Receptors

• Photoreceptors


  The retina contains 2 types - rods and cones, which have the same basic structure but differ in shape and the pigment they contain
• Rods and cones


  1. Make synapses with bipolar neurones

  2. Bipolar neurones synapse with sensory neurones

  3. Sensory neurones are bundled together to form the optic nerve

  4. Optic nerve transmits impulses from the retina to the brain
• Light is focussed by the lens on the part of the retina opposite the pupil - the fovea
• The fovea receives the highest light intensity, and only cone cells are found there
• At the outside edge of the retina, where light intensity is the lowest, only rod cells are found
• Rod cells


  • Allow vision in dim light
  • Sensitive to very low light intensity
  • Many rod cells share a single bipolar neurone (spatial summation)
  • Do not help us distinguish different colours
  • Do not help us see in detail
• Cone cells


  • Sensitive to high light intensity
  • There are 3 different types, each containing a different form of iodopsin that absorbs a different wavelength of light
  • Allow us to see in colour
  • Give us very accurate vision
  • Give us high visual acuity
• If all 3 types of cones are equally stimulated, we see white light
• Types of cone cells

  • Red-sensitive
  • Green-sensitive
  • Blue-sensitive
• The colour seen depends on the relative degree of stimulation of the three different types of cone cell