Neuroimaging and pain: a window on the autonomic nervous system.
Pain is one of the most common experiences of humans. Neuroimaging techniques can visualize the main brain areas involved in pain modulation, the pain matrix. It is noteworthy that many of the brain areas forming the pain matrix are also involved in modulating autonomic nervous system (ANS) activity that in turn plays a major role in determining the best adaptive response to the pain experience. The tight connection between the pain system and ANS is also evident from neuroanatomical studies indicating that the lamina 1 neurons receive both painful and visceral stimuli from all visceral organs giving rise to the spinothalamocortical pathway concerned with conveying interoceptive information to central structures. The resulting interoceptive stream projects to the viscerosensory cortex in the mid-insula and onto the right anterior insula and orbitofrontal cortices. Right anterior insula activation is involved in the sympathetic arousal associated with mental tasks. This brain region receives numerous other inputs including pain and painful stimuli are conveyed somatotopically to both insulae. A similar somatotopic organization of painful stimuli has also been shown in the basal ganglia involved in cognitive, affective, motor and autonomic states. This highly specialized organization of nociceptive information in these brain areas may subserve a number of functions, particularly of coupling pain with the most appropriate autonomic states and affective/emotional states. The anterior cingulated cortex, another brain area playing a crucial role in nociception, is also directly involved in the control of autonomic functions such as arousal during volitional behaviour, including effortful cognitive processing. It is evident that the nociceptive system and ANS closely interact in many processes involved in maintaining internal homeostatis and in order to give the most appropriate biological substrate for cognitive, affective and emotional states.