It has long been held that a person can no longer see again if the optic nerve responsible for vision is damaged. A new animal study, however, is challenging this idea as researchers were able to help mice with damaged optic nerves see again.
Although there are no guarantees that the results can be replicated in humans, the research points scientists to promising new directions that may possibly help millions who suffer from vision loss.
For the research reported in the journal Nature Neuroscience on July 11, Andrew Huberman, from Stanford University, and colleagues destroyed the optic nerve in one eye of lab mice.
Optic nerves transmit impulses from the eye to the brain. Once damaged, they sever connection to the brain, and this results in blindness.
"When those cells' axons are severed, it's like pulling the vision plug right out of the outlet," explained Huberman.
The animals' condition resembled glaucoma, the second top cause of blindness after cataracts and which affects 70 million people worldwide. It is caused by excessive pressure on the optic nerve and unlike cataract, glaucoma currently has no cure.
Huberman and colleagues found that subjecting the mice to a regimen of high-contrast visual stimulation of the nerve and biochemical manipulations can rescue and regenerate the axons, the thread-like part of the nerve cells that serve as the transmission lines of the nervous system.
The researchers also found that these optic-nerve cables can retrace their former routes and reestablish connections with the appropriate areas of the brain, resulting in restoration of vision in the blind animals.
The restoration of eyesight was proven when the animals passed two tests where they detected large objects and movements, albeit they failed in tests that required more detailed vision.
"RGC axons navigated back to their correct visual targets and avoided targets incorrect for their function," the researchers wrote in their study. "Moreover, these regenerated connections were successful in partially rescuing a subset of visual behaviors."
University of Washington ophthalmology professor Russell Van Gelder, who was not involved in the study, said that visual input and growth activation stimulates the brain cells to grow so it makes sense that both of these processes would be required for regeneration.
He said that it is possible to revive the communication between the eye and brain in development via a combination of the right growth factors to the retinal cells and a pattern of activities that the brain can utilize.
