大脳皮質―大脳基底回路の構造―平行ループ回路と収束・発散回路

Cortico-basal Ganglia Circuits―Parallel Closed Loops and Convergent/Divergent Connections

はじめに

　大脳基底核は，パーキンソン病，ハンチントン舞踏病などの神経疾患の責任部位であるほか，強化学習，手続き学習などの認知機能にも深く関わっていることが示唆されている。解剖学的には，基底核が大脳皮質，視床を含む平行ループ回路を形成していることがさまざまな研究から指摘され，これが基底核の機能の基盤であるとされている。しかし，近年の研究により，異なる平行ループ間を結ぶ神経連絡も無視できないことが指摘されてきている。本稿では，皮質－基底核回路の異なるループ（あるいは機能ドメイン）間を結ぶ神経連絡に焦点を当てて，今日までの解剖学的な研究を概観する。

Abstract

　The basal ganglia play important roles not only in motor control but also in higher cognitive functions such as reinforcement learning and procedural memory.　Anatomical studies on the neuronal connections between the basal ganglia,cerebral cortex,and thalamus have demonstrated that these nuclei and cortical areas are interconnected via independent parallel loop circuits.　The association,motor,and limbic cortices project to specific domains in the striatum,which,in turn,project back to the corresponding cortical areas via the substantia nigra/globus pallidus and the thalamus.　Likewise,subregions in the motor cortex representing different body parts project to specific regions in the putamen,which project back to the original motor cortical regions.　These parallel loops have been thought to be the basic anatomical structures involved in the basal ganglia functions.　Furthermore,neuronal projections communicating between different loops (or functional domains) have also been discovered.　A considerable number of corticostriatal projections from functionally interrelated cortical areas (e. g.,hand representations of the motor cortex and somatosensory cortex) converge at the striatum.　It has also been suggested that the location of the substantia nigra is in such that it can transmit information from the ‘limbic loop' to the ‘association loop',and from the ‘association loop' to the ‘motor loop'.　Furthermore,a recent transsynaptic neuronal tracing study conducted at our laboratory demonstrated that the ventral (limbic) striatum sends divergent outputs to multiple regions in the frontal cortex.　These ‘inter-loop' connections would be important for the integration of information to achieve goal-directed behaviors.