Michael Stevens asked:
I went to see one of the plasticized human exhibits. [This is where they get a particular organ or system and they effectively fill it with plastic so youíre left with a model of a human system]. I was amazed at the number of peripheral nerves there are in the body. I thought that any injury could easily cut through some of these nerves and I have always been told that nerves canít regenerate. So if all of these little injuries can damage your nerve then how do nerves regenerate?
This was asked along with Or's question:
The nervous system is divided into two camps. There is the central nervous system which is your brain and your spinal cord. Then there is the peripheral nervous system which is everything else. The central nervous system (the brain and spinal cord) if you injure that as far as we know, itís permanent. The nerve cells may die, they may not die but they certainly donít reconnect where they should connect. That stops signals getting through which is why you get problems of paralysis of loss of sensation depending on where the damage is. Thatís why a stroke is so disabling.
In the skin the nerve cells there seem to be able to survive. They also seem to be able to re-grow their targets so they go back to where they should have gone in the first place. They reconnect to, if it was a muscle they were supposed to be supplying, theyíll reconnect with the muscle. If it was a patch of skin they can branch out and re-supply the skin so you do get sensation back. Nerves grow quite slowly, probably a couple of millimetres a day. So if youíve got a big injury the length of your arm it can take a few weeks before the nerves can get back to your arm. The sensation may not be absolutely perfect because some nerve cells might die but you should get coverage of the skin back afterwards.
What happens when you break the nerve the actual cell inside is just one massive long cell. The distal bit (the bit downstream of the cut site) will degenerate. It retracts and forms this little lump bulb. This thing grows back along the original path of the nerve so it uses the original pathway of the nerve as a guide. Rather like a motorway cone. It uses the cones and lays down a new road surface, which is the nerve, and it gets back to where it was supposed to attach. The distal site it was supposed to attach to switches on various markers so it can recognise it and off it goes!