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If we had the technology to directly interface electronic devices with the nervous system and we wanted to connect a robotic arm to a person that lost his/her real arm, would we be able to find the same "wires" that control(led) that arm in every person?

Or does every human control their organs in completely unique, "incompatible" way?

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Probably the best way to answer this question is with a counterexample to humans: the nematode worm C. elegans has 302 neurons. Every individual has exactly that number, and each is found in the same place with the same function in every individual. This is one of the things that makes C. elegans an attractive model organism for study.

Humans definitely do not develop the same way. There are certainly groups of neurons that are found in every individual, and many of the tracts have at least a typical anatomical course that can be found from individual to individual, but it's not as if you can arrange each of the tens of billions of neurons in a list and find the same "Neuron #10,934,035,587" in two different people.

The most successful prosthetic operation that interfaces with the neuromuscular system these days is through myoelectric control, where rather than using electrical activity from neurons, one uses electrical activity in muscles, such as muscles in the chest and shoulder to control a prosthetic arm or hand.

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    $\begingroup$ @Barmar They can connect nerves, but not neurons. The individual neurons need to re-grow through the nerve structure. "Re-wiring" the hand from that point onward is a result of extensive physical therapy and more about plasticity in the CNS than rewiring in the periphery. A similar process would happen if a nerve were severed and reattached in the same patient without a transplant; it's also somewhat similar to how those connections are developed in the first place. Human infants are born with fairly incomplete motor control and so are an (adorable) excellent insight into this. $\endgroup$
    – Bryan Krause
    Sep 22 at 14:32
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    $\begingroup$ (it's also worth noting that a lot of hand control occurs outside the hand) $\endgroup$
    – Bryan Krause
    Sep 22 at 14:36
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    $\begingroup$ @Barmar Sure, but isn't it a bit superficially obvious that if you have a finger, it has to have nervous innervation? "People have fingers" is enough to tell you that; the interesting part would be considering whether there is some one-to-one relationship between the neurons and structure of the finer nerve endings in two people; there isn't. $\endgroup$
    – Bryan Krause
    Sep 22 at 14:44
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    $\begingroup$ @Barmar Only at a very gross level. For example see this question: biology.stackexchange.com/questions/82918/coronary-circulation about a common difference in the coronary circulation, and that's in a couple of major vessels together supplying over half of the blood to the heart. Have a look at the superficial veins in your arm that are quite visible from the surface: you'll find they vary quite a lot. $\endgroup$
    – Bryan Krause
    Sep 22 at 14:51
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    $\begingroup$ WRT C. elegans, neurons are actually a fairly high level of nervous system organization. Neurons have many dendrites (IIRC something like 1-10K branches in humans) & many synapses whose state changes due to synaptic plasticity. And researchers keep finding more low-level complexity: quantamagazine.org/… $\endgroup$
    – jamesqf
    Sep 23 at 2:31

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