How Does the Retina Work?

[peter.ell]

I'm wondering how it's possible for vertebrates like us to see so well, or at all, if our photoreceptors are actually pointed away from the light focused on the retina. I would assume that the light would just pass right by the photoreceptors and we wouldn't be able to see much except for the presence or absence of light.

The fact that photoreceptors are pointed away from the light seems like it wouldn't allow for good vision similar to how you can't see the sun if your head is turned away from it. Yet it works for us and many other vertebrates with even better vision but the same design. How?

[Ygggdrasil]

Part of the answer seems to be that some of the glial cells in front of the retina act like fiber optic cables to channel the light from the retina to the photoreceptor cells. See the following paper:

Franze et al. (2007) Müller cells are living optical fibers in the vertebrate retina. Proc Natl Acad Sci USA 104(20): 8287. doi:10.1073/pnas.0611180104 (http://dx.doi.org/10.1073/pnas.0611180104), PMC:1895942 (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1895942/)

Abstract:
Although biological cells are mostly transparent, they are phase objects that differ in shape and refractive index. Any image that is projected through layers of randomly oriented cells will normally be distorted by refraction, reflection, and scattering. Counterintuitively, the retina of the vertebrate eye is inverted with respect to its optical function and light must pass through several tissue layers before reaching the light-detecting photoreceptor cells. Here we report on the specific optical properties of glial cells present in the retina, which might contribute to optimize this apparently unfavorable situation. We investigated intact retinal tissue and individual Müller cells, which are radial glial cells spanning the entire retinal thickness. Müller cells have an extended funnel shape, a higher refractive index than their surrounding tissue, and are oriented along the direction of light propagation. Transmission and reflection confocal microscopy of retinal tissue in vitro and in vivo showed that these cells provide a low-scattering passage for light from the retinal surface to the photoreceptor cells. Using a modified dual-beam laser trap we could also demonstrate that individual Müller cells act as optical fibers. Furthermore, their parallel array in the retina is reminiscent of fiberoptic plates used for low-distortion image transfer. Thus, Müller cells seem to mediate the image transfer through the vertebrate retina with minimal distortion and low loss. This finding elucidates a fundamental feature of the inverted retina as an optical system and ascribes a new function to glial cells.