頭頂葉と痛みの認知

Pain perception in the parietal cortex

　これまで，多くの臨床病理学的報告や基礎研究により頭頂葉は痛み情報の伝達機構のなかで，sensory-perceptionだけではなく，異なる感覚収束，感覚と運動の統合のなかで果たす役割があると考えられてきた。近年，画像解析研究の発展により，頭頂葉における痛みの認知機構解明の努力がされており，特にallodyniaなどの異常な痛みにおける頭頂葉の活性化が新しい話題となっている。一方，基礎研究では，頭頂葉7b野で記録される侵害受容ニューロンは，体性感覚刺激（機械的刺激）と視覚刺激の両方に反応する，多感覚受容細胞であることがわかっている。さらにこの細胞の反応は，被験体の恐怖心などの内的意識状態に影響される。これらの研究から，頭頂葉における痛み認知機構は，種々の内的意識状態により修飾を受けるが，他の感覚同様，感覚の収束や運動との統合に関与し，神経の可塑的な変化が関与していると考えられている，非生理的な痛みにも役割を果たすと考えられる。

　Many areas of the brain, including the parietal lobes, are activated by noxious stimulation in human brain imaging studies using PET and fMRI. These areas could be divided into three groups, according to different aspects：（1）sensory-perception, （2）affectional and attentional aspects of pain, （3）sensory-motor integration. The insula cortex and the parietal operculum, which were activated by noxious stimulation bilaterally or unilaterally, may play roles in sensory perception. Lesions of the areas induce changes in pain sensitivity. A remarkable decrease in pain threshold was observed in patients with a tumor of the posterior insula and the operculum（Greenspan, 1999）. Brodmann area（BA）40 of the parietal cortex represents hemispheric lateralization of activity evoked by noxious stimulation. Responses were recorded dominantly on the right（Coghill, 2000）. We also observed responses on the right side, when heat stimulation was applied to either the right or the left ankle. Moreover, BA 40 was simultaneously activated by flexion of the ankle joint. BA 40 is considered to participate in multisensory convergence and sensory-motor integration in pain mechanism. Experimental allodynia induced by capsaicin injection or neuropathic pain increased responses in the BA 5/7. In the experiments using primates（Dong, 1989, 1994）, nociceptive neurons in the 7b area are characterized as multisensory, responding to heat, mechanical and visual stimulation. Most of them encode stimulus intensity. The population of nociceptive neurons in the parietal cortex is low；however, they have important roles to not only in perception but affection and attention in pain processing.