How Do Nerve Signals Differentiate Muscle Movements in the Arm?

[hisham.i]

Hello..
Am an Electrical Engineer, my FYP concerns the artificial limbs... I made a small research about the nervous messages, and how it flows from the brain until it reaches the muscles. In the arm i found that there are 3 nerves (Ulnar, Median, and Radial nerve) which are responsible for arm movements, but what i didn't understand, if these nerves are common to all muscles in the arm, forearm, and the hand, so what is the difference between the message which cause our muscle to contract, and the message which causes the thumb to move?

 

[bobze]

Hello..
Am an Electrical Engineer, my FYP concerns the artificial limbs... I made a small research about the nervous messages, and how it flows from the brain until it reaches the muscles. In the arm i found that there are 3 nerves (Ulnar, Median, and Radial nerve) which are responsible for arm movements, but what i didn't understand, if these nerves are common to all muscles in the arm, forearm, and the hand, so what is the difference between the message which cause our muscle to contract, and the message which causes the thumb to move?

Good question!

The short answer is there are more than just those "three nerves". While those serve as the main "trunk", they each have many branches coming off of them, and each are composed of many axons which carry the message from the central nervous system to the motor end plate (neuromusclar junction). So while at first it would appear to be problematic, its a level of detail that is often missed by the layman.

Think of each of those three "trunk nerves" as giant braided ropes, made of braided ropes, made of braided ropes, etc. So at the smallest level you would have an individual braided rope made of strings, similarly at the smallest level you have nerves made of small collections of axons, which branch off to different parts of a muscle.


Hope that helps.

 

[mugaliens]

And if you'd really like to get into it, in the words of a good friend, a magna cum laude of my alma mater, turned biological engineer (both masters and PhD level (and I haven't been able to locate him since)), once told me (in 1990), I don't know what we'll be able to do ten years from know. The human nervous system is so incredibly complex. Our best approach would be if we can simply put the neurons back into a chemical environment where they learned to connect in the first place, and retrain the patient by putting them through the same stages they went through as babies, adolescents, etc., I think that would be the best approach, although obviously, it would take only a few months, not 12 years. On the other hand, for severe neurological damage, we're looking at tapping into salamander stuff, you know, the ability of them to regrow entire limbs?"

I really wish I could get hold of this guy...

 

[hisham.i]

Well, i understood what you said bobze, but it still not clear.
If these main nerves have branches to different muscle, when they conduct a message why should it pass through branch "x", and not in branch "y"? :S
I need more details about the difference of different hand movements and nervous messages responsible for each action...
Mugaliens thanks for your comment, really the technique we are going to use is called "targeted muscle re innervation", and you can see this link for more details about this technique.
http://www.sciencedaily.com/releases/2007/11/071111182522.htm

 

[hisham.i]

Any comments?!

 

[jaseh86]

Hi Hisham.i,

What bobze said was correct but I think you may have the misunderstanding that nerves "branch" like a tree. Like he said try to think of them as a braided rope.

The smallest unit of a nerve is the axon of the motor neuron. A single nerve fibre such as the median nerve of the arm contains over hundreds of individual axons. Each axon are carries bursts of electrical potential that travel from the spinal cord all the way to a set of muscle fibres in ONE muscle (usually).

Take the median nerve for example, it innervates both flexor carpi radialis (flexes wrist) and flexor digitorum profundus (flexes finger). Obviously we can activate each muscle independetly but the axons that control them both travel down the median nerve. This is because the individual axons for the wrist start at different locations in the spinal cord compared to the fingers, but they all travel down the same "highway" or "route" which is the median nerve. They then leave at different levels down the arm. Each set of axons can also be differentially activated by specific neurons from brain called pyramidal neurons, each set controlling a different set of muscles. Thus the body's muscle movements are arranged in a thing called a "somatotopic map", which basically means different muscles are activated by location-dependent cues.

As for the actual nervous system messages, they are sent as bursts of high frequency electric currents called action potentials (initiated by the influx of Na). The frequency of these action potentials is what controls the strength of muscle contraction.

Hope that helps

 

[hisham.i]

This is because the individual axons for the wrist start at different locations in the spinal cord compared to the fingers, but they all travel down the same "highway" or "route" which is the median nerve. They then leave at different levels down the arm.

Ok that's fine, but at the down side to the muscle, how does the message know that it should innervate the finger muscle or the hand muscle, really i still cannot imagine, and understand it, please if you can give me more details...
Best regards

 

[bobze]

Ok that's fine, but at the down side to the muscle, how does the message know that it should innervate the finger muscle or the hand muscle, really i still cannot imagine, and understand it, please if you can give me more details...
Best regards

So going back to the rope analogy. Suppose at one end of the heavy braided rope we have the "bodies" of each individual sting. Transmitting a message from "body end" to the correct muscle would simply be a matter of knowing which "body" to active to send the message down the string in the "big braid" until the point it branches off to the correct muscle.

Do you have kids Hisham? Or have you ever observed a baby and how "twitchy they are"? Those twitches are their brains figuring out the "best" pathways available to them to activate different muscle groups. Excess and unnecessary nerves are culled via apoptosis, while the "good connections" remain.

So if my brain wants to send a message to move my finger, it does so by dumping neurotransmitters on the cell bodies who's axons will run to the correct muscles.

Edit: This of course, is a simplified overview of a very complex process.