The eye - rod and cone cells

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  • Created by: Laura
  • Created on: 14-04-14 15:36
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  • The Eye
    • The actual light detection is carried out by photo receptors in the retina. There are two types; rod and cone cells.
      • These cells form synapses with special interneurones called bipolar neurones which in turn synapse with sensory neurones called ganglion cells.
      • They cover the inner surface of the retina and form the optic nerve
    • Rod cells
      • Rod cells enable us to see in black and white. Rod cells contain light sensitive pigment rhodopsin which converts into opsin and retinal.
      • Vitamin A is needed for the pigment rhodopsin. In the dark the opsin and retinal will recombine to form rhodopsin again where as in the light they are just constantly being broken down
      • In dim light it is possible that opsin and retinal are being broken down and recombining at the same time.
      • Rods are not very useful in bright light as the substances are broken down quicker than they are reformed
      • Rods connect to the brain in groups which means they provide poor acuity as there are many connections to one neuron so the brain cannot tell precisely where the signal is coming from.
      • However as the rod cells are grouped it means impulses can be sent even when there isnt much light. They can put all the rhodopsin together to produce a big enough impulse so rods have an extreme sensitivity to light.
    • Cone cells
      • Cone cells allow us to see in colour. They contain the pigment iodopsin which there are many forms of.
      • Cone cells produce a one to one connection, they are not grouped together. This produces a good acuity as the brain can tell exactly where the signal is coming from.
      • As they cannot combine their iodopsin each cone cell has to breakdown a large amount of iodopsin for an impulse to be sent.
      • The yellow spot on the retina or the fovea contains only cones. This spot then produces the best vision due to the one to one connection.
      • There are three different types of cone cells; red, green and blue. They are all no use in dull light they need bright light.
      • They are all sensitive to different wave lengths of light. Yellow light is broken down by red and green cones. Orange by all red and some green.
  • Rods are not very useful in bright light as the substances are broken down quicker than they are reformed


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