To Karl's right is Dr. Robert Kowatch, child psychiatrist, back for a visit. In the 1970s, when he was an undergrad, he and I published the first paper on the lack of effect of morphine on pain induced by stimulating central pain pathways in the same rats whose ordinary peripheral pain was totally blocked by the drug. This was at the time a major piece of support for the view that opiates act via the turning on of central systems which descend through the cord to inhibit secondary afferent pain neurons at the primary-to-secondary afferent synapses in spinal cord and trigeminal nucleus. (We don't do the opiate stuff anymore.)