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. Author manuscript; available in PMC: 2022 Jul 8.
Published in final edited form as: Annu Rev Neurosci. 2021 Mar 17;44:221–252. doi: 10.1146/annurev-neuro-100520-012117

Figure 1.

Figure 1

The composition of neocortical L1. L1 is the only layer lacking excitatory cell somata. It contains several structures that mediate the integration of top-down and bottom-up information in PCs, the output neurons of the cortex located in deeper layers, via synapses on their distal dendritic tufts in L1. Top-down information is conveyed to L1 by dense axonal projections, including feedback projections from cortical areas higher in the processing hierarchy, axons from higher-order thalamocortical neurons mediating cortico-thalamo-cortical loops, and axons from neurons in neuromodulatory centers. L1 also contains a rich population of GABAergic INs, including four types of INs with somata in L1: NGFCs, α7, CanCs, and VIPs. L1 also contains a significant proportion of the dendrites of at least three populations of INs with somata in L2/3, including VIP/Bips, VIP/CCK, and ChCs. Lastly, L1 contains the ascending axon of SST-expressing Martinotti cells located in L2/3 and infragranular layers. L2/3 and L5—6 SST INs receive powerful local excitatory input, but so far they do not seem to receive long-range inputs. Abbreviations: α7, α7 nicotinic receptor–expressing interneuron; CanC, canopy cell; ChC, chandelier cell; IN, interneuron; L, layer; NGFC, neurogliaform cell; PC, pyramidal cell; SST, somatostatin; VIP, vasoactive intestinal peptide–expressing interneuron; VIP/Bip, vasoactive intestinal peptide–expressing bipolar interneuron; VIP/CCK, cholecystokinin- and vasoactive intestinal peptide–expressing interneuron.