While nerves in the arms, legs and peripheral organs can sprout back at the rate of up to 1 millimeter per day, adult neurons in the brain or spinal cord, the so-called gray matter, were long assumed incapable of growing back after an injury.

FORBES: To Walk Again

But in the early 1980s McGill University researcher Albert Aguayo showed that rat spinal nerves would grow like crazy inside a piece of peripheral nerve that was grafted onto the spine.

FORBES: Magazine Article