Leergaard T B, Alloway K D, Mutic J J, Bjaalie J G
Department of Anatomy, Institute of Basic Medical Sciences, University of Oslo, N-0317 Oslo, Norway.
J Neurosci. 2000 Nov 15;20(22):8474-84. doi: 10.1523/JNEUROSCI.20-22-08474.2000.
Subcortical re-entrant projection systems connecting cerebral cortical areas with the basal ganglia and cerebellum are topographically specific and therefore considered to be parallel circuits or "closed loops." The precision of projections within these circuits, however, has not been characterized sufficiently to indicate whether cortical signals are integrated within or among presumed compartments. To address this issue, we studied the first link of the rat cortico-ponto-cerebellar pathway with anterograde axonal tracing from physiologically defined, individual whisker "barrels" of the primary somatosensory cortex (SI). The labeled axons in the pontine nuclei formed several, sharply delineated clusters. Dual tracer injections into different SI whisker barrels gave rise to partly overlapping, paired clusters, indicating somatotopic specificity. Three-dimensional reconstructions revealed that the clusters were components of concentrically organized lamellar subspaces. Whisker barrels in the same row projected to different pontine lamellae (side by side), the somatotopic representation of which followed an inside-out sequence. By contrast, whisker barrels from separate rows projected to clusters located within the same lamellar subspace (end to end). In the neostriatum, this lamellar topography was the opposite, with barrels in the same row contacting different parts of the same lamellar subspace (end to end). The degree of overlap among pontine clusters varied as a function of the proximity of the cortical injections. Furthermore, corticopontine overlap was higher among projections from barrels in the same row than among projections from different whisker barrel rows. This anisotropy was the same in the corticostriatal projection. These findings have important implications for understanding convergence and local integration in somatosensory-related subcortical circuits.
连接大脑皮层区域与基底神经节和小脑的皮层下折返投射系统具有拓扑特异性，因此被认为是平行回路或“闭环”。然而，这些回路内投射的精确性尚未得到充分表征，无法表明皮层信号是在假定的区室内还是在区室之间整合。为了解决这个问题，我们利用顺行轴突示踪技术，从生理上定义的初级体感皮层（SI）的单个胡须“桶状区”进行研究，该技术研究的是大鼠皮质-脑桥-小脑通路的第一个环节。脑桥核中标记的轴突形成了几个清晰界定的簇。将双重示踪剂注射到不同的SI胡须桶状区会产生部分重叠的成对簇，表明存在躯体定位特异性。三维重建显示，这些簇是同心组织的层状子空间的组成部分。同一排的胡须桶状区投射到不同的脑桥薄片（并排），其躯体定位表征遵循由内向外的顺序。相比之下，来自不同排的胡须桶状区投射到位于同层状子空间内的簇（首尾相连）。在新纹状体中，这种层状拓扑结构则相反，同一排的桶状区与同层状子空间的不同部分接触（首尾相连）。脑桥簇之间的重叠程度随皮层注射部位的接近程度而变化。此外，同一排桶状区的投射之间的皮质-脑桥重叠高于不同胡须桶状区排的投射之间的重叠。这种各向异性在皮质-纹状体投射中也是相同的。这些发现对于理解体感相关皮层下回路中的汇聚和局部整合具有重要意义。
