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ePaper created 2012-11-06, 16:43:12 | version 1.24.0
CreatingKnowledge Annual Report 2011 | Forschungszentrum Jülich 33 pulses. The cones, which register such light stimuli, could not transmit these signals, because the neural pathways were blocked by the excessive activity of the rods as soon as the first light im- pulse was registered (see image). “It’s a bit like a telephone line that’s engaged – if somebody is constantly on the phone, other information simply won’t get through,” says Frank Müller. That the rods are responsible for blocking the lines, so to speak, was demonstrated by Müller’s team in anoth- er experiment. They studied mice who not only lacked the molecular anti-glare protection HCN1 but whose rods were The eye has two types of visual cells in the retina: cones, which allow us to see colours in daylight (coloured green in the inset), and highly sensitive rods (red), which allow us to see at night (incl. blue, green and red bipolar cells connecting the visual cells). In dim light, both types of visual cells are active. To ensure that the rods do not react excessively, their signals are reduced. A key role is played by the ion channel HCN1. It ensures that the normal retina is not blinded and that individual light flashes are transmitted as separate signals (below left). If the HCN1 “switch” is missing, the rods react so strongly to the very first light stimuli that they block the neural pathways with “sustained firing” (below right). also simultaneously incapable of func- tioning due to a mutation. No interfering signal could therefore be released by the rods. In these mice, the cones react- ed normally to the flickering light. Perfect balance “In order to see optimally, we need a high sensitivity to light. At the same time, it is also essential that the retina is not saturated by a moderate inci- dence of light – causing us to be blind- ed,” says Müller in summary. “Our work has improved our understanding of how the eye carefully balances the conflict- ing requirements.” And not just in mice. “The HCN1 molecule is also present in human rods and cones and has been found in all others investigated to date,” says Müller. “I am convinced that we have discovered a universal mechanism here.” Nature Communications (doi:10.1038/ ncomms1540) Retina without HCN1Normal retina