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#17 The optical fibers in our retina

Various animals evolved light-sensing structures. Among them, vertebrates and molluscs evolved sophisticated simple eye structure. However, due to the constraint of embryonic development, in vertebrates, the direction of light transmission is opposite to the direction of neuronal signal propagation. That is to say, the light must pass through layers of structures in retina to encounter the light-sensing receptors. Different layers of structures have different refractive indexes. Besides, the diameters of nerve fibers are around 500 nm, which is comparable to the wavelength of visible lights. These factors cause significant diffraction and scattering. How could we see the high-quality images that we have been taken for granted?

There are a group of specialized glia cells called Muller cells. Muller cells are cylindrical cells which span across the whole layers of retina. Scientists believe that Muller cells serve as the supporting structures of retina and modulate the concentration of neural transmitters and potassium. However, are these the main purpose of these cells?

Some researchers discovered that the refractive index of Muller cells is significantly higher than its peripheral tissues. The refractive index of the soma of Muller cells is 1.380, compared to 1.336, the refractive index of vitreous body, and 1.358, the average refractive index of peripheral retinal tissue. That means Muller cells are capable of entrapping light by total reflection. Studies performed by laser found that light transmitted in Muller cells are enhanced by more than 2 fold compared to the light transmitted in culture medium. Furthermore, experiments performed on isolated retina concluded that light transmitted between Muller cells suffered from serious lateral scattering and lost its resolution, while light transmitted in Muller cells are not strongly scattered. That means at least in some animals (some vertebrates have Muller cells that are too thin to transmit light), Muller cells may be the living optical fibers!

Aside from Muller cells, the outer nuclear layer of retina may also act as a lens array, and the inner segment of rods and cones may also serve as light collectors. The physical details of above discussion would be left for the readers to search, and we may come back to it someday.

K. Franze, J. Grosche, S. N. Skatchkov, S. Schinkinger, C. Foja, D. Schild, O. Uckermann, K. Travis, A. Reichenbach., and J. Guck. Muller cells are living optical fibers in the vertebrate retina. PNAS 104(20): 8287-8292. (2007)

#biophysics #3minBiophysics #生物物理 #三分鐘生物物理
#retina #Muller #cell #optics #optical #fiber