I’ve never heard of any research in this area. It’s obviously beyond reach of current technology. But is anyone even working on it?
We’d have to figure out how to sever and reattach nerves first. That’s still a ways away.
I know it’s probably opening a big can of worms to discuss, but what is the major hurdle in doing so with nerves?
Apparently a transplant operation more similar to a complete eye transplant than to routine corneal transplants was performed at a Chennai, India hospital just last February:
Yeah, this still avoids the whole optic-nerve issue, but we’re looking at replacing a fair chunk of the original eye there.
Size (waaaay tiny), and complexity (a huge number of individual connections, each of which has to be right).
We haven’t invented Dr. McCoy’s “sonic separator.” 
Well, this was just a transplant of the ‘optical’ part of the eye. Neither the optic nerve nor the retina were transplanted.
The problem with ‘reconnecting’ the nerve is that:
- we don’t know how to ‘reconnect’ nerve fibers.
- even if we did, the optic nerve contains about 1 million fibers, that you would have to cut and then reconnect… in the right order ! Forget about the adult brain adapting to a totally disrupted visual input.
There are some projects of retina prosthesis:
Visual prosthesis - Wikipedia
Retinal implant - Wikipedia
Here the approach is different, as you are simply sending electric stimulations to the retina
Because this is what the connections look like.
Because an eye is no use without connection to the appropriate part of the brain. To functionally transplant an eye, you would need to reconnect each axon (extension of a single nerve cell) correctly without killing those cells, or find a way to encourage the optical cortex (part of the brain that processes visual information) to regrow the correct nervous connection to the retina.
So . . . it comes down to nanotechnology, then? Are there any promising avenues there?
To create, and abuse, a metaphor, if you compare nanotechnology to transportation, we’re at the Roman chariot. We need a supersonic jet.
Hang on, what about microsurgery? I was under the impression that they could reconnect severed fingers or hands, and get back most, or at least some of the movement and feeling. Don’t they reconnect the nerves there?
Or are you specifically talking about optic nerves, as opposed to other types? Granted that the optic nerve might be more complex than nerves in your fingers, but is there a fundamental difference between them?
Yes.
The optic nerves develop directly from the central nervous system, whereas the other sensory and motor nerves throughout the body are parts of the peripheral nervous system.
The CNS and the PNS are supported by different types of glial cells with different responses to injury. The PNS, which is much more likely to be injured, has a pretty good routine; the Schwann cells secrete a bunch of growth factors and stay lined up in an orderly, tube-like fashion to direct regrowth.
In the CNS (and the optic nerve), on the other hand, the glial cells (oligodendrocytes and astrocytes) freak out and start producing scar tissue and growth inhibitors, in effect trying to isolate the damage instead of fixing it.
Even if you could line up the 1.2 million fibers correctly, you’d still have to contend with that hostile response to injury.
Well, first of all, wunderkammer points out some physiological aspects:
Furthermore, the complexity is a fundamental issue. The sensory nerve in the had will send some sensory information back to the brain. This is a relatively complex information as various sensations (pressure, heat, rubbing) coming from various part of the hand have to be transmitted, and the whole organization of connection between the hand and the brain will be disrupted. However, this is something that the brain can deal with:
- we know this from studies of limb prosthesis. Lot of prototypes offer some sensory feedback which improve motor performance.
- furthermore, the information which is sent back to the brain does not have to be of amazing quality. If the brain can simply manage to adapt to the transplanted hand, and recover some basic information about the positions of the hand (opened, close, thumb-up), and maybe the overall pressure inside the hand while grasping an object, this will greatly improve motor performance.
In other words, all you expect from a reconnected hand nerve is that the brain adapts to the disrupted input and manages to extract, say, 10 bits/s out of it.
In contrast, you need to be able to extract much more information out of the optic nerve if you want to get something that you can call a visual sensation as we call it in everyday’s life. With the current knowlege, there is no way that an adult visual cortex could adapt so much to a drastic change of its input. Indeed:
- obviously, the neurons in the visual cortex would have to aquire a new configuration which would be very complex.
- furthermore, they would have to ‘forget’ the previous one. Visual cortex is a highly organized structure. This probably limits its plasticity to a large extend.
An example of this limitation if that children with a strong strabismus have to be treated in the first year of their life. If the strabismus is compensated later by glasses or surgery, it is too late for them to adapt to the modified visual input and, for instance, they never regain stereo vision.
So, imagine that you would transplant an eye and reconnect the optic nerve somehow, and that the brain adapts nicely, what would you obtain ? My feeling is that you may restore a sensation like ‘there is some light in front of me but the left side is darker’. You would maybe also get motion information (‘something is moving to my right’) since the visual system deals a lot with motion information.
Sure, that would be helpfull for someone who is blind. However, you can do as good as that by simpler means, for instance but putting a camera and/or a laser rangefinder on top of the head and connected it to a matrix of tactile stimulator (e.g. a array of 10x10 little vibrators that you fix somewhere on the skin, on the shoulder for instance). Btw, this exists.
So, as a conclusion of this long post, you can restore senroy inputs which have been lost, by the mean of transplantation of prosthesis. The brain will adapt to a certain extent, but it will be a difficult training and the amount of information being aquired by the brain will always be limited. You would need to find a way to boost brain plasticity if you want to get better than that.