Hyperpolarisation of Rod Cells.

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  • Hyperpolarised Rod Cells (trichromatic vision provided).
    • Photoreceptors convert light into electrical impulse.
      • 1. Light enters eye, hits photoreceptor and absorbed by light-sensitive pigments.
      • 2. Light bleaches these pigments and cause a chemical change in them.
      • 3. This triggers a nerve impulse along a bipolar cell connecting the photoreceptors to the optic nerves.
      • 4. There are two types of photoceptors- Rod cells (for monochromatic vision) and cone cells (for trichromatic vision).
      • 5. Rods are mainly found in the peripheral parts of retina, cones are found in fovea in packs (and found in green/red/blue-sensitive version.
    • When Dark:
      • 1. Sodium ions are pumped into the outside membrane but diffuse back into the cell by sodium ion channels.
      • 2. The cell membrane is therefore depolarised as there is only a slight difference in negative charge between both sides of the membrane.
      • 3. This triggers release of neurotransmitters which inhibit the bipolar cell firing an action potential therefore no information is sent to the brain.
    • When Light:
      • 1. Light energy hits rod cells, stimulating them and causing rhodopsin to bleach into retinal and opsin.
      • 2. This closes sodium ion channels in the membrane therefore actively transport Na+ ions can't diffuse back in.
      • 3. This hyperpolarises the cell as the outside membrane becomes more positive than the inside one.
      • 4. Neurotransmitter release is inhibited meaning the action potential in the bipolar cell is sent along.
      • 5. The bipolar cell is therefore depolarised, with a potential difference reaching the threshold for an action potential to be sent along the optic nerve to the brain.

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