Throughout our bodies are pain receptors, called nociceptors. These receptors are stimulated by the release of various chemicals from damaged tissue such as histamine, substance P, serotonin, bradykinin and prostaglandins.
When stimulated, the nociceptors release of various chemicals by the damaged cells including histamine, substance P, serotonin (5HT), bradykinin and prostaglandins transmit the pain signal along sensory nerves to the spinal cord.
In the spinal cord the impulses are processed by the dorsal horn and sent to the brain. Sometimes signals sent to the spinal cord communicate directly with motor nerves to cause, for example, one’s arm to quickly withdraw after touching a hot plate. This is called an automatic reflex and does not involve the brain or conscious thought.
Most of the time pain signals are sent upwards in the spinal cord from the dorsal horn via ascending nociceptive tracts, such as the Spinothalamic tract.
These tracts terminate in the medulla, midbrain and thalamus. Further processing in the thalamus leads to signals being sent to areas in the brain that control heart rate, blood pressure and emotions.
Thalamic neurons also extend to regions of the cortex including the somatosensory cortex, where fine discrimination of pain occurs.