detection of light

• eye

  • eye is a sense organ with receptors
  • light enters the eye via the pupil and is focused onto region of retina called fovea
  • amount of light entering eye is controlled by muscles in iris
  • cilary muscles attached to lens via suspensory ligaments
  • fovea has many light and photoreceptors
• photoreceptors


  rod cells

  • outer retina
  • sensitive to light intensity
  • only black and white images

  cone cells

  • grouped together in fovea
  • sensitive to different wave lengths of light so detects colour
  • can be red,green or blue sensitive-images in colour
• • action potentials generated in photoreceptors are sent to the brain via optic nerve
  • optic nerve leaves via the blind spot(no receptors present)
• photoreceptors+nerve impulses

  • generate AP when stimulated by bright enough light(rods)or particular wavelengths(cones)
  • light sensitive pigments in photoreceptors are bleached when light falls on them
• rhodopsin

  • rod cells contain light senstive pigment RHODOPSIN
  • when light hits rhodopsin it breaks into retinal and opsin
  • this breaking apart=bleaching- causing a chemical change in photoreceptors and results in generation of nerve impulses
• nerve impulse-bipolar neurone-optic nerve-brain
• the action of rod cells

  • rod cells initiate an AP in neighbouring bipolar neurones when they are hyperpolarised
• in the dark:


  1)NA+ is actively pumped out of rod cells creating a concentration gradient

  2)NA+ diffuses back down this concentration gradient and into rod cells via sodium channels- rod cell is depolarised

  3)depolarised rod cell released an inhibitory neurotransmitter which diffuses across synapse to the bipolar neurone

  4) NT inhibits depolarisation in bipolar neurone, no generation of AP=preventing a nerve impulse being sent to optic nerve
• in the light

  1)light bleaches rhodopsin=retinal and opsiN

  2)bleaching causes NA+ channels in surface membrane of the rod cells to close- preventing sodium ions from diffusing back into the rod cell as opsin binds to membrane

  • active transport of NA+ out is still taking place so NA+ removed and not returned

  3)lack of + ions in rod cell causes interior to become more negative until it hyperpolarises

  4)hyperpolarised rod cell stops releasing inhibitory NT so generation of AP in neighbouring bipolar neurone no longer inhibited

  5)AP generated in bipolar neurone- impulse sent to optic nerve