This study will evaluate brain pathways involved in feeling pain and pain relief following administration of pain medication. Magnetic resonance imaging (MRI) will be used to measure brain activity at sites that become more active following oral surgery and then to see if the activity changes after pain medication is administered. MRI combines a powerful magnet with an advanced computer system and radio waves to produce detailed pictures of organs and tissues. During the scan, the subject lies on a table in a narrow cylinder containing a magnetic field. He or she can speak with a staff member via an intercom system at all times during the procedure.