Robert O. Becker M.D., a bone man, while at a V.A. hospital in northern New York state, used salamanders while trying to discover how come they can grow a new leg or tail, while we can't. It turned out that the difference is that they have relatively huge diameter nerves compared to brain size, while we have wee skinny nerves in comparison - so the electrical field generate by the ends of the cut nerves is too weak (and, he wrote, switches to the wrong polarity).
He invented the electrical field-generating widget that is used to this day to make bones heal better and quicker in hospitals, and actually got human amputations to regrow an inch of bone before his grant was cancelled.
His book was called "The Body Electric".
Nothing to do with nerve cells, it's relatively easy to grow back fingers etc. as long as you catch the injury in time
