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Say your brain wants to retract your right hand.

If there is some information coding playing a major role here, then what is the mechanism to find the correct path to reach the correct muscle?

Or is it every part has a single nerve connecting to the brain?

Any difference when it comes to withdrawal reflex comparing to brain initiating action? i.e. how the reflex round trip reaching the correct muscle?

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    $\begingroup$ This is a fairly basic question. Have you tried google? Do you know the difference between neurons and nerves? $\endgroup$ – Oliver Houston Apr 10 '17 at 6:58
  • $\begingroup$ Yes, tried but no avail. "how nerve impulse finds the correct path" is what I tried. But I only found results about how a nerve impulse travels, but not the mechanism on how it can select the correct path to reach the correct destination. $\endgroup$ – user1589188 Apr 10 '17 at 7:33
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    $\begingroup$ Here's a recent video I made in response to a similar question. It's called "Spinal cord is (not) a bus". youtube.com/watch?v=e5TRfya3FPk ---------- >! Basically, spinal cord is pretty much a labeled line! $\endgroup$ – Memming Sep 5 '18 at 1:17
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OK, I think I can answer at the right level now.

Firstly, a nerve is a bundle of neurons, each neuron is like a fibre. So the sciatic nerve in the leg will carry fibres from the spinal cord to the muscles in the leg, like a data cable might connect many computers in an office to a server on another level.

Each neuron will form connections with a specific area on a specific muscle, at a specialised synapse called the "neuromuscular junction". When a signal travels down that particular neuron, it will release neurotransmitters that activate that particular muscle region. So by sending signals to the right neurons, the muscle to be activated and how much to activate it can be selected by the CNS.

A nerve impulse is a slightly misleading term as it refers to the collective activity of signals within the many neurons of the nerve, so an individual neuron won't register on an impulse. The signals in neurons are called "action potentials".

There is some information coding in the frequency of action potentials within neurons, but it is used to control the response of the muscle, rather than locate the correct muscle.

Wikipedia is pretty good on the subject if you know where to look:

neuron

withdrawal reflex

sciatic nerve

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  • $\begingroup$ Right ok. The question is still there: "How to select the right neuron so that the correct destination can be reached"? Using computer terms, we have router to send signal to the correct computer and the router can look at the signal header to know which ip to send. $\endgroup$ – user1589188 Apr 10 '17 at 15:34
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    $\begingroup$ The motor cortex in the brain is divided into regions associated with each part of the body: memrise.com/mem/3977517/primary-motor-cortex so activity in the part associated with the hand will lead to movement of the hand. i.e. if you stick an electrode into someones motor cortex you can trigger twitches in certain muscle groups. It involves a massive amount of processing, and learning to generate smooth, controlled movements. $\endgroup$ – Oliver Houston Apr 10 '17 at 22:29
  • $\begingroup$ Thanks. You mean the only selection point is at the brain? And the rest is just signal propagation to the nerve end point? Isn't there some intermediate relay nodes like from the brain to the spinal cord? Or these relay nodes are also 1-to-1 mapping to the brain cortex? $\endgroup$ – user1589188 Apr 11 '17 at 1:14
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    $\begingroup$ Sort of, there is a relay in the spinal cord, where upper motor neurons pass signals to lower motor neurons, but yes I believe it's one to one. There is also input from other brain areas, such as the cerebellum and brainstem that helps fine-tune movements, but I can't remember much about it. Also sensory neurons (usually pain) can directly activate motor neurons in the spinal cord, e.g. during the withdrawal reflex, so no brain input whatsoever. $\endgroup$ – Oliver Houston Apr 11 '17 at 14:35

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