Institute for Neurosciences, Technische Universität München, 80802 Munich, Germany;
Graduate School of Systemic Neurosciences, Ludwig-Maximilians Universität München, 82152 Martinsried, Germany.
Proc Natl Acad Sci U S A. 2017 Aug 15;114(33):8853-8858. doi: 10.1073/pnas.1704302114. Epub 2017 Aug 3.
Neurons in cortical layer 5B (L5B) connect the cortex to numerous subcortical areas. Possibly the best-studied L5B cortico-subcortical connection is that between L5B neurons in the rodent barrel cortex (BC) and the posterior medial nucleus of the thalamus (POm). However, the spatial organization of L5B giant boutons in the POm and other subcortical targets is not known, and therefore it is unclear if this descending pathway retains somatotopy, i.e., body map organization, a hallmark of the ascending somatosensory pathway. We investigated the organization of the descending L5B pathway from the BC by dual-color anterograde labeling. We reconstructed and quantified the bouton clouds originating from adjacent L5B columns in the BC in three dimensions. L5B cells target six nuclei in the anterior midbrain and thalamus, including the posterior thalamus, the zona incerta, and the anterior pretectum. The L5B subcortical innervation is target specific in terms of bouton numbers, density, and projection volume. Common to all target nuclei investigated here is the maintenance of projection topology from different barrel columns in the BC, albeit with target-specific precision. We estimated low cortico-subcortical convergence and divergence, demonstrating that the L5B corticothalamic pathway is sparse and highly parallelized. Finally, the spatial organization of boutons and whisker map organization revealed the subdivision of the posterior group of the thalamus into four subnuclei (anterior, lateral, medial, and posterior). In conclusion, corticofugal L5B neurons establish a widespread cortico-subcortical network via sparse and somatotopically organized parallel pathways.
皮质 5B 层（L5B）中的神经元将皮层与众多皮质下区域连接起来。研究得最好的 L5B 皮质下连接可能是在啮齿动物的桶状皮层（BC）的 L5B 神经元和丘脑后内侧核（POm）之间。然而，POm 和其他皮质下靶区中的 L5B 巨终末的空间组织尚不清楚，因此尚不清楚该下行通路是否保留了躯体图组织，即上行感觉通路的标志。我们通过双色顺行标记研究了来自 BC 的 L5B 下行通路的组织。我们在三维空间中重建和量化了来自 BC 相邻 L5B 柱的终末云。L5B 细胞靶向中脑前脑和丘脑的六个核，包括丘脑后部、未定带和前顶盖。从 BC 的 L5B 细胞发出的亚皮质投射在终末数量、密度和投射体积方面具有靶标特异性。这里研究的所有靶核共有的是从 BC 的不同桶状柱维持投射拓扑，尽管具有靶标特异性的精度。我们估计低皮质下汇聚和发散，表明 L5B 皮质丘脑通路稀疏且高度并行化。最后，终末的空间组织和胡须图组织揭示了丘脑后群分为四个亚核（前、外侧、内侧和后）。总之，皮质传出 L5B 神经元通过稀疏且具有躯体图组织的并行通路建立了广泛的皮质下网络。
