Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA.
Department of Biomedicine, Aarhus University, Aarhus, Denmark.
Nat Rev Neurosci. 2021 Jul;22(7):389-406. doi: 10.1038/s41583-021-00459-3. Epub 2021 May 6.
Functions of the neocortex depend on its bidirectional communication with the thalamus, via cortico-thalamo-cortical (CTC) loops. Recent work dissecting the synaptic connectivity in these loops is generating a clearer picture of their cellular organization. Here, we review findings across sensory, motor and cognitive areas, focusing on patterns of cell type-specific synaptic connections between the major types of cortical and thalamic neurons. We outline simple and complex CTC loops, and note features of these loops that appear to be general versus specialized. CTC loops are tightly interlinked with local cortical and corticocortical (CC) circuits, forming extended chains of loops that are probably critical for communication across hierarchically organized cerebral networks. Such CTC-CC loop chains appear to constitute a modular unit of organization, serving as scaffolding for area-specific structural and functional modifications. Inhibitory neurons and circuits are embedded throughout CTC loops, shaping the flow of excitation. We consider recent findings in the context of established CTC and CC circuit models, and highlight current efforts to pinpoint cell type-specific mechanisms in CTC loops involved in consciousness and perception. As pieces of the connectivity puzzle fall increasingly into place, this knowledge can guide further efforts to understand structure-function relationships in CTC loops.
新皮层的功能依赖于其与丘脑的双向通讯，通过皮质-丘脑-皮质（CTC）回路来实现。最近对这些回路中的突触连接进行的解析工作，为它们的细胞组织提供了更清晰的图像。在这里，我们回顾了在感觉、运动和认知区域的发现，重点关注主要类型的皮质和丘脑神经元之间的细胞类型特异性突触连接模式。我们概述了简单和复杂的 CTC 回路，并指出了这些回路中似乎是普遍的和专门的特征。CTC 回路与局部皮质和皮质内（CC）回路紧密相连，形成了扩展的回路链，这些回路可能对跨层次组织的大脑网络中的通讯至关重要。这种 CTC-CC 回路链似乎构成了组织的一个模块化单元，作为特定区域结构和功能修饰的支架。抑制性神经元和回路嵌入在 CTC 回路中，塑造兴奋的流动。我们根据已建立的 CTC 和 CC 回路模型来考虑最近的发现，并强调了目前努力确定参与意识和感知的 CTC 回路中细胞类型特异性机制的努力。随着连接拼图的越来越多的部分逐渐到位，这些知识可以指导进一步努力理解 CTC 回路中的结构-功能关系。
