Member (Idle past 4083 days)
From: Brisbane, Australia
Message 1 of 3 (418451)
08-28-2007 8:50 AM
I doubt this subject has been dealt with yet, given its very, very specific nature. Maybe it came up once or twice, but I doubt it.
Anyway, mentioned here is one creationist claim that isn't false, isn't a lie and isn't misrepresenting science. No, this is, as far as I can tell, is true.
The optical system of the human eye is such that ambient light tends to fall with peak intensity on the macular area of the retina with much less on the retinal periphery. It must be significant therefore that not only is melanin more abundant in the macular region because its RPE cells are taller and more numerous per unit area than elsewhere but there is also in the retina’s central area the yellow pigment xanthophyll (Greek: xanthos, yellow). In this region of the retina, xanthophyll permeates all layers of the neurosensory retina between its two limiting membranes and is concentrated in the retinal cells, both the neurons and the supporting tissue cells. Recently attention has been drawn to the presence of a collection of retinal supporting tissue cells (called Müller cells after the person who first described them) over the internal surface of the fovea and forming a cone whose apex plugs the foveolar depression Besides providing structural support for the fovea, it is thought that the Müller cells, particularly in this location, act as a reservoir of xanthophyll.
Retinal xanthophyll is a carotenoid, chemically related to vitamin A, whose absorption spectrum peaks at about 460 nm and ranges from 480 nm down to 390 nm It helps to protect the neurosensory retina by absorbing much of the potentially damaging shorter wavelength visible light, i.e. blue and violet, which is more scattered by small molecules and structures. Studies have shown that the retina’s sensitivity to photic damage increases exponentially with decreasing wavelength, being six times more sensitive to ultraviolet radiation (UVR) than to blue light. However, almost all of the non-ionising radiation with wavelengths shorter than 400 nm is blocked by the combination of cornea and lens, leaving a remaining dangerous band of wavelengths, 420–450 nm in the blue part of the spectrum, against which xanthophyll is an effective shield.
The presence of this pigment was well demonstrated in practice when ophthalmologists used to use the argon laser in which the emitted blue light was at first not subtracted; xanthophyll would absorb the blue light and produce an unwanted burn in the neurosensory retina. Modern ophthalmic argon lasers are made to emit only green light for this reason.).
I guessed that like the rest of the paper, it would be nonsense or misinformed. But, it isn't.
Landrum, J. T. and Bone ,R. A., "Lutein, Zeaxanthin, and the Macular Pigment" Archives of Biochemistry and Biophysics 385(1), 1 January 2001, p28-40 http://dx.doi.org/10.1006/abbi.2000.2171
PROTECTION AGAINST PHOTODAMAGE
It is well known that intense light can produce damage in the retina . The action spectrum for light-induced damage shows a distinct maximum at wavelengths between 400 and 450 nm, consistent with the absorption spectrum of the [macular pigment] (MP). Several studies show clear evidence that MP attenuates photic damage in the human retina. Haegerstrom-Portnoy has reported that the age-related decline of retinal sensitivity of the short-wavelength (blue) cones is reduced in areas where MP levels are highest. A clinical condition, known as Bull’s eye maculopathy, associated with photosensitizing drugs, is characterized by retinal degeneration in an annular pattern which surrounds but significantly spares the macula. Photic damage by the operating microscope has also been reported to result in lesions, but the damage is least in illuminated regions that overlap the MP.
MP protection of the retina from photic damage has been postulated to occur through two different functional roles. The first of these is through absorption of blue light as it enters the inner retinal layers thereby attenuating the intensity and potential for photo-oxidation
of reactive unsaturated lipid components of photoreceptor disk membranes. This might be referred to as the passive protection mode. Lutein and zeaxanthin present in the inner retinal layers are distant from the photoreceptors and the underlying retinal pigment epithelium
where photic damage is believed to produce pathological effects. The lutein and zeaxanthin in rod outer segments are potentially intimately associated with photo-sensitive components. While 1O2 has not been directly detected in the retina, its presence has been postulated based upon the sensitivity of the retina to photic damage and the demonstrated ability of natural heme to function as a sensitizer.
And also, from the same article:
As discussed below, absorption of blue light is a potentially significant function of the macular pigment. At normal levels, between 20 and 40% of light at 460 nm is being absorbed in the macula.
So there you have. Finally, a benefit to our peculiar retinal arrangement. Not sure if the protection this provides compensates for the risk of retinal detachment, but it's a start.
Unless, of course, I'm missing something. What do you guys and gals think?
(as an example of unintelligent design, I'm guessing the eye belongs in that forum).
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