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When photons hit the photoreceptors

When photons hit the photoreceptors. These cGMP gated channels close. --> membrane potential hyperpolarizes Why do the cGMP gated channels close following photon capture?. Signal amplification. 1 photon activates 1 rhodopsin molecule Closes up to 200 ion channels (2% of a rod’s channels)

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When photons hit the photoreceptors

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  1. When photons hit the photoreceptors • These cGMP gated channels close. • --> membrane potential hyperpolarizes Why do the cGMP gated channels close following photon capture?

  2. Signal amplification • 1 photon activates 1 rhodopsin molecule • Closes up to 200 ion channels (2% of a rod’s channels) • Change of 1 mV of membrane potential.

  3. Light adaptation • With closed channels, the intracellular calcium concentrations decrease. • How does this affect signal transduction?

  4. Light adaptation • Low calcium • --> increases the activity of guanylate cyclase • --> more cGMP intracellular • --> can keep cGMP gated channel open with some light • Mb still depolarized during light adaptation

  5. Light adaptation • Low intracellular calcium • Increases the affinity of the cGMP gated channels for cGMP • --> the channels can stay open with light even with low cGMP

  6. Light adaptation • Also neural interactions between photoreceptors and horizontal cells.

  7. After photon capture • How are open channels restored? • Protein arrestin blocks rhodopsin from activating transducin, and helps to break down activated rhodopsin • All trans retinal dissociates from opsin and diffuses into the cytosol, ultimately into the pigment epithelium, where it recycles into rhodopsin.

  8. Illumination • Low light levels, scotopic vision (rods) • Photopic vision - normal indoor lighting, rod channels are closed, cones dominate • Mesopic vision - both rods and cones, twilight

  9. Rods vs cones • Rods - large current change with a single photon • Cones - smaller current change, compared to noise.

  10. Rods vs cones • Cones do not saturate at high illumination. • Cones adapt more to high illumination than rods

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