Sensitization and kindling are types of learning characteristic of certain parts of the nervous system. In sensitization, a stimulus (S) that initially causes little or no observable response (R), eventually, with repetition, produces a full response. Given further exposure to the same stimulus, the full response will soon occur even with lower stimulus levels than the original stimulus. Eventually, with further repetition, the response occurs spontaneously, in the absence of any stimulus. Once the response begins to occur automatically, without external stimuli, kindling has occurred.
It seems that kindling, as a property of different functional systems of the central and peripheral nervous system, is involved in hypertension, seizure disorders, and, according to most evidence (Post, 1992), in the unipolar (depression only) and bipolar (mania and depression) affective disorders. The concepts of sensitization and kindling have great significance for the treatment of affective disorders.