Institute of Neuroinformatics, UZH/ETH, Winterthurerstrasse 190, 8057, Zurich, Switzerland.
Brain Struct Funct. 2018 May;223(4):1811-1828. doi: 10.1007/s00429-017-1588-7. Epub 2017 Dec 12.
Pyramidal cells in the superficial layers of the neocortex provide a major excitatory projection to layer 5, which contains the pyramidal cells that project to subcortical motor-related targets. Both structurally and functionally rather little is known about this interlaminar pathway, especially in higher mammals. Here, we made sparse ultrastructural reconstructions of the projection to layer 5 of three pyramidal neurons from layer 3 in cat V1 whose morphology, physiology, and synaptic connections with layers 2 and 3 were known. The dominant targets of the 74 identified synapses in layer 5 were the dendritic spines of pyramidal cells. The fractions of target spiny dendrites were 59, 61, and 84% for the three cells, with the remaining targets being dendrites of smooth neurons. These fractions were similar to the distribution of targets of unlabeled asymmetric synapses in the surrounding neuropil. Serial section reconstructions revealed that the target dendrites were heterogenous in morphology, indicating that different cell types are innervated. This new evidence indicates that the descending projection from the superficial layer pyramidal cells does not simply drive the output pyramidal cells that project to cortical and subcortical targets, but participates in the complex circuitry of the deep cortical layers.
大脑新皮层浅层的锥体神经元向第 5 层提供主要的兴奋性投射，第 5 层包含向皮质下运动相关靶区投射的锥体神经元。关于这个层间通路，人们知之甚少，无论是在结构上还是功能上，尤其是在高等哺乳动物中。在此，我们对来自猫 V1 第 3 层的 3 个锥体神经元向第 5 层的投射进行了稀疏的超微结构重建，这些神经元的形态、生理学以及与第 2 层和第 3 层的突触连接都是已知的。在第 5 层中，74 个鉴定出的突触的主要靶标是锥体细胞的树突棘。这 3 个细胞的靶标棘突树突的分数分别为 59%、61%和 84%，其余的靶标是光滑神经元的树突。这些分数与周围神经胶质中未标记的不对称突触的靶标分布相似。连续切片重建显示，靶标树突在形态上具有异质性，表明不同的细胞类型被支配。这一新证据表明，来自浅层锥体神经元的下行投射并不仅仅驱动向皮质和皮质下靶区投射的输出锥体神经元，而是参与了深层皮质层的复杂回路。
