Retina.
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   The retina contains two types of photoreceptors:
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<BR><LI> Rods are light-sensitive but don't distinguish colors.
<BR><LI> Cones distinguish colors but are not as sensitive to dim light.
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   Action potentials triggered by light are transmitted to bipolar cells.
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   The  action potentials are then relayed to ganglion cells, whose axons
   make  up the optic nerve and propagate the signal to the brain via the
   optic disk.