Interactions Between Descending Pathways from the Dorsal and Ventrolateral Periaqueductal Gray Matter in the Rat

Abstract

Many psychological and environmental factors are known to influence responsiveness to noxious sensory stimuli. Until recently, the physiological mechanisms which enable such changes to take place were not understood. Current knowledge of endogenous analgesic mechanisms stems largely from the observation by Reynolds (1969) that analgesia could be produced in rats by electrical stimulation in the midbrain periaqueductal gray matter (PAG). This important paper was the first to suggest a neurophysiological basis for altered pain perception and provided a major impetus for further studies of pain control from the PAG. Although there is now a wealth of data concerning the neurophysiological and neuropharmacological mechanisms which underly the stimulation-produced analgesia, the physiological role of pain control from the PAG and the conditions under which this system becomes activated are less well understood. The PAG is known to be involved in initiating and integrating many aspects of behavioral, somatomotor and autonomic activity of which analgesia is but one component. In the past the tendancy has been to study the individual aspects of PAG function in isolation. This approach has yielded much information regarding the physiological mechanisms by which such changes are mediated but has been less profitable in terms of our understanding of the functional significance of the responses.