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First off, there are lots of causes that could lead to someone not being able to walk, and severed nerves or damaged neural pathways are only one of them.

Nerves cannot be naturally regrown by the body, and are delicate and difficult to work with. So far it hasn’t really been possible to do the kind of reconnection you’re thinking of. But there are groups working on that! For example there are projects that implant sensors “upstream” of the break to measure signals and then stimulating electrodes “downstream” of the break to match those measures signals. They’ve started having some pretty remarkable success too.

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> > > > > Nerves cannot be naturally regrown by the body,

Not entirely true, but in the CNS the regeneration capacity is extremely limited. Peripheral neurons can regenerate to a reasonable degree, typically guided along the existing path of supporting Schwann cells.

Thank you for the correction!

Would you be able to cite these projects please?

We think of walking as a rather simple task that the majority of the population does everyday without thinking. In reality, its actually quite a complex process that requires numerous brain areas as well as your senses constantly providing feedback.

The main motor system consists of your motor cortex with upper motor neurons (in the brain), your spinal cord with lower motor neurons, and the muscles they innervate. All those need to be firing in a complex pattern (tense the quad of one leg and release the hamstrings while doing the opposite on the other leg for example).

We also need to maintain balance. We do that by taking in 3 sensory inputs: the feel of the ground beneath our feet, the orientation of our environment with our eyes, and the placement of the fluid within our vestibular organs in our ears. All of those systems combine within the basal ganglia (like the thalamus) and the cerebellum to drive changes in our gait that allow us to stay upright even on uneven ground.

Walking is also a rhythmic motor movement so we need to maintain an even pace for each step. You can imagine how difficult it'd be to walk if every step went a different distance or your speed constantly increased or decreased. The cerebellum also is in charge of that and modifies our gait to maintain a certain pace.

All that is required just to walk on a treadmill. If we're also in a complex environment, let's say a sidewalk, we're also going to need to use our prefrontal and premotor cortices for planning future movements. If someone is walking in front of us, we'll need to plan a route around them and maybe speed up a little to pace them. If you see someone start to cross the street and they may intersect our path, we now need to calculate how fast they're going, the most probable path they'll take, and how we can change our own gait to prevent collision. Of course nearly all of this is done unconsciously without you even realizing it but it all needs to happen to successfully navigate just a sidewalk.

All of this is to say when people can't walk, there could be a multitude of problems and all these pathways contain millions if not hundreds of millions of connections. So there isn't a simple solution of reconnecting the pathways since its incredibly complex. Spinal cord injuries, uncontrolled diabetes, strokes, Parkinson's disease, amyotrophic lateral sclerosis, cancer, vertigo, etc are all disease that make it hard to walk but they affect different pathways.

Pyramidal system (motor cortex) - https://www.ncbi.nlm.nih.gov/books/NBK540976/

Extrapyramidal (everything else) system - https://www.ncbi.nlm.nih.gov/books/NBK554542/

Of course! This is something I’m mostly familiar with from the popular press, so I’ll leave it to someone else to dig up the more technical papers but:

Here’s an article in Science about recent projects and results.

Here’s a more technical article from Nature Medicine from the research team that pioneered this technique

Here’s a human interest story from a hospital network that performed one of these operations which gives a look at what this is like for a patient.

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Thank you!

Focusing more on your second sentence and taking it down ro the simplest level: imagine after a haircut someone was tasked with reattaching each cut strand of hair to the strand on your head that it *originally" came from. That would be an easier task.

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