Suter Benjamin A, Shepherd Gordon M G
Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611.
Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611
J Neurosci. 2015 Feb 18;35(7):2959-74. doi: 10.1523/JNEUROSCI.4287-14.2015.
Primary motor (M1) and secondary somatosensory (S2) cortices, although anatomically and functionally distinct, share an intriguing cellular component: corticospinal neurons (CSP) in layer 5B. Here, we investigated the long-range circuits of CSPs in mouse forelimb-M1 and S2. We found that interareal projections (S2 → M1 and M1 → S2) monosynaptically excited pyramidal neurons across multiple layers, including CSPs. Area-specific differences were observed in the relative strengths of inputs to subsets of CSPs and other cell types, but the general patterns were similar. Furthermore, subcellular mapping of the dendritic distributions of these corticocortical excitatory synapses onto CSPs in both areas also showed similar patterns. Because layer 5B is particularly thick in M1, but not S2, we studied M1-CSPs at different cortical depths, quantifying their dendritic morphology and mapping inputs from additional cortical (M2, contralateral M1, and local layer 2/3) and thalamic (VL nucleus) sources. These results indicated that CSPs exhibit area-specific modifications on an otherwise conserved synaptic organization, and that different afferents innervate M1-CSP dendritic domains in a source-specific manner. In the cervical spinal cord, CSP axons from S2 and M1 partly converged on middle layers, but S2-CSP axons extended further dorsally, and M1-CSP axons ventrally. Thus, our findings identify many shared features in the circuits of M1 and S2 and show that these areas communicate via mutual projections that give each area monosynaptic access to the other area's CSPs. These interareally yoked CSP circuits may enable M1 and S2 to operate in a coordinated yet differentiated manner in the service of sensorimotor integration.
初级运动皮层（M1）和次级躯体感觉皮层（S2）虽然在解剖结构和功能上有所不同，但却共享一个有趣的细胞成分：5B层中的皮质脊髓神经元（CSP）。在此，我们研究了小鼠前肢-M1和S2中CSP的长程回路。我们发现，区域间投射（S2→M1和M1→S2）单突触地兴奋了包括CSP在内的多层锥体神经元。在输入到CSP子集和其他细胞类型的相对强度上观察到了区域特异性差异，但总体模式相似。此外，这些皮质-皮质兴奋性突触在两个区域的CSP上的树突分布的亚细胞图谱也显示出相似模式。由于M1中的5B层特别厚，而S2中并非如此，我们研究了不同皮质深度的M1-CSP，量化了它们的树突形态，并绘制了来自其他皮质（M2、对侧M1和局部2/3层）和丘脑（腹外侧核）来源的输入图谱。这些结果表明，CSP在原本保守的突触组织上表现出区域特异性修饰，并且不同的传入神经以源特异性方式支配M1-CSP的树突域。在颈脊髓中，来自S2和M1的CSP轴突部分汇聚在中层，但S2-CSP轴突进一步向背侧延伸，而M1-CSP轴突向腹侧延伸。因此，我们的研究结果确定了M1和S2回路中的许多共同特征，并表明这些区域通过相互投射进行通信，使每个区域能够单突触地接触到另一个区域的CSP。这些区域间相连的CSP回路可能使M1和S2能够以协调但有差异的方式运作，以服务于感觉运动整合。
