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mckulty t1_j45hlvp wrote

There are no pain receptors in the trunk of the nerve, only at the ends. Pinch the ulnar nerve at the elbow and you get pins and needles along the underside of the arm and last 2 fingers, not at the elbow.

You can open the skull and stimulate the postcentral gyrus directly, and the sensation will be from the foot, not the head.

When the nerve loses its function (or you lose a limb), the brain it was attached to hallucinates and gives you phantom paresthesias that feel like the limb itself, and not the stump.

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_AlreadyTaken_ t1_j46u0oi wrote

I read about phantom limbs and it seems that the brain expects noise from nerve fibers. It normally ignores this noise and considers it just a nerve fiber at rest. When the nerve is cut now there is no noise but there is no signal either. Without noise something must be happening but what? So the brain fills in the blank.

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ThatOtherGuy_CA t1_j470iwd wrote

Seems like an evolutionary side effect that would normally be beneficial. Pinching a nerve for a prolonged time isn’t good, so when your brain loses feeling to an area it gives you pain signals to trigger you into moving, so that you don’t sleep or sit in a way that could lead to paralysis, and once it gets other feedback it stops.

Unfortunately it can’t tell the difference between a pinched nerve and a lost limb, so it’s instead like “bro move your hand, bro, BRO!!!” And now your left hand that’s been missing for 6 years is suddenly on fire.

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Rombolio t1_j46ul2d wrote

I work with a pain doc that is an anesthesiologist and they've done sympathetic blocks to help with phantom limb. They've had varying success, but sometimes "shutting off" the nerve resets it. We do a lot for CRPS from crush injuries in the feet.

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_AlreadyTaken_ t1_j46v8e4 wrote

Check out Oliver Sacks' writing on phantom limbs, it is very interesting. One guy had a prosthetic limb. It seems to get agile with a prosthetic you need this phantom limb effect. So this guy had to make his "appear" by slapping the leg stump and he could make it turn on. Some people even have severe pain in the phantom limb or it feels like the limb is contorted.

One interesting therapy for phantom limb problems is mirror therapy.

https://www.amputee-coalition.org/resources/mirror-therapy/

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Front-Erection t1_j46wuu9 wrote

There are no pain receptors in the body whatsoever. Pain is an experience, a complex output of the mind formed from many various factors, one of which is the input received by nociceptors that effectively detect harmful or potentially harmful change.

That said, neuropathic pain is also somewhat complicated but typically if a nerve is damaged at some point then the pain/altered sensation will follow the course of the nerve from that point to the furthest distal end of the affected nerve.

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lavachat t1_j45lehy wrote

Everywhere that sensory nerve has its branches end, and sometimes over every sensory quality it transmits. So the feelings can switch between different sorts of pain, or temperature, or vibration, or pressure. It depends on what kind of damage occurred where along the nerve.

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Current-Ad6521 t1_j45shpi wrote

What you feel is situational dependent, and most areas of the body to not have the acuity for there to be a difference. What I mean by this is that the majority of the body is not sensitive enough for you to tell exactly where you are feeling something, and most nerves are so small that it is well below the threshold for being able to tell where the sensation is coming from exactly. There are also different types of mechanoreceptors that perceive touch, vibration, pressure, etc. differently and occur in different amounts over the body, so there is no one size all fits answer for perception related questions. In many cases you wouldn't feel anything, it would just be numb. The pain does physically "happen" in the pain receptors which are at the end of the nerves, though our sensation and perception is not fine tuned enough for us to feel a difference.

For example, if you put two pins next to each other on someones fingers with their eyes closed they would feel that there are two pins. The fingers have high acuity because of the type of mechanoreceptor + there are many small receptors and a large amount of brain dedicated to fingers. If you put two pins next to each other on someones back, they will feel as if there is only one. The nerve anatomy is not fine tuned enough on your back to be able to tell that there are two pins, and only one receptor is activated. Now imagine you put two microscopic pins a fraction of a millimeter away from each other. You wouldn't feel two different pins because the distance is so small, which is how nerves usually are. There are bigger nerves, like the ulnar nerve aka funny bone where you can feel more localized pain though it still is not that obvious where you are feeling the pain in terms of ending vs stem.

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LedgeEndDairy t1_j472rmy wrote

As someone with a herniated lower disc that presses up against my spine (and is doing so right now), it radiates to the actual receptors.

But those receptors can be anywhere and everywhere. I feel pain in different locations depending on "how" my disc is currently herniated, inflammation in the area affects this as well.

Right now my actual butt is in a lot of pain, but it can radiate all the way down to my toes sometimes. This is all from the disc pressing against the spinal cord in different micro locations, nothing is actually touching the pain receptors in my bum (other than my clothes, I guess).

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gravelbee t1_j46l0e4 wrote

Bodies are so complex... it really depends on the type of damage and the location. For example, mild nerve damage - let's say from extreme cold exposure in the fingertips - will cause sensations of pain or discomfort only in the affected area of the nerves involved. On the other hand, a very common example of nerve damage is sciatic pain. The actual site of inflammation is in the lower back/hip area but people will get pain that extends all the way down their leg.

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FineRevolution9264 t1_j46poax wrote

Vert true. But why does the pain radiate like that? The nerve is clearly not damaged in the calf for example.

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_AlreadyTaken_ t1_j46ugpm wrote

This may help:

https://www.houstonmethodist.org/blog/articles/2021/feb/why-does-hitting-your-funny-bone-hurt-so-much/

>When you stub your toe, slam your finger in a door or smack your head, irritation and damage to your tissue triggers the surrounding nerves to coordinate nociceptive pain. These nerves inform your brain you're in pain — cue those sharp or throbbing sensations. Moral of the story: Stop doing the thing you're doing, it's causing damage.

>Nerve pain is a bit different. The pain is still coordinated by a nerve, but the irritation or damage is occurring directly to the nerve itself. In addition, the pain feels different — more electric, burning or stinging.

>In the case of hitting your funny bone, squashing your ulnar nerve into your medial epicondyle bone is irritating. And you feel this nerve pain in the areas where your ulnar nerve provides sensation, resulting in an unpleasant, shocking sensation shooting down your forearm and into your fingers.

So one is from nerve fiber sensory organs at the nerve endings. The other is affecting the nerve directly so it registers it along the entire region the nerve covers.

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[deleted] t1_j469gsw wrote

[removed]

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beechbummus t1_j47ge58 wrote

I just want to say, that's a great question, and a lot of accurate answers 😁. I work in a related field as well, and many people cannot wrap their minds around 'yes, there are symptoms in you leg (for example), but the cause is located at the peripheral nerves that become the sciatic nerve. Your leg is actually physically okay, at least for now.' Obviously when the nervous system isn't functioning properly, all of the other bodily functions will be affected as well. Resulting in a multitude of other issues.

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_AlreadyTaken_ t1_j46tga3 wrote

I had an interesting experience with appendicitis. When I first felt it I didn't feel it in my lower right abdominal quadrant, I felt it right below my navel. At first I thought it was just something like constipation. Then by the next day it migrated to the LRQ.

I wondered why I had this experience and the answer was that the nerve for that region joins the spine at just below the navel. So at first I felt it at the spinal region then as the region around my appendix got more inflamed the pain sensation moved to that region.

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