Neural mechanisms of pain: an overview.

There are two essential components of pain: discriminative and affective. The discriminative component includes the ability to identify the stimulus as originating from somatic or visceral tissue, determine some of the physical properties of the stimulus, and localize it in space, time, and along a continuum of intensities. The affective component is the experience of aversiveness which motivates escape, avoidance, and protective behavior. Both of these components of pain were acknowledged by Sir Charles Sherrington (1947) and must be considered in any discussion of the neurophysiological basis of pain. The neural mechanisms subserving discrimination and affect are different and may be differentially affected by drugs or surgical procedures. A consideration of pain mechanisms must also include the neural systems modulating pain, for it is well known that pain can be profoundly influenced by other somatic stimuli and by attentional, emotional, and cognitive factors. A thorough and detailed discussion of pain mechanisms is beyond the scope of this brief overview, but I will cover major features of the neural mechanisms currently thought to underlie the discriminative and affective dimensions of pain and the mechanisms by which pain may be modulated.