ramabadran seshadri iyengar

After tissue damage following any kind of injury there is release of membrane Phospholipids. By the action of enzyme PhospholipaseA2 this is converted to Arachidonic acid. It again is converted to Prostaglandins (PGs) by the action of Cyclooxygenase or COX. These PGs and other chemical mediators like bradykinin, subP serotonin, histamine, potassium and hydrogen ions sensitize A delta & C fibres (they are pain bearing nerves). So they are up regulated & impulse is easily generated and carried to the dorsal horn of spinal cord. There they synapse with cells of the spinothalamic tract, which carry the impulses up the spinal cord, through the brain stem to the thalamus. From the thalamus, impulses are delivered to various areas of the cerebral cortex that allow the perception of pain and reaction to pain. 

In the dorsal horn of the spinal cord the afferent sensory nerves synapse with fibres of the ascending spinothalamic tract, either directly or via a complex system of connecting fibres