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?

181 views

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

This is from massgeneral.org

Surgical nerve repair involves exploration of the injured nerve and removal of injured tissue or scar from the nerve endings. After that, a nerve can be directly reconnected if there is enough length on the ends to allow for a good quality repair without tension.

Another website says after healing begins the nerves regrow and “reach out” to stimulate the surrounding tissue allowing for reattached limbs to work.

(Edit: I’m hoping to create a discussion blog, where we can discuss biology/medical related topics + read about interesting findings.)

Quick answer:

Recipient’s and new limb’s nerve fibers are connected => Recipient’s nerve fibre grows along the new limb’s nerve fibre and stimulate the surrounding muscles, making it functional (able to feel & move) => it takes months until the recipient’s nerve fibers have fully grown into the donated limb, making the whole new limb functional.

Long answer:

Limb transplant is a very complicated process and takes hours to finish the surgery. For a limb transplant to be able to regenerate function and sensation, the recipient must first have the nerves for the limb (literally), meaning the limb to be replaced was broken due to an accident or a disease. People who are born without that limb usually cannot perform this limb transplant, as they do not have the correct nerves.

The transplant surgery consists of 3 steps:
1) Fixating bones
2) Reattaching arteries & veins
3) Repairing tendons and nerves.

The third step is the answer to your question:

The nerves of the new limb will be surgically connected to the body’s nerve fiber. That’s when nerve regeneration and the healing process begins.

Healing process: After connecting the old and new nerve together, the recipient likely isn’t able to move right away. The recipient’s nerve fibre will take the new nerve as a template, the fibre grows outwards from their body into the donated limb. Until complete regeneration of nerve signals in the new limb, it can take months. That’s also one of the reasons recipients have to go to therapy to practice care for their new limb.

A bit off topic, but one of the most controversial body transplant-plan has been head transplant. But this is a whole new discussion.

.

Welcome back for updates on frequently asked questions

.

Edit:

1) How to surgically connect nerves?

“The surgical nerve repair will be done by stitches.

A nerve, in simple terms, is composed of the axon/nerve fiber (which transports nerve signals) and the myelin sheath (which protects the axon).

When reconnecting nerves it’s only possible to stitch the sheaths together and not the nerve fibers itself. However, stitching the sheaths together creates a pathway for the new nerve bundle/fibers to grow along.”

Answer from u/surprisestupidity in the comments: “Nerve bundles are composed of many neurons surrounded by myelin sheaths with various layers of connective tissue sheaths. Think of it like how a bundle of individual wires can be wrapped together to form a thick a thick cable. It is actually the connective tissue sheath(the wrapping) around a BUNCH of neurons that are stitched together. The individual neurons are still disconnected within the stitched sheath but ideally, over time, each recipient neuron connects with the ends of donor neurons in the transplanted limb.”

Answer from u/IceEngine21 in the comments: “Plastic Surgeon here: You can also use nerve tubes when there is a small gap between two nerve ends, as a sort of bridge and guidewire.”

2) How to stitch nerves together when they are so small?

“The process is called microsurgery.

The art is mastered by few top surgeon experts in the world, through the use of microscopes and special techniques.”

3) Why can’t we regrow the spinal cord?

“There are 2 nervous systems: the CNS (central nervous system) and the PNS (peripheral nervous system). A damage to the CNS is irreversible in most cases due to various factors, whereas if to the PNS the regeneration rate is around 30%. A limb transplant affects the PNS, therefore nerves can regenerate. However, a spinal cord injury affects the CNS where nerve fibers almost definitely won’t be able to regenerate. However, there have been many ground-breaking studies on this topic, making spinal cord injuries seem less impossible than before.”

4) How well does the limb work?

“Whether or not the patient can regain complete control is complex. Assuming the new limb isn’t rejected by immunity, the patient will need months to retrain their new limb. If the nerves regenerate successfully + adequate physical therapy, it’s possible for the new limb to function very well though, complex movements such as tying shoelaces would be possible too!”

5) How come the body doesn’t reject the new limb?

“1. Find a compatible donor: matching blood type, limb size, medical conditions.
2. Immunosuppressants after the transplant.”

6) Is it possible to regrow the optic nerve?

“The eye’s optic nerve is much much more complicated than other limbs’ nerves. For once, they are tiny and packed tightly together, each photoreceptors (rods and cones) have their own nerves to detect colours and brightness. And when there are about 125 million photoreceptors in a few mm space, that can get quite difficult, not to mention the optic nerve is more complex than just a bunch of nerve fibers:)”

7) How does neuron regeneration feel?

“A good explanation is:

When you have been sitting on your leg for a long time and stand up, your leg feels numb, unpleasant, tingling. This is how neuron regeneration can feel like around the time. The tingling feeling in one area means new nerve endings are formed there.

Pain would be a sign of irritation.”

8) Have head/body transplant ever been done?

“Head transplant have been done on cadavers and some animals successfully, even though they had to die shortly after that new transplant due to ethical issues.

It’s never been done on live human, but the Italian Dr. Canavero has worked on this goal for a very long time. A head transplant directed by him almost happened last year, but the patient decided to not commit to the transplant anymore”

.

Thank you for reading.

Here is a short youtube video about a hand transplant procedure

[YT-Hand Transplant](https://www.youtube.com/watch?v=d5bwRhHdgCU)

From what i know after removing dead tissues and recconecting everything people start to feel and sometimes even move the new limb.

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.

It slightly depends on the level (I.e. wrist/forearm etc).
Recently I reattached an amputated arm at mid-humerus level (about halfway through where the bicep is).
At this level there are a few named nerves that everyone has. The main three are the median, ulnar and radial nerves. So we find these and stitch them together using a microphone to try and line them up as well as we can. Then, as others have said, we hope the nerves grow down the sutured nerves.

If we’re lucky enough the nerves will grow across the join and down into the hand to give sensation and motor function

A subject I have personal experience with!

I broke my humerus playing roller derby a few years back and the fracture severed my radial nerve, meaning I lost the ability to lift my wrist and extend my fingers.

As other posters have noted your nerves are protected by a myelin sheath, which is basically a tube around the actual nerve. If you think about your cell phone charging cable the nerve is like the metal wire inside that carries the electrical current and the myelin sheath is like the materials covering and insulating it.

When a nerve is damaged the entire portion of the nerve from the point of damage to the termination (or “end”) dies as well. As long as the myelin sheath remains intact or heals properly the nerve will begin to regrow from the point of damage to the termination point.

So when performing a transplant or reconstructive surgery all the surgeons can do is connect these tubes and wait for the nerve to regenerate within.

Nerves grow very slowly, about 1 inch per month. The radial nerve which I severed in my accident invenerates the hand muscles at the top of your forearm. There was approximately six inches between this point and the damage site so it took about six months to fully regain control of my hand functions.

As far as I know this only applies to nerves branching off the spinal column. The spinal nerve itself will not regenerate or heal itself in most circumstances, which is why spinal injuries usually result in permanent disabilities.

But long story short, we can’t directly correct the nervous system through surgery or other intervention, we can only manipulate the tubes in which the nerves reside and wait for the nerves to repair themselves.

I didn’t have a transplant but I cut my finger off and feel reattached it. Their are a dozen or so vein clamps and nerve clamps in there. They told me they basically pull the nerves and vein together and the clamp it with the clamps to hold it and either stitch it together or do something else. I can post a pic of my x-ray and you can see the hardware in there.

I cut my left thumb nearly clean off, and had it reattached by a plastic surgeon. Took two surgeries.

They hook the nerves together with sutures. They are expected to regenerate sensitivity at a rate of 1 mm per month.

With my thumb, there are still numb areas 43 years after the operation. It doesn’t always work 100% evidently.

I’ve known about this since a young age.

My dad when he was 16 worked in a factory. It had a shop saw that would automatically come down and make 3 cuts.

One unfortunate day the chop saw caught his shirt and sliced his arm in 3 right at the elbow. The saving grace was that it went right at the elbow and couldn’t slice through the elbow as easily as the rest of his arm.
He basically had to constrict it and call 911 himself as several of his coworkers fainted when they saw what happened, and that his arm was literally hanging by skin.

When this happened to him it was early 70s in Toronto 1972/1973. They had very very very very limited experience with this procedure(he was one of the first 3-5 people they had done it on, but never for pretty much an entire arm) and for the first year of his recovery they honestly didn’t know if they had connected all the nerves and tendons back properly and if it was “wired” correctly.

He was very fortunate and they did manage to do that so he infact got full use and mobility of his arm back after about 2 full years of recovery(it took 1 year for the tendons and everything to heal properly before he could fully start rehab).

He has 3 insanely gnarly scars in succession on his arm. One right above his elbow one directly on it and one directly below it.

one of the nerves is so exposed on the back of his elbow that if he would normally hit his funny bone as one does it literally buckles him over in pain. (There were a couple times as a young child(4-6) where I thought it was funny and would go for that nerve when we were play wrestling, I would hit that nerve and feel like the David defeating Goliath, but when I was 6 and did it I saw how much pain I caused my dad while he was just trying to have fun and play wrestle and never intentionally did it again).

Considering how insanely vulnerable that nerve is I’m both surprised and impressed with how few times I’ve seen him tap it. But I guess it’s like Achilles protecting his week spot, he knows it’ll take him down so he’s extra cogniscent of it.

I didn’t lose a limb but came very close to it. I ripped a slightly larger than an adult male’s fist, sized hole out of my forearm, exposing the bone and severing nerves and tendons to some of my fingers and my hand.

From what my dad told me, the surgeons cut my arm open wider and using microsurgery, stitched the nerves and tendons back together and I got a skin graft over the gaping hole in my arm, from my thigh.

The hardest part was lots of physio later, but they were so amazing and I was playing guitar and piano a little after a year. All the while realising how easily that it could have been my head (accident was on a mountain bike and impacting a Roughcast wall)

I had a sheet of glass fall on me at work last year, they wouldn’t intubate me for the repair surgery fearing they would need the machines for covid patients.
spent about 3 hrs watching two “micro plastic surgeons” (I think that was their titles) they literally cut my wound wider and used a metal clamp to hold the it open. then used a combination of methods to stitch and glue my nerves an tendons back together…. the craziest event out of it all was feeling my nerves “recalibrating”. I spent an entire night in bed unable to sleep, it felt like my hand was possessed an moving around wildly, yet I was watching it sit there perfectly still, wrapped tight in a fiberglass splint.
the sensations came and went for about a week and now about a year later I would say I’m pretty much recovered other then some numb spots on my palm.