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Background

When any bone/muscle of body is hit badly by a blunt object, it hurts pretty bad of course for some time. But, after some time the pain goes off completely or to some extent.

Then, if we touch the wounded spot, the terrible pain returns.

Why?

If the pain goes off as the healing process of severed tissues, blood capillaries, then why does it return at the event of touching?

My reasoning: I think there is a sophisticated pain mechanism responsible for this phenomenon.Firstly, pain warns our nervous system to be cautious against any new injury. As time goes on, sensory adaptation diminishes the pain. But the event of touching is a new pressure for the blunt wound.So, the pain comes back again. But, I need well-referenced answer to support my view.

Papers: The most relevant paper I've found till now: 1 which is behind paywall

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  • $\begingroup$ perhaps the first shock cause some invisible injury $\endgroup$ – Always Confused Oct 12 '16 at 7:02
  • $\begingroup$ Invisible? The pain is in the same place? $\endgroup$ – Mockingbird Oct 12 '16 at 7:53
  • $\begingroup$ I meant injury in cells and tissues, that can't be seen from macroscopic vision. Pain is commonly a response to tissue-damage. $\endgroup$ – Always Confused Oct 12 '16 at 7:56
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    $\begingroup$ I am not even sure why the pain goes in the first place if it's not completely healed. Maybe our brain has this weird pain mechanism which after sometime reduces pain and then triggers pain when touched to give a signal that something is wrong. Maybe it's evolution. $\endgroup$ – Mockingbird Oct 12 '16 at 8:13
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    $\begingroup$ There might evolutionary significance so that we can protect the injured organ from further damage. $\endgroup$ – Always Confused Oct 12 '16 at 8:15
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BASIC REASON WHY THE PAIN DISAPPEARS FOR SOMETIME:

There are gating mechanisms involving pain both in the spinal cord and in the brain.

Although the pain gating pathways in the brain are multifold complex,the spinal pathway can be simplified. The latter may be understood in a simplified manner in 2 ways:

  • When there is a nociceptive stimulus,the epicritic receptors are stimulated.Sensory fibres from them synapse with internuncial neurons which inturn transmits impulses via the ascending tract to the periaquiductal grey. This region is capable of inhibiting pain impulse transmission to the spinal cord or thalamus by modulating impulses via the descending tracts. This type is called supraspinal control

  • Another spinal reflex-type gate control operates in which stimulus like rubbing over the pained area ,application of warmth or cold causes inhibition of pain. This is due to the impulses by large type (myelinated)A-fibres which arises from the rubbed areas synapse with the internuncial neurons which are already synapsed with the epicritic sensory neurons of the dorsal root.The ends of such type of fibres release neurotransmitters that inhibits transmission of pain impulses through the internuncial neurons . This is termed as segmental suppression

WHY DOES THE PAIN RETURN AGAIN:

This is because the pain receptors are the slowest to adapt.It's exactly as you said because it keeps the body apprised of tissue damaging stimulus. So when there is the return of the noxious stimulus we can understand the tremendous summation effect which takes precedence over the gate control.

This occurs by a greater output of neurotransmitters from the synapse of the pain sensory fibres which outdoes the inhibitory effect of the neurotransmitters favouring gate control of pain.

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