system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does paralysis occur? Why don't nerve cells regenerate and reconnect like other cells?
gpt
Okay, so you know how cells are really really small?  Well, in nerve cells that's not quite true. Nerve cells have normal sized bodies, but then really really long tails (axons) that connect them to other cells or parts of your body. The nerve that feels a spot on your big toe lives all the way up in the spinal cord... It's part of a single cell, not a chain of cells. 

That long tail doesn't travel alone... The body bundles it together with other nerve cells going to a similar place, and surrounds the whole thing with connective tissue to protect it. Very similar to a bundle of cables in a computer, but with many many more cables. 

Now imagine cutting through part of that bundle. Well, the cells themselves are alive and well, but the tail that has been cut off will die.

The cell will try to regrow along the connective tissue guide that's left behind, and sometimes it can if the injury is a nice clean slice. Sometimes the nerve will reconnect to the wrong tube, but the brain will sort that out (mostly).  So you can totally regain some movement/function after an injury like this.  

But usually there's just not enough of a guide for the nerve to know where to grow to. Often the new nerve tail just ends up making a messy ball at the injury site (which causes all new problems). The nerve tails don't end up where they need to be, so you lose the function of that nerve. If the injury is bad enough, and enough nerves can't find there way back to the right places, you permanently lose function.