if the colon has no pain receptors, why does trapped gas hurt so much?

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I’ve had a colonoscopy (without pain relief) where they took biopsies. The doctors said the biopsies wouldn’t hurt because the colon couldn’t feel pain, and they were indeed painless. The amount of air they pumped in was horrifically painful however.

Trapped gas sounds trivial, but can also be extremely painful. Ulcerative colitis also hurts. So does diarrhoea.

So how do these pain mechanisms work? What causes the pain, if the interior of the colon is unfeeling?

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There are muscle fibers on the outside of your colon that help with peristaltism. A gas filled colon makes this fibers stretch and that’s where the pain is coming from.

Part of the intestines are covered in a thin layer of extremely sensitive tissue called the peritoneum. When gas presses your intestine it also presses that thin layer and that’s why it hurts so bad.

The reason why you don’t feel every gas or growth (cancer) is that the obstruction needs to be of quite great mass so that it stretches the peritoneum to much.

I’m not sure about Colitis and Diarrhea. I’m guessing Colitis hurts because instead of pressurizing nerves cells are actually dying.

Diarrhea could hurt because your body is applying the pressure to get whatever you ate out of your body.

Nocioceptors are the sensory neurons that we call pain receptors. There are several different types. Not all types of receptors are present in all places of the body. The ones in the colon can sense mechanical stress (i.e., distention from gas), inflammation, and ischemia (tissue death). Gas can cause pain because it’s physically putting pressure on your colon, and inflammatory bowel diseases like Crohn’s disease or ulcerative colitis cause inflammation, which will also trigger those nerves.

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We just talked about this in class yesterday!! So you dont have pain receptors in your gut but you do have a lot of baroreceptors or pressure receptors to tell you when the pressure gets too high and response of high pressure from these is essentially translated as pain

As it swells it starts pushing on other muscles, organs, and lining that does have nerves.

It’s like a person who is too big for an airplane seat who is pushing into the other person. The big person is mostly fine but the other person now is uncomfortable and tells the stewards what’s wrong

A simple way to think of the colon and pain is to relate it to the skin. Lots of different things can cause different types of pain in the skin. There’s pressure, heat, cold, cuts and probably a few more. There is a specific type of nerve ending to sense each type of pain.

The colon (the entire intestine, in fact) has only two types of pain receptors – ones that sense stretching, and ones that sense lack of oxygen (which happens when the colon is starved of blood). Trapped gas will make the colon wider, which sets off the stretching pain receptors. Biopsies don’t hurt because the colon lacks the nerve endings to sense cuts and bruises.

Víscera like the intestine, kidneys, liver, lungs etc, have non precise pain receptors on the outer layer. That’s why this kind of pain is diffuse and not very precise. This means that there is no pain in the inside. That’s why a tumor doesn’t hurt until it affects the outside layer, either by stretching or damaging it. In the case of the intestines, they get stretched causing pain.

You ever stub your toe really hard and have the thought, “crap, this is really gonna hurt.” The reason you “know” you stubbed your toe before you feel any pain is because the nerves that carry the sensory information that tells you where your foot is in space, what your foot is touching and that you just bumped it are different nerves than the nerves that tell your brain that there is pain. The first type send a signal much much faster than the pain type (the ELI5 for nerve conduction speed is a completely different conversation but also fascinating).

In short, different nerves send different signals and the nerves that send the signal for “pain” in the traditional sense (eg, cutting your finger) simply aren’t present in the bowel.

For more info, you can explore topics like: somatic vs. autonomic nerves, autonomic nervous system, enteric nervous system, referred pain, proprioception. I also recommend writing by Oliver Sacks and VS ramachandran if this type of stuff is interesting to you. Enjoy!!

In addition to the other mechanisms described, these are actually important diagnostically for different conditions as we learn in medical school.

Pain from stretching of abdominal organs generally causes what we call “visceral pain”–this is generally deep, squeezing, discomfortable pain that’s not really well localized to a single spot, since the way that those sensory nerves run doesn’t allow you to easily tell where exactly the pain is.

On the other hand, if you have really sharp, stabbing pain in a particular spot of the abdomen, this is because something is damaging the peritoneum (the thin tissue lining the abdominal wall on the inside). This is innervated with sensory nerves more similar to the ones on your skin, so you can localize it well. This is usually a pretty big red flag, because it means there’s some kind of disease processing happening *outside* of the gastrointestinal tract, like a ruptured bowel, a really swollen/burst appendix, etc.

Doctor here, please let me know if I haven’t explained well enough:
Pain in organs (visceral pain) is very different to the pain you’re used to experiencing in your skin/muscle (musculoskeletal pain). We have very sensitive pain receptors which localize all sorts of pain in the skin/muscles (including touch (pressure, cutting)/temperature/inflammation) – we have evolved this because it is very important we know quickly and accurately if there’s a problem in our musculoskeletal system.

Internal organs have pain receptors, but not for touch – someone could be cutting your internal organs and you wouldn’t feel it. They do have pain receptors for stretch (e.g. Bad gas), inflammation (e.g. Appendicitis) and when they’re oxygen starved (e.g. Heart attack). These pain receptors are poorly mapped by the brain, which is why heart attack pain can be felt as jaw or arm pain, and pain in the liver can be felt as shoulder pain.

Gastroenterologist here.

Since when does the colon not have pain receptors? Your entire GI tract is innervated heavily, both with nerves inside (the enteric nervous system), and nerves that project onto your skin (somatic nerves). Distension and gas absolutely cause pain, and your body is very finely tuned to detect when a portion of your bowel is distended. One of the most common causes of chronic pain is [irritable bowel syndrome](https://en.wikipedia.org/wiki/Irritable_bowel_syndrome), which causes intense pain with contraction of the colon and distension with gas. In short, there are about as many nerves in your GI tract as there are in your spinal cord. Many of these nerves are pain fibers.

Furthermore, there is something called the brain-gut axis that connects all this machinery to your brain, which is constantly being informed of the status of your GI tract. Hence the reason that your stomach hurts when you have anxiety.

The reason the doctor said that is because the part we biopsy, called the mucosa, is above the shallowest nerve layer, called the submucosa. Deeper biopsies can certainly hurt, but the biopsy forceps are very tiny, and we just get a little pinch of tissue.