A protein needed to re-grow injured nerves in limbs has been identified, raising the prospect of new treatments.
The findings, in mice, have implications for helping patients recover from peripheral nerve injuries.
They also open up new pathways for investigating how to regenerate neurons in the spinal cord and brain.
Peripheral nerves provide the sense of touch and drive the muscles that move the arms, legs and feet.
Unlike central nervous system nerves of the spinal cord, they can regrow after being cut or crushed. But how this happens is still not well understood.
Scientists conducting the new research, reported in the journal Neuron, identified a signalling protein that helps switch on the regeneration process.
The molecule, called leucine zipper kinase (DLK), regulates signals that tell a nerve cell it has been injured, often communicating over distances of several feet.
Mice lacking DLK were unable to regrow severed nerves.
Lead researcher Professor Aaron DiAntonio, from Washington University in St Louis, US, said: "DLK is a key molecule linking an injury to the nerve's response to that injury, allowing the nerve to regenerate.
"How does an injured nerve know that it is injured? How does it take that information and turn on a regenerative programme and regrow connections? And why does only the peripheral nervous system respond this way, while the central nervous system does not? We think DLK is part of the answer."
Understanding the role of DLK may also be relevant to the central nervous system, say the researchers.
"Since this sort of signalling doesn't appear to happen in the central nervous system, it's possible these nerves don't 'know' when they are injured," said Prof DiAntonio.
"It's an exciting idea - but not at all proven - that activating DLK in the central nervous system could promote its regeneration."