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?
In: Biology
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.
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.
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.
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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.
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.
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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
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
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!!
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