Pain pathways in the primate.

From the work reviewed here, it appears that the classical view that there is a sensory channel for pain sensation rather like sensory channels for other sensations seems plausible. However, pain has the property of producing more prominent motivational-affective behaviors than do other sensations (although there are certainly motivational-affective components of the responses to many sensory experiences, such as a verbal attack or the odor of a favorite perfume). It may be that certain nociceptive neurons, such as the STT cells that project to the medial thalamus that have total body receptive fields and many similar spinoreticular neurons, are concerned not so much with sensory events but rather with motivational-affective responses. Nevertheless, there are specific nociceptive afferent fibers, nociceptive spinothalamic tract cells with restricted receptive fields, nociceptive VPL thalamic and SI cortical neurons that presumably could play a crucial role in the sensory-discriminative aspects of pain (signalling, for example, stimulus intensity, location, duration, rate, and quality). Interestingly, many nociceptive neurons receive a convergent input from both sensitive mechanoreceptors and from nociceptors and so can be classified as "wide dynamic range" or multiconvergent neurons. It is not at all clear what the significance is of this kind of multimodal convergence. One possibility is that the weaker tactile input is treated as noise and largely ignored by higher processing centers in the brain. Another possibility is that WDR cells are switched in function by the action of descending pathways originating in the brain stem or cerebral cortex (cf., Gerhart et al., 1984; Yezierski et al., 1983). In any event, the solution of this problem is likely to be very important for the full understanding of the coding properties of nociceptive neurons, and this issue is reminiscent of the coding problem discussed by David Smith in this volume with respect to the gustatory system.