Chronic pain is an immense unsolved clinical problem. Current approaches to this condition are limited by uncertainty about mechanisms of acute and chronic pain in man. Although much progress has been made toward understanding peripheral neural mechanisms of human nociception (Willis, 1985; Price & Dubner, 1977), we have a poor understanding of CNS pain mechanisms. Spinal mechanisms of pain processing are the subject of the previous chapter. The purpose of the present chapter is to review the anatomy and physiology of the ascending spinal pathways and supraspinal centres with painrelated activity. This chapter will focus on the primate nervous systems since there are significant differences between pain transmission in primates and other species such as cats and rats.
It is widely recognized that there are different components to the pain sensation (Melzack & Casey, 1968; Casey, 1978). The sensory–discriminative aspect of pain refers to the intensity of the sensory experience of pain. The motivational– affective aspect of pain refers to the unpleasantness of the pain and how likely it is that the pain will motivate the organism to escape the pain. Throughout this chapter we will refer to these different components of the pain sensation.
Ascending spinal pathways
The two main output somatosensory tracts from the spinal cord are the anterior–lateral and the dorsal column spinal systems. The anterior–lateral system terminates in the brainstem and thalamus while the dorsal column system terminates in the dorsal column nuclei (Willis, 1985).
The anterior–lateral spinal column
The cell bodies of origin for the anterior–lateral system are found in the spinal dorsal horn, particularly in lamina I, outer layers of lamina II and in laminae III to V; though some cells are also found in laminae VI to IX (Willis, 1985; Willis & Coggeshall, 1991). The spinal pathway from the spinal cord to the thalamus, the spinothalamic tract (STT) is partly located in the anterior lateral column. The STT system consists of two tracts, one positioned in the ventral lateral and the other in the dorsal lateral spinal funiculus (Cusick et al., 1989; Apkarian & Hodge, 1989b; Craig, 1997; Ralston & Ralston, 1992) and most commonly referred to as the ‘ventral’ and ‘dorsal’ STTs, respectively (Ralston & Ralston, 1992; Zhang et al., 2000).