How does the body react to pain?

When a stimulus induces a reaction from the pain receptors, an immediate firing occurs in a reflexive reaction.  Pain receptors and nociceptors have significantly higher firing rates than receptors for temperature and touch.  Pain receptors react due to physical stimuli such as cuts, scrapes, heat, chemical substances causing tissue damage and lack of blood circulation in a specific area.  The production of prostaglandins begin, causing an irritation and signaling the body of an unpleasant feeling and of traumatic pain.

Prostaglandins, a member of a group of lipid compounds, derived from the essential fatty acid arachidonic acid, serve in a variety of bodily functions.  These functions include smooth muscle contraction and relaxation, dilation and constriction of blood vessels, blood pressure control and modulation of inflammation. 

Pain can also be inhibited or masked at this point where the use of drugs such as aspirin act as prostaglandin receptor antagonists.  Inhibiting the synthesis of prostaglandins is a very effective way to fight pain.  More information on this topic follows in the treatment section. 

This reflexive mechanism that initially occurs involuntarily sends impulses to the spinal cord triggering a muscle response.  This occurs regularly in daily life, however with little thought processing.  Say while closing the window, the pulling force accidentally knocks the window down quickly onto your fingers.  You immediately pull your fingers away not because your brain recognizes that your fingers are stuck, and thus it is desirable to move them, but because there is almost instantaneous impulses sent from receptors at the injury site which activate the flexion withdrawal reflex, causing the removal of your fingers.  You may even experience a crossed-extension reflex in a moment such as this where say you do not want the window to close completely and when the hand bracing the window becomes pinched, you quickly not only reflexively remove that hand, but the opposite hand moves in to take the weight over and brace the window.  This reflexive mechanism will occur prior to an intermediate neuron which signals the brain of a pain message, and thus you remove your hand before the pain sensation actually sets in.