Complicated architecture of cortical microcircuit: a comprehensive review.

Since Mountcastle's discovery of vertically oriented cells responding to a particular sensory modality, the radially aligned columnar organization has been extensively studied. The concept of columnar organization in the cerebral cortex has been accepted as a fundamental principle. Anatomically, different types of columns are recognized, which vary in size, components, organization, and across species. However, the precise definition of a column and its role as a functional unit remains debatable. As the brain functions as a network, understanding how columnar organization is integrated within cortical circuits is essential, but many aspects of its integration into cortical circuits remain elusive despite considerable investigation. In the cerebral cortex, connected pyramidal neuron pairs often share common synaptic inputs. This fine-scale specificity of synaptic connections within cortical neurons creates subnetworks that are thought to be embedded within functional columns. However, how to embed into the functional column and what type of columnar structure or unidentified structure to embed in a subnetwork remain largely unexplored. This review comprehensively introduces both anatomically and physiologically identified functional units and discusses potential approaches for integrating these concepts.