Unit analysis of nociceptive mechanisms in the thalamus of the awake squirrel monkey.

Abstract

THE PREVAILING CONCEPT of pain sensation has arisen from von Frey’s (14) extension of Miiller’s (37) doctrine of specific nerve energies. According to this view, there are cutaneous receptors which, when stimulated, generate impulses which are transmitted via central pathways anatomically distinct from those subserving other forms of somatic sensation. Evidence from animal (8, 20, 33, 46, 48, 57) and human (9, 25) experiments has indicated that activity in small diameter peripheral fibers is associated with pain responses and sensation. The pain-specificity concept is further supported by the work of Poggio and Mountcastle (43) in which it was found that, in the anesthetized cat, nearly 60yo of the units recorded from a posterolateral thalamic region responded only to noxious stimuli. The cortical projection of this thalamic area has subsequently been found to contain neurons which are apparently exclusively nociceptive (7). Other electrophysiological work (2, 22, 41) has suggested that certain of the medial and intralaminar thalamic nuclei may also contain exclusively nociceptive units. However, the concept of pain-specific elements has been challenged (23, 35, 39, 47, 51, 52) for a number of reasons; among these is the lack of evidence for morphologically distinct “pain receptors” (26, 52), the paucity of specifically high threshold mechanoreceptive neurons (23, 50) or fibers, (10, 19, 20, 46, 57) and the failure to account for clinical phenomena such as congenital abnormalities in pain perception and the persistence of pain following spinothalamic tractotomy (5,29,31,39,40). One alternative view has been that pain results from a unique patterning of impulses from nonspecific cutaneous receptors (51). bines certain elements of ‘ Still another concept of cutaneous sensation com‘specificity” and “pattern” theory (35) . The purpose of this study was to test the hypothesis that, in the awake animal, there are thalamic neurons which respond only to noxious stimuli. An additional purpose was to explore the possibility that certain thalamic neurons, responsive to innocuous stimuli, may show a quantitatively distinct response to noxious stimuli. Awake animals were used because there is evidence that anesthetics may profoundly influence the response properties