9.7 the eye

• structure of retina

  • light detection going down each structure
  • ganglion cells (from optic nerve)
  • bipolar cells
  • photoreceptors (rods and cones)
• 2 types of photoreceptors in the retina

  
  

  • rod cells
  • cone cells
• Rod cells

  • distribution = in the periphery
  • pigment = rhodopsin
  • rhodopsin bleached by low light intensity
  • not sensitive to colour. absorbs all light wavelength
  • visual acuity = low - many synapses in one bipolar cell
• cone cells

  • distribution = in the fovea
  • pigment = iodopsin
  • iodopsin bleached by high light intensity
  • 3 types - absorb red, green and blue light (trichromatic)
  • visual acuity = high - one synapses in one bipolar cell
  • slower than rods
• retinal

  
  

  2 forms:

  • cis - binds to opsin, converted to trans (with energy from photon)
  • trans - cannot bind to opsin
• role of rhodopsin in initiating action potential (in light)

  • breaks down into opsin and trans retinal
  • opsin causes Na+ channels to close
  • Na+ actively transported out of inner segment
  • rod cells hyperpolarise
  • so glutamate release stops (no inhibitory signal)
  • biopolar neuron depolarises
• why do you need time to get used to the dark?

  • time needed to regenerate rhodopsin
• role of rhodopsin in initiating action potential (in dark)

  • Na+ ions flow out of cell
  • Na+ ions flow into the cell through open Na+ channels
  • inside of cell more negative
  • membrane slightly depolarised
  • glutamate released
  • which is inhibitory - no info transmitted to brain
• summation -

  • at low light intensities, stimulus from 1 rod cell isn't enough to reach threshold
  • multiple rods share 1 bipolar neurone
  • combined stimulus is enough to reach threshold