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DocTreeMedicinal t1_jdtyfnj wrote

Motor commands come from the brain into the periphery.

If you were somehow able to press your primary motor cortex in your brain, you'd see a loss of muscle function (paralysis)

It is most likely that when pressing on a body part you are temporarily compressing a peripheral sensory nerve. Hope this helps

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Mord42 t1_jdtyoy1 wrote

But why does the sensory neurons go before motor neurons? The same is true for peripheral hypothermia, for example. When your hands are exposed to cold temperatures and go numb, you can still move the muscles but not feel anything, even with your proprioceptors.

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smallbutlazy t1_jdu1n9y wrote

That is a really good question, I would like to know more myself.

This page has some clues, https://www.openanesthesia.org/keywords/peripheral_nerves_sensory_vs_motor/

regarding the motor nerves they do have a different composition and conduction velocity compared to nerves associated with sensory activity.

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lollroller t1_jdvpuyf wrote

It is a good question. It is also true regarding both spinal disc herniation on nerve roots, and disc herniation on the spinal cord itself, that sensory pathways are involved far, far, earlier than motor pathways. And in fact, motor involvement is considered much more severe and is often what leads to surgical repair

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Any_Respond_9011 t1_jdvlcef wrote

It could be that they are more important for survival, therefore it's "worth" making them more robust. It's unpleasant to wake up with tingly/numb legs, but it's much better than not being able to run/defend from danger because you can't control them.

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Mord42 t1_jdvwy0p wrote

That makes sense but ultimately isn't a satisfying answer for me. Personally I'm more concerned about the specific biochemical pathways that control and cause this effect.

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keestie t1_jdu6sfo wrote

When I sleep on my arm for long enough, I lose the ability to move it until the blood flow is restored. It takes longer tho, and as the blood returns, movement comes back first, then the the sensation.

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Black_Moons t1_jdud0qa wrote

I always worry when that happens that its going to be permanent, or damage the nerves...

How important is it not to sleep on your arm like that? Like is it a minor annoyance or major health hazard?

Any tips for how to make your arm not fall asleep when sleeping on your side? Like a certain shoulder position or place to avoid pressure? Pillow position?

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RobleViejo t1_jdvblxw wrote

On a soft surface is not a big deal.

However on a hard surface it can lead to necrosis on muscle tissue due to hypoxia, which then needs to be surgically removed, which causes permanent lose of functions. (This is common on people who pass out on concrete and sleep on their arm, due to alcohol or drugs)

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QuerulousPanda t1_jdvo9vl wrote

Sleeping on it is less likely to cause permanent damage because you're going to tend to move around enough that you probably won't cut off enough circulation or squeeze an individual nerve for long enough.

Where it can become an actual problem is for alcoholics and drug users who pass out to the point of total unconsciousness and don't move at all.

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Implausibilibuddy t1_jdx7q0z wrote

It's nerve compression, not blood flow that causes that. Restricting blood flow for any length of time would result in atrophy.

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keestie t1_jdx8h2p wrote

Interesting. Is enough known about nerves that we can understand the actual physical and/or chemical processes at work? The gradual return makes me think it's some sort of displaced fluid that slowly flows back in, but is that really it?

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