system
Given the following post, respond as if you're commenting on Reddit:
human
If a nerve in your body is severed, can it grow back 
together?

I really don't want medical advice or anything, something just got me thinking recently... can a severed nerve repair itself? I really know very little about the nervous system of the body, but if someone were to cut their arm bad enough that it slices through a nerve and perhaps makes their hand partly numb, obviously the ER would be step one, but after that, would the nerve begin to re-knit itself and give the hand feeling once again?
gpt
The axon of a nerve cell—the long part that travels through nerves and conveys signals—can indeed regrow. There are some caveats. (Boy are there caveats.)

* The axon gets all its cellular nutrients from the cell body.
* The part of the broken axon no longer connected to the cell body lacks nutrients and gets disintegrated.
* The axon has to regrow from the point of injury.
* The new axon sprout has to find its way through the same pathway of Schwann cells to get to its original target.
* The further it is from the target, the less likely it is to make it there.
* Also, regeneration occurs at about 1mm/day.
* Without the guidance of the old nerve structures like Schwann cells and whatnot, the axon ain't gonna make it.
* Plus, slicing through a nerve disrupts those supporting structures, too.
* And, when talking about a whole nerve, you need _all_ the axons to sprout and regrow into the proper places through the right maze of supporting nerve tissue (which has also been damaged).
* So in the case of having a peripheral nerve actually severed, regrowth is basically impossible; a little less unlikely if the nerve is "reattached" surgically.