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. Author manuscript; available in PMC: 2022 Jul 1.
Published in final edited form as: Nat Rev Neurosci. 2021 May 6;22(7):389–406. doi: 10.1038/s41583-021-00459-3

Fig. 1 ∣. Major classes of excitatory projection neurons in cortico-thalamo-cortical loops.

Fig. 1 ∣

a ∣Reconstructed dendrites are shown for the major excitatory cell classes involved in forming cortico-thalamo-cortical loops — thalamocortical (TC) neurons in the thalamus (purple), and three broad classes of excitatory neurons in the neocortex: intratelencephalic (IT; red), pyramidal tract (PT; blue) and corticothalamic (CT; green) neurons. The IT neurons are a large and diverse class, comprising multiple subtypes across and within cortical layers, from layer 2 to layer 6. The PT neurons, sharing layer 5B with IT neurons, have large apical dendritic tufts in layer 1. The CT neurons, sharing layer 6 with IT neurons, have relatively small dendritic arbors. b ∣ The axonal projection patterns of the same four classes are depicted schematically, following the same colour scheme. The IT neurons project bilaterally to other cortical areas and striatum. The PT neurons form descending projections with branches to multiple subcortical and subcerebral destinations. The CT neurons are specialists with axons projecting almost entirely to the thalamus. The cortical projection to the thalamus is composed of the axons of both PT and CT neurons. c ∣ Axonal arbors of IT, PT and CT neurons in motor cortex, and of TC neurons projecting to motor cortex, are shown separately (top row; twelve examples of each) or together (bottom row, in side, top and front views). The axons are from the database of the MouseLight project, which developed methods for reconstructing axonal arbors in their entirety across the brain 31,197 The images were generated with the MouseLight interface (www.janelia.org/project-team/mouselight) by selecting multiple axon IDs for display (IT axons AA0662, AA0627, AA0622, AA0592, AA0582, AA0003, AA0656, AA0588, AA0442, AA0184, AA0102, AA0002; PT axons AA1050, AA0927, AA0926, AA0923, AA0617, AA0587, AA0584, AA0261, AA0135, AA0134, AA0132, AA0131; CT axons AA0863, AA0833, AA0770, AA0652, AA0640, AA0623, AA0548, AA0545, AA0185, AA0043, AA0041, AA0038, and, TC axons AA0692, AA0591, AA0586, AA0581, AA0391, AA0342, AA0809, AA0719, AA0675, AA0451, AA0176, AA0137). Reconstructions in part a adapted with permission from REFS. 74-76,78,196). Reconstructions in part c adapted with permission from REF.31.