Explaining the functional differences of rods versus cones

Abstract

Our visual sensation is mediated by two types of photoreceptors, rods and cones. Both respond to light electrically. Rods are highly light‐sensitive but cones are not. Because of this sensitivity difference, rods mediate night vision and cones mediate daylight vision. While a response to a brief light flash is rather slow in rods, it is brief in cones. These rod and cone differences in their light sensitivity and the response time course arise in the differences in the reactions in the enzyme cascade to evoke light responses in these cells. This cascade (phototransduction cascade) in rods is now rather well understood at the molecular level in a quantitative way. In cones, similar cascade has been known to be present. However, details are not known yet because it was difficult to obtain purified cones in an amount large enough to study these issues biochemically. Fortunately, we succeeded in the purification of enough amounts of cones from the retina of carp (Cyprinus carpio), which enables us to compare the qualitative and quantitative differences in the phototransduction cascade between rods and cones. The results so far we obtained explain lower light sensitivity and briefer light responses in cones than in rods. Furthermore, an additional difference between rods and cones was found in the retinoid metabolism. This reaction, specifically found in cones, ensures effective regeneration of visual pigment so that cones can operate even under very bright light. WIREs Membr Transp Signal 2012, 1: 675–683. doi: 10.1002/wmts.8 For further resources related to this article, please visit the http://wires.wiley.com/remdoi.cgi?doi=10.1002/wmts.8.

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