Introduction

It is well understood that there are different components to the sensation of pain (Melzack and Casey,1968). The sensory-discriminative aspect of pain refers to the location, intensity and quality of the sensory experience of pain. The affective-motivational aspect of pain refers to the unpleasantness of the pain and how likely it is that it will motivate the animal to escape the pain. We refer to these different components of the pain sensation throughout this review as we examine the possibility that these different components are mediated by different structures in the brain.

The spinothalamic tract (STT) is the spinal tract projecting toward the brain which is most often associated with the sensation of pain (Price and Dubner, 1977; Willis, 1985; Price et al., 2003). Cells of origin of the STT can be divided into those which respond to low-threshold stimuli (LT cells), those which respond to stimuli across the intensive continuum into the noxious range (wide dynamic range, WDR), and those that respond only to noxious stimuli (nociceptive specific, NS). Evidence that any structure mediates the sensory aspect of pain is grouped into four lines: that the structure is connected to other structures known to demonstrate pain-related activity; that neural elements in that structure respond to noxious stimuli; that stimulation of that structure produces pain; and that interventions which interfere with the function of that structure interfere with the sensation of pain evoked by noxious stimuli (Price and Dubner, 1977).