When a person receives a limb donation, how do surgeons “wire up” the nerves so that the recipient can use the limb and feel sensation from it?
In: Biology
They don’t. Not directly. The correct nerve end on the recipient’s remaining limb is attached directly to the nerve fibres of the donor limb, but this doesn’t ‘rewire’ the nerve. Instead, the recipient’s nerves then use the donor nerve as a kind of scaffold, and will grow down the same path over many months, gradually connecting with muscles and skin as they go. This is an extension of what nerves do all the time.
For example, when you exercise and your muscles get stronger, one of the ways they do this is by increasing how strongly a muscle receives a signal from a nerve. This isn’t about sending a stronger signal – its the same signal, but our nerves grow in response to being used, and create more nerve endings, meaning the signal is repeated multiple times along the same muscle fibre. Nerves and muscles will shrink by default, but will grow if the brain keeps asking them to do things. In the case of a severed limb, the brain is still asking for information on that area but not getting it, and it’s still trying to send movement signals to that area, so the nerves will still try and grow. In the case of our fully developed nerves, for example the nerves on your hand right now, the brain is getting all the information it asks for, so there’s a balance between the brain saying ‘stretch out and tell me stuff’ and the nerves saying ‘relax and stretch less, so we use fewer resources’.
The challenge is that it takes time for nerves to regrow. They grow at a rate of around 2mm a day. For a hand transplant, this means it takes roughly 2 years for the nerves to reach all the different muscles and skin surfaces. In a larger transplant, it takes even longer. However, when muscles aren’t used, our body breaks them down and absorbs them – this is called atrophy. In the case of a full arm transplant, the muscles would fully atrophy long before the nerves reached them. This is the same issue with spinal cord injuries. The nerves can and do regrow naturally, but so slowly that muscle atrophy outpaces them.
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