2015 February ; 18(2): 170–181. doi:10.1038/nn.3917. | Kenneth D. Harris and Gordon M. G. Shepherd
The neocortex, a brain structure crucial for cognitive abilities, exhibits remarkable similarities in circuit organization across different areas and species. These similarities suggest a common strategy for processing diverse types of information, including sensory modalities, motor control, and higher cognitive functions. Cortical neurons can be classified into a small number of major classes, each with similar properties such as connectivity, developmental history, gene expression, intrinsic physiology, and in vivo activity patterns. These classes are further divided into subclasses, and conserved patterns of input and output connections are emerging for many of these subclasses.
The neocortical excitatory circuits (ECs) are composed of three major classes: intratelencephalic (IT) neurons, pyramidal tract (PT) neurons, and corticothalamic (CT) neurons. IT neurons form recurrent connections with local neurons and interconnect extensively within and between hemispheres. PT neurons integrate local and thalamic inputs and project to subcortical structures. CT neurons, found in layer 6, receive inputs from various sources but project primarily to the thalamus. The connectivity of these ECs is highly conserved, but quantitative differences allow for specialized functions in different areas.
Inhibitory circuits, mediated by interneurons, also exhibit serial homology. These circuits are organized sequentially, with vasoactive intestinal peptide (Vip) neurons being the most upstream. The activity of interneurons is modulated by behavior, and quantitative differences in these circuits may explain how different cortical regions adapt to specific modalities and behaviors.
The developmental basis of serially homologous circuits involves homologous genetic programs that specify cell types and their connectivity. Differences in gene expression and extrinsic innervation patterns sculpt the circuits in different areas, leading to functional specialization.
While significant progress has been made, further research is needed to clarify the input and output connectivity of subclasses, understand the role of IT subclasses in inter-areal connectivity, and explore how specific connectivity and physiology contribute to in vivo firing patterns. These efforts will help establish general principles of cortical circuit organization and function across species.The neocortex, a brain structure crucial for cognitive abilities, exhibits remarkable similarities in circuit organization across different areas and species. These similarities suggest a common strategy for processing diverse types of information, including sensory modalities, motor control, and higher cognitive functions. Cortical neurons can be classified into a small number of major classes, each with similar properties such as connectivity, developmental history, gene expression, intrinsic physiology, and in vivo activity patterns. These classes are further divided into subclasses, and conserved patterns of input and output connections are emerging for many of these subclasses.
The neocortical excitatory circuits (ECs) are composed of three major classes: intratelencephalic (IT) neurons, pyramidal tract (PT) neurons, and corticothalamic (CT) neurons. IT neurons form recurrent connections with local neurons and interconnect extensively within and between hemispheres. PT neurons integrate local and thalamic inputs and project to subcortical structures. CT neurons, found in layer 6, receive inputs from various sources but project primarily to the thalamus. The connectivity of these ECs is highly conserved, but quantitative differences allow for specialized functions in different areas.
Inhibitory circuits, mediated by interneurons, also exhibit serial homology. These circuits are organized sequentially, with vasoactive intestinal peptide (Vip) neurons being the most upstream. The activity of interneurons is modulated by behavior, and quantitative differences in these circuits may explain how different cortical regions adapt to specific modalities and behaviors.
The developmental basis of serially homologous circuits involves homologous genetic programs that specify cell types and their connectivity. Differences in gene expression and extrinsic innervation patterns sculpt the circuits in different areas, leading to functional specialization.
While significant progress has been made, further research is needed to clarify the input and output connectivity of subclasses, understand the role of IT subclasses in inter-areal connectivity, and explore how specific connectivity and physiology contribute to in vivo firing patterns. These efforts will help establish general principles of cortical circuit organization and function across species.