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    Intense noxious sensations are perceived in the human body by specialized sensory receptors that are called nociceptors.  In the human body, nociceptors are found in almost all tissues.  There are four subclasses of nociceptors; thermal, mechanical, polymodal and silent nociceptors.  Each of these different nociceptors are usually all found together and operate in conjunction with one another to sense various kinds of stimuli that are harmful to human tissue. Thermal nociceptors sense extreme temperature differences ( > 45 * C  or  < 5*C ) while mechanical nociceptors are activated by intense pressure applied to body tissues.  The  polymodal nociceptors act as both thermal and mechanical nociceptors.  Silent nociceptors are found in the viscera of the human body and are sensitive to chemical fluctuations only.  For the most part nociceptors are just free nerve endings that are scattered throughout human tissue.  All signals from the nociceptors travel to the spinal column ( see picture above ) where these signals are taken directly to the brain to be processes.  Not much is known about the signaling mechanism that causes the the sensation of pain other than it is caused by " substance P ".
    However, we do know that local enkephalin containing interneurons  are located in the same proximity where nociceptors transmit "pain signals " at the spinal column.  Local enkephalin containing interneurons release enkephalin which inhibits the pre synaptic and post synaptic signaling from the nociceptors to the neurons in the spinal column.
    Even though it is not known exactly how pain is caused, the recent discovery of opiate receptors on neurons in the spinal column and brain stem have shown how pain can be inhibited by drugs such as opium and morphine.  The opiate receptors are activated by morphine and signal enkephalin to be released from the local enkephalin containing interneurons.  This slows the signaling between nociceptors and neurons in the spinal column and effectively prevents the sensation of pain.

* Courtesy of the Pain Relief Foundation *