Oral--facial pain: old puzzles, new postulates.

Dental examples are given (e.g. referred pain, V neuralgia) which exemplify the multifactorial and puzzling nature of pain. Pain results from a noxious stimulus activating some of the small-diameter myelinated and unmyelinated afferent nerve fibres that innervate skin, mucosa, teeth etc. It is complex experience including the sensation experienced as well as the emotional, cognitive and motivational reactions evoked by the stimulus. Pain is not a simple function of the magnitude of damage caused by the stimulus, but also depends on factors such as the person's emotions, past experience of pain, and other concomitant sensory experiences (e.g. with acupuncture). Early postulates of pain failed to take all these factors into account. In the V system (Fig. 1), the specificity theory would maintain that there exists a 'private pain path' from oral-facial tissues to the cerebral cortex, with nerve fibres and brain cells in this path responsive only to stimulu of a noxious character (Fig. 2). But anatomical, physiological and psychological observations have failed to give full support to this theory and other postulates of pain. Although some details of the more recently proposed gate control theory have not been sustantiated, this theory has provided a good general framework for viewing pain and has stimulated much recent research. It emphasizes sensory interaction between large and small-diameter afferent nerve fibres in the gate control system, and regulation over central pain transmission through the gate by descending central controls (Fig. 3). In the V system, brain cells involved in central pain transmission have now been found (Fig. 4), and sensory interactions and descending controls on these brain cells noted (Fig. 5). Neural mechanisms such as these have been implicated in pain, and in its control by procedures such as acupuncture, suggestion, distraction and narcotic analgesia.