Why do green rods of frog and toad retinas look green?

Amphibian “green” rods express a blue-sensitive cone visual pigment, and should look yellow. However, when observing them axially under microscope one sees them as green. We used single-cell microspectrophotometry (MSP) to reveal the basis of the perceived color of these photoreceptors. Conventional...

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Bibliographic Details
Published in:Journal of Comparative Physiology 2014-09, Vol.200 (9), p.823-835
Main Authors: Govardovskii, Victor I, Reuter, Tom
Format: Article
Language:English
Subjects:
Amphibia
Animal Physiology
Animals
Anura
Biomedical and Life Sciences
Bufo bufo - anatomy & histology
Bufo bufo - physiology
color
color vision
frogs
granules
humans
Life Sciences
lighting
melanin
Microspectrophotometry
Models, Neurological
Neurosciences
Original Paper
Photic Stimulation
photoreceptors
Rana ridibunda - anatomy & histology
Rana ridibunda - physiology
Rana temporaria - anatomy & histology
Rana temporaria - physiology
reflectance
retina
Retinal Pigments - metabolism
Retinal Rod Photoreceptor Cells - cytology
Retinal Rod Photoreceptor Cells - physiology
rhodopsin
toads
transmittance
Zoology
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Summary:Amphibian “green” rods express a blue-sensitive cone visual pigment, and should look yellow. However, when observing them axially under microscope one sees them as green. We used single-cell microspectrophotometry (MSP) to reveal the basis of the perceived color of these photoreceptors. Conventional side-on MSP recording of the proximal cell segments reveals no selective long-wave absorbing pigment explaining the green color. End-on MSP recording shows, in addition to the green rod visual pigment, an extra 2- to 4-fold attenuation being almost flat throughout the visible spectrum. This attenuation is absent in red (rhodopsin) rods, and vanishes in green rods when the retina is bathed in high-refractive media, and at wide illumination aperture. The same treatments change the color from green to yellow. It seems that the non-visual pigment attenuation is a result of slender green rod myoids operating as non-selective light guides. We hypothesize that narrow myoids, combined with photomechanical movements of melanin granules, allow a wide range of sensitivity regulation supporting the operation of green rods as blue receptors at mesopic-to low-photopic illumination levels. End-on transmittance spectrum of green rods looks similar to the reflectance spectrum of khaki military uniforms. So their greenness is the combined result of optics and human color vision.
ISSN:0340-7594
1432-1351
DOI:10.1007/s00359-014-0925-z