I thought nerves aren't regenerated either?
I had some nerve damage a while back and went to a neurologist. He did an EMG test (https://en.wikipedia.org/wiki/Electromyography) and confirmed the nerve was damaged. He used electrodes to show that it did not conduct electricity like it was supposed to, nor did it conduct electricity like the corresponding one on the other side of my body. (Somewhere, I have a printout showing voltage and latency numbers.)
He then told me that if the damage is not too great, nerves can repair themselves, that the repair proceeds outward from the brain toward the periphery of the body, and that it happens at a rate of roughly 1 millimeter per day. So, he said, my problem would probably resolve itself but it would be a long time, think 6 months or a year, maybe even longer to finally be back to 100%.
And it basically happened just like he said it would. 6 months later, it was a lot better, and it continued to gradually improve from there.
He also mentioned that using the affected muscles would help encourage healing. I think he meant the nerve itself would heal faster this way, not just that it would help reverse muscle atrophy.
He said that if the outer layer is intact, the inner layer can be repaired. I'm not sure if that means more nerve fibers form or that existing nerve fibers are repaired but that the repair takes place incrementally instead of all at once. Intuitively it seems to make sense that a new fiber is growing, but I'm wary of speculating based on what seems to make sense to me.
We do not want nerves to regenerate in most cases because it could cause even worse results. If you start regrowing a batch of nerve, you have a chance of growing out nerves that transfer pain signals into places you don't want them to be, which could cause persistent neuropathic pain (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5290455/).