Photoreceptor

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  • Photoreceptors
    • Cones
      • Allow colour vision in bright light
      • Found in the centre of the retina
      • Synapse with bipolar neurones
        • Synapse with ganglion neurones
    • Rods
      • Only give black and white vision
      • Also work in dim light
      • Found in the retina
      • Synapse with bipolar neurones
        • Synapse with ganglion neurones
      • Rhodopsin (purplish pigment) absorbs light and results in a chemical change
        • Located in membranes of vesicles found in the outer segments of the rod
      • In the dark
        • Na+ ions flow into the outer segment through open cation channels
          • Na+ moves down the conc. gradient into the inner segment
            • Na+ actively pumped out of the cell
              • Membrane slightly depolarised (-40 mV)
                • Triggers the release of a neurotransmitter
                  • Neurotransmitter binds to the bipolar cell
                    • Stops the bipolar cell from depolarising
      • In the light
        • Rhodopsin broken down into retinal and opsin
          • Na+ channels close
            • No/less Na+ entering the rod
              • Inner segment still pumps out Na+ ions
                • Cell hyperpolarised
                  • Neurotransmitter is no longer released
                    • Bipolar cells depolarised
                      • Optic nerve neurones depolarised
          • Retinal = non-protein
          • Opsin = protein component
            • Leads to the hydrolysis of a molecule attached to the cation channel in the outer segment
              • Results in the closure of the cation channels
                • Na+ channels close
                  • No/less Na+ entering the rod
                    • Inner segment still pumps out Na+ ions
                      • Cell hyperpolarised
                        • Neurotransmitter is no longer released
                          • Bipolar cells depolarised
                            • Optic nerve neurones depolarised
          • Has to be quickly converted back into rhodopsin once broken down
            • Dark adaptation

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