why do we get pain?

[Charlie07]

what happens when we do? i mean what goes on inside the body and what reactions does the body give??

[Kit]

This is the third time I've tried to answer this - Answerbank won't let me post because the answer is too big!
I'll therefore try to cut it into bits. First, the Why: there is only one kind of pain that serves any useful purpose, and that is Acute pain, which warns the body of damage, and immobilises the part to prevent further damage and promote healing. There is also Chronic pain, which persists long after all healing has finished. It serves no useful purpose, and is mainly due to changes that occur wihtin the body. People who have had an amputation often feel Phantom pain, which is pain felt in the part of the body which has been amputated. Thisis partly due to abnormal firing of cut nervs, and partly due to body image.
Part 2: What happens: to follow.

[Kit]

Part Two: what happens. Various things can prod a nerve into firing off an impulse. These include: trauma (eg surgery or injury), pressure (eg swelling or tumour pressing on a nerve), stretch, chemicals (eg htose produced during inflammation. Capsaicin, found in chillies, is als a potent pain-producing chemical, which is why some curries can be painfully hot.) The nerve impulse travels to the spinal cord, and the Pain Gate. The pain gate is a sort of switch - it can be turned off by skin stimulation, eg warmth, massage, ice, and also by endorphins, the body's natural painkillers. It can be turned on by sufficient numbers of pain impulses reaching it. So not all pain impulses get past the pain gate. Those that do then travel up to the brain, where they are scrutinised. The brain compares them to other experiences and sensations, eg it may decide that it feels more like heat or pressure than pain. It also evaluates what it means, and how important it is - this is why sportsmen may not be aware of injury until after the match has finished.
Part Three: how does the body react: to follow...

[Kit]

Part Three: how the body reacts. Once the brain has decided that it is pain, various reactions occur. To prevent further injury, there is withdrawal from whatever is causing the injury - eg if you touch something hot you pull your hand back very quickly. Another way of preventing injuy is to stop using the painful part, eg limping and not putting weight on a painful foot. You may also shield the part form further damage by holding it protectively. You may also pull a face to signal to other people htat you are in pain, and need them to take notice so they can help, avoid bumping into you etc. Various chemicals are released into the bloodstream, eg adrenalin and cortisol, and the sympathetic nervous system becomes more active. Result is fast heartbeat, shallow rapid breathing, sweating, feeling sick, going pale.
Hope this answers it - it's taken me long enough to get it posted!

[Headtime]

Kit, what a fantastic answer!

[cucumbers]

bit vague mate !

[cucumbers]

only kidding. 5 star answer.

[JohnPPotts]

I think the idea of long term pain is to help memory formation, so that you learn not to (for example) put your hand into a flame. Possibly also why you feel pain in an amputated limb - your body assumes the lack of information from the nerves indicates severe damage, so is telling you to be careful with it.

[Kit]

Good theories, JohnPPotts - and it's true, phantom pain is partly due to the body image, your brain "expects" you to have all parts of your body present. However, in many cases, the person feels as if their limb is still present, so phantom pain could cause you to try and take weight on it rather than a warning to keep off it.
Also, you do not need chronic pain to develop a memory of what not to do. All of us have had many episodes of acute pain, and have learnt as a result not to touch hot things etc. However, only a minority have chronic (long term) pain.
In addition, chronic pain is also to do with nerves becoming over-responsive, so even normal movement can give rise to pain. Normal movement is just that - normal, so a phenomenon that stops us from doing normal things doesn't make sense. So why do we have chronic pain? Well, one theory is that man has always been a social animal and cared for old and infirm individuals. Inability to feel acute pain is a life-threatening problem, as it makes the individual prone to damage and infection - a huge evolutionary disadvantage. However, chronic pain is disabling rather than life-threatening; Stone Age man with long-term pain would have been cared for by other members of the group, so we havn't evolved a way out of it.
For anyone interested in pain and its mechanisms, "The challenge of pain", Ronald Melzack and Patrick Wall, is an absolute must. My copy's at work at the moment so I can't give publishers or years, but Melzack and Wall invented the Pain Gate theory in 1965 and know their stuff!

[d+dwidow]

Kit, that is so true what you said about chronic pain serving no useful purpose. I've suffered with M.E now for 3 years and get terrible leg pains but my GP told me that it's nothing to do with me having done something I shouldn't, it's because my brain is sending signals out telling me i'm in pain even though i'm not really. It's funny what the brain'll do to your body!

[melody2002]

d+dwidow - Your pain is very real and your Doc seems to have fallen in to the category that lots of GP's do when you mention M.E or Fibromyalgia (which I have). Your pain is not 'in your head' and it is not your brain 'fooling you.' You have real pain. My knees are the worst. Good luck and change your Doc! My pain is due to not being able to maintain zone 4 sleep (deep sleep - Delta sleep) which stops my joints, muscles everything from mending . I try for 15 hours sleep a day and have stopped working which has lessened the pain but unfortunately this is due to less stress - even with 15 hours sleep I still can't deep sleep.

[d+dwidow]

Melody - I think your right. The trouble with most GP's is because there's nothing physically wrong with me, by that I mean you can't see that i'm ill, they presume that it's all in my mind! My knees and legs are absolute agony most of the time and I dose myself up on co-proxamol and amitriptyline. I used to sleep for 16 hrs + a day, but now circumstances have changed and I have to work full time, which doesn't help. I rarely ever have a decent nights sleep and feel like i've been 3 rounds with Audley Harrison!