You're able to see the world — from words on a screen to clouds in the sky — because of some specialized nerve cells in the retina at the back of your eye.
Cells called photoreceptors respond to light, turn that response into an electric signal, and transmit that signal to the brain via nerve cells called retinal ganglion cells. The brain interprets those signals into what we know as vision.
When everything is working right, the relay looks something like this animation from The National Eye Institute, which shows photoreceptors as long skinny cells and retinal ganglion cells as the round ones:
Certain rare genetic diseases and common non-genetic ones damage photoreceptors and retinal ganglion cells, which then don't function as well and eventually die. When these cells are missing or not doing their jobs, we can't see.
The body only has limited means to repair nerve cells and no way to replace dead ones, so damage to the nervous system that causes blindness is considered permanent.
Damage and death of photoreceptors is behind the most common cause of blindness in developed countries, age-related macular degeneration.
Without photoreceptors, the eye can't begin to convert light into an electric signal the brain can interpret, and the world appears to go dark.
Blindness from glaucoma, one of the most common eye diseases around the world, is caused by retinal ganglion cell damage.
Broken retinal ganglion cells cut off photoreceptors from the brain, and their signals about light can't get through.
We have ways to treat vision problems from issues at the front of the eye, like surgery for cataracts and LASIK surgery to change the shape of the cornea for clearer vision. But fixing the damage at the back of the eye from glaucoma and age-related macular degeneration will require repairing or replacing nerve cells, something we're not able to do quite yet.
Stem cell research in animal models and gene therapy clinical trials for people with genetic diseases have begun to show promise, however. Scientists at the National Eye Institute are optimistic that, with continued research, certain types of blindness from nerve damage may be treatable in the near future and no longer permanent.
Watch the video from the National Eye Institute here.
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