only detect wavelength contrast
  • No S-cones in fovea