Why do some injuries not hurt at first?

Yesterday, I got a small but deep scrape while helping a friend move. It didn’t hurt at the time, but about an hour later, it sure did. I have noticed this many times before, and we’ve all read accounts of people who were seriously injured who didn’t realize it until later. I am trying to figure out two things:

  1. Why do some injuries hurt right off the bat, while with others, it takes awhile to feel the pain? Does it have to do with the type of injury? Why would that be the case? I know shock can affect pain (and vice-versa), but I’m also referring to injuries too slight for shock to be a factor. Also, I know that full-thickness burns typically don’t hurt because the nerve endings have been destroyed; I’m not referring to injuries that never hurt.

  2. What would be the biological advantage of not feeling pain from an injury immediately? I am trying to avoid the word “purpose,” which would be a rookie error. Or is it not a matter of advantage; it’s just the lack of pain has to do more with how our nerve endings work or something?

There’s probably an obvious answer that I can’t see. Someone please set me straight.

I think the obvious advantage to not feeling pain immediately is that you have the potential of still functioning and surving. Many times a person can be shot or badly wounded and still keep moving somewhat. It may only be a few seconds in some cases, but something nevertheless.

Many athletic injuries take place without showing a sign until the person stops the activity. One of the Tigers I believe reinjured a fractured ankle and was running fine in the field until after the inning when he sat down.

Physiologically why this happens, I don’t know. Perhaps blood is flowing and fighting inflammation. Then at rest the circulation slows and the inflammation runs amok.

I would think that increased blood flow would increase inflammation. That’s why you put ice on a recent injury: to reduce blood flow and thus decrease inflammation.

When you stub your toe, or some similar incident, you don’t feel the pain right away. It’ll take a few seconds for the nerve impulses to reach your pain center in the brain. Reflexively, you withdraw the injured part, but the pain ensues shortly thereafter. The reflex has a shorter route, needing only to reach the nearest plexus in your spine.

Athletes injure themselves and continue playing, I think, because of adrenalin. The adrenalin rush overpowers the pain center.

When I put my arm through our kitchen door window (I was pushing hard because it had swelled up (this was during the week of constant rain after TS Lee came through) and it was sticking), it didn’t hurt for more than an hour, and then only because the ER doc was sticking me with a needle to anesthetize for the stitches. I only knew I’d cut myself because I saw the blood.

When I got hit from behind at 45mph while I was stopped, and bounced off the back wall of my pickup cab, I was fine for an hour or more before everything started locking up and bruising. Dunno what was up with that.

I think this is probably correct. In the Army we were taught always to aim center mass at a human target. This is partly to increase the chance of simply hitting the enemy soldier anywhere at all (even if your aim is off), but also because the body’s vital organs are in the torso (or head). And unless you hit one of those vital organs, an adrenaline-filled, running enemy soldier is not likely to go down from a non-lethal bullet wound, despite what you see in some movies. Humans who are fighting for their lives have always been able to utterly disregard amazingly serious injuries in the heat of combat. I’m not a big-game hunter, but friends of mine who are have told me that this is the case with deer, as well. You either have to kill them with one shot, or off they go into the forest, quick as lightning, no matter how bad the flesh wound.

A true story that illustrates this sort of phenomenon: During the Philippine-American War, the US standard-issue .38-caliber handgun proved remarkably ineffective at bringing down charging Moro warriors (the Moros were a tribe of Philippine natives fighting the Americans). The fast, small-diameter bullet would simply pass through the natives’ torsos and keep going, and the warriors kept charging without seeming even to notice the wound. So the US adopted the .45-caliber Colt automatic as the new GI handgun. With its slower velocity and larger caliber, it imparted far more kinetic energy to the enemy, causing sufficient damage to literally stop him in his tracks.

So an injury has to pretty much be to a vital organ or you might not notice it at all, if you’re involved in intense physical activity at the moment of injury.

I think this is accurate, too. Whenever I have injured myself running, I haven’t noticed until a few hours later.

I think it’s probably because of adrenaline and endorphins blocking the pain during the activity.

Some injuries cause more of a delayed response than others -

Leakage across injured vessels causing swelling.

Immune responses to the injury, cytokines, etc. … also causing more swelling and pain receptor sensitivity. These substances need to be released, to migrate across the vessels, ad to then result in other cells responding. This inflammatory response is part of healing but and also causes greater pain.

The immediate pain felt is to the skin torn and the pain receptors directly affected; all this other stuff happens, to variable degrees, over the next minutes to hours.

And yes there is a central factor in how we perceive pain, and which even can regulate the peripheral cytokine response to some degree.

The reason some really serious acute injuries, like being shot or having a leg blown off, sometimes don’t hurt is because the body can kick in a huge amount of adrenaline to suppress the pain in an attempt to allow you to survive the situation.

Why minor injuries like scrapes sometimes don’t hurt to start with is a different question, I’m not sure about the answer to that one.

Interesting question. I distinctly remember as a teen helping a neighbor clean up some renovation debris and stepping square on a (substantial) nail sticking out of board and feeling absolutely nothing. In fact, the only way I noticed I’d even done so was that when I went to take a step the board stuck to my foot and moved with me, like a ski.

I wonder about the non-life threatening, relatively minor injuries, too. When I was in my early teens, I was helping my dad and my uncle clear brush from behind our house. I was walking behind my uncle, and saw that he had a large twig impaled in his back, right beside his right shoulder blade (no idea how he managed that). He had no idea it was there until I told him.

Given that this is GQ I do feel a need to correct this.

Actually, everything else being equal, the direct effect of peripheral adrenaline (a.k.a. epinephrine) is to slightly increase subjective pain sensation (hyperalgesia). OTOH central attention mechanisms activated during times of crisis (not directly caused by epinephrine) may dramatially decrease pain sensation. Remember that pain, like other sensations such as vision and hearing, is ultimately a brain experience; it can be felt less even when the receptors are being fully stimulated just as it can be felt in a limb that does not even exist (phanom limb).

(It also may be true that endogenous opioids are released in those sorts of circumstances as well.)

And again, the minor scrapes are likely a function of peripheral events that take a while to happen. As for the twig in the back, well the back actually is pretty sparsely innervated; pretty easy to not realize a puncture type of wound there when there is lots of other input being actively attended to and other input being filtered out.

Last May I cut myself on the side of my index finger and it needed 8 stitches. There was never any pain at all. I saw the knife hit my hand and was at first stunned that I didn;t get cut (I was wearing gloves and could not actually see my skin). Then I saw the blood poring out of the cut in the glove.

I was amazed that I never felt anything from it.

I also don’t get why so many of the small open/skin injuries I get (especially my most recent which required surgery and now months of splinting and PT until I can use my hand properly) barely hurt, or even honestly don’t hurt at all at any point! But then things like papercuts and plucking my eyebrows make my nose run and can smart for an hour…

The most awfully painful thing I do to myself regularly is stub my toes and whack my feet and shins on things. I often yelp or curse it hurts so much, and have to sit down to recover. Yet I don’t even get bruises from this usually. And cutting pieces off my fingers or blistering burns to my hands are no biggie. I just hope I never get hit really hard or crushed, cause apparently that gets thru loud and clear.

The nervous system is weird.

I’m not a doctor and have no medical training, but I think that the onset of swelling brings more pain. Years back, I was reframing a window and left the old trim, nails up, lying around my feet.

You guessed it. When I stepped on the board, I felt just a tiny pinch. I looked down and the nail was protruding from the top of my foot (shoe and all). I pulled it out with just a minor bit of pain.

After walking around, apologizing to all of the neighbors for my creative use of a multitude of profanities, I sat down to look at the wound. When I stood back up, I fell because to put weight on the foot caused incredible pain. For the next 3 nights I couldn’t sleep because everytime my heart beat, it shot pain through the foot.

But, I think that is because the swelling kicks in which puts pressure on an already damaged area. Feel free to shoot down this hypothesis.

Interesting responses! Maybe swelling increases pain in some cases, but that wasn’t the case with my minor little scrape. I’ve also thought about some areas having more pain receptors, but the poster who had to have eight stitches in a finger seems to indicate that that isn’t always the case. Aren’t finger tips one of the most sensitive parts of the body? And when I was 13, i looked down to see that I somehow a straight pin had become stuck into the topside of my toe (I was in my bare feet.) without me feeling it. I never figured out how it got there, nor how a pin could get buried up to the head in my toe without my being aware of it.

Yet the few times I’ve been stung by bees or yellow jackets, it hurt like hell immediately. Our nervous systems are weird.

Another factor besides adrenaline is that nerve endings are not distributed equally among the body. Some surfaces, like finger tips, have a lot of receptors (hence Braille), others, like the arms, have less. A clean cut or surface abrasion might thus not hit any nerve points/ pain receptors until pressure, cold or similar affects the wound and thus make you notice it.

So what is it that can cause the body to temporarily block out the pain when it experiences some sort of massive trauma, like a soldier having a leg blown off?

Well that’s a bit different than minor skin traumas since they are injecting us with poison, which different people have different reactions to. In my case, I get only about 3 minutes of some pain from a sting, and it turns right into severe itching which lasts days. Other people seem to have much more intense pain from stings, sometimes it lasts a long time and sometimes they don’t have itching at all…

I got kicked in the knee by a friend’s horse while we were out trail riding together. It hurt about as much as smacking your shins on a coffee table initially, so I didn’t think it was that badly injured. Aside from the weird acid-trippy things my brain was doing (shock?), I could sort of walk on it and after a while, got back on my horse and rode home. Went to the hospital, got it checked out, was told it was just a bruise and went home a few hours later. I put my toe down to balance on the stairs to my apartment and just about puked from the pain. Turned out to be much more serious damage than just bruising.

I think adrenaline and shock kick in with a serious injury, perhaps to buy us time to escape whatever injured us. Then swelling and inflammation occur causing pain, which reminds us to not use the injured part so it can heal.

Or maybe someone with real medical knowledge can weigh in?

Without derailing this into a guns thread…
A minor clarification. The army was using the 38 Long Colt cartridge not the better known later 38 Special which is slightly more powerful. It was also far slower than the 45ACP, generally 775fps vs 1000+ Thus a larger, heavier bullet moving faster was introduced.
Training for proper shot placement is still critical.
The army decision was based on the Thompson-LaGarde_Tests of 1904