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CarbonRobot CarbonRobot
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A month ago
Is there that much different between rods and cones? I think rods lack maybe 80nm from the upper range of the red cones but otherwise covers the cones wavelengths well. Cones give fine detail and color vision, but rods are like 200 times more sensitive to light. Is there any conceivable way to create a cell type that has the benefits of all? Three or four cone/rod cells that all give fine detail, color, and maximum light sensitivity?
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Staff Member
A month ago
There are far too many differences between the two cell types biologically that lend to their form and function. Speaking from a evolutionary standpoint, DNA comparisons from a large number of vertebrate pigments show that cone pigments emerged before the rod pigment evolved. This and other evidence indicates that cones are older and that rods evolved from cells with at least some of the properties of cones.

But why did nature choose to have this two photoreceptor model instead of a single photoreceptor that can do it all, as you suggested. One hypothesis is that the principal advantage of two distinct kinds of photoreceptors is that this model uses less ATP energy than animals that have one or the other. The only animals known to have no cones at all, and therefore that are incapable of color vision, are skates, which are cartilaginous fishes related to rays and, more distantly, to sharks. Sharks are also monochromatic, but rays are thought to have relatively good color vision. Here are some interesting facts I gathered about rods and cones.

1. Number :

Rod cells - more numerous, 125 millions in each eye.

Cone cells - less than rods, 7 millions in each eye.

2. Pigment :

Rod cells - rhodopsin

Cone cells - iodopsin

3. Sensitivity :

Rod cells - sensitive to light

Cone cells - less sensitive to light

4. Vision :

Rod cells - dim light (night vision)

Cone cells - bright light

5. Visual pigment :

Rod cells - only one, black white

Cone cells - 3 visual pigments, to differentiate colours

6. Location :

Rod cells - periphery of retina

Cone cells - center of retina

7. Speed of response to light :

Rod cells - slow response, long integration time

Cone cells - fast response, short integration time

8. Shape of outer segment :

Rod cells - cylindrical

Cone cells - conical

9. Size :

Rod cells - small

Cone cells - large

10. Light level :

Rod cells - scotopic

Cone cells - photopic

11. Absolutely sensitivity :

Rod cells - high

Cone cells - low

12. Visual acuity:

Rod cells - poor, many rods share common neuron connection to brain.

Cone cells - good, each cone cell has it's own neuron connection to brain.

Functions of rod and cone cells :

Rod cells :

1. Rods are very sensitive to light and have a low threshold, so the rods are responsible for dim light vision or night vision or scotopic vision.

2. Rods don't take part in resolving the details and boundaries of objects (visual acuity) or the color of the objects (color vision). Vision by rod is black, white or in the combination of black and white namely grey. Therefore, the colored objects appear faded or greyish in twilight.

Cone cells :

1. Cones have high threshold for light stimulus. So, the cones are sensitive only to bright light. Therefore, cone cells are called receptors of bright light vision or daylight vision or photopic vision.

2. Cones are also responsible for acuity of vision and the color vision.
Source https://royalsocietypublishing.org/doi/10.1098/rstb.2016.0074
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A month ago
Very through. Thank you much.
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