The neurobiology of facial and dental pain: present knowledge, future directions.

This review outlines recent research which has identified critical neural elements and mechanisms concerned with the transmission of sensory information related to oral-facial pain, and which has also revealed some of the pathways and processes by which pain transmission can be modulated. The review highlights recent advances in neurobiological research that have contributed to our understanding of pain, how acute and chronic pain conditions can develop, and how pain can be controlled therapeutically. Each section of the review also identifies gaps in knowledge that still exist as well as research approaches that might be taken to clarify even further the mechanisms underlying acute and chronic oral-facial pain. The properties of the sense organs responding to a noxious oral-facial stimulus are first considered. This section is followed by a review of the sensory pathways and mechanisms by which the sensory information is relayed in nociceptive neurones in the brainstem and then transmitted to local reflex centers and to higher brain centers involved in the various aspects of the pain experience--namely, the sensory-discriminative, affective (emotional), cognitive, and motivational dimensions of pain. Reflex and behavioral responses to noxious oral-facial stimuli are also considered. The next section provides an extensive review of how these responses and the activity of the nociceptive neurones are modulated by higher brain center influences and by stimulation of, or alterations (e.g., by trauma) to, other sensory inputs to the brain. The neurochemical processes, involved in these modulatory mechanisms are also considered, with special emphasis on the role of neuropeptides and other neurochemicals recently shown to be involved in pain transmission and its control. The final section deals with recent findings of peripheral and central neural mechanisms underlying pain from the dental pulp.