2024/07 - Minicolumns, References Frames and Models

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@jhawkins, @vclay, @nleadholm and @rmounir review the previous day’s brainstorming from the 2024 summer retreat and expand upon reference frames, minicolumns and modeling.

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Very interesting discussion. Since layer 6 was discussed, I thought I would share a few papers I have read recently which discussed the subdivisions in this layer. Both papers also contain some good figures.

Martinetti LE, Autio DM, Crandall SR. Motor Control of Distinct Layer 6 Corticothalamic Feedback Circuits. eNeuro. 2024 Jul 9;11(7):ENEURO.0255-24.2024. doi: 10.1523/ENEURO.0255-24.2024. PMID: 38926084; PMCID: PMC11236587.

URL: Motor Control of Distinct Layer 6 Corticothalamic Feedback Circuits - PMC

Some key points:

  • This is from a study done on the rat barrel somatosensory cortex. Studies neurons from primary motor cortex (vM1) projecting to the primary somatosensory cortex (vS1). This is more evidence that layer 6 receives motor/location related input.
  • There are two sublayers within layer 6A (upper and lower). A class of layer 6A upper excitatory neurons feedback to the primary thalamus (VPm), while another class of layer 6A lower excitatory neurons project to both the VPm and higher order thalamus (POm).
  • The two subtypes have little spatial overlap, suggesting distinct circuits.
  • Motor cortex input to L6 somatosensory cortex more strongly excites the dual projecting neurons compared to VPM-only neurons. Dual project neurons have higher intrinsic excitability than VPM-only neurons.
  • VPm-only neurons have compact dendritic trees.
  • Dual-projecting neurons have larger, vertically extended dendritic arbors, better suited to integrate distal inputs (like from vM1). The Axon trajectories also differed: dual neurons had wider axonal spread, implying broader influence.
  • Layer 6A upper is organized into infrabarrels which align with layer 4 barrels. In rodent vS1, L6aU CT cells appear to project to topographically aligned VPm regions (barreloids), whereas L6aL neurons have more complicated projections that include both aligned and non-aligned VPm regions as well as POm.
  • GABAergic inhibitory neurons, presumably in L6, are also engaged by vM1, providing disynaptic feedforward inhibition proportional to the vM1 excitation onto CT cells.

Frandolig JE, Matney CJ, Lee K, Kim J, Chevée M, Kim SJ, Bickert AA, Brown SP. The Synaptic Organization of Layer 6 Circuits Reveals Inhibition as a Major Output of a Neocortical Sublamina. Cell Rep. 2019 Sep 17;28(12):3131-3143.e5. doi: 10.1016/j.celrep.2019.08.048. PMID: 31533036; PMCID: PMC6941480.

URL: The Synaptic Organization of Layer 6 Circuits Reveals Inhibition as a Major Output of a Neocortical Sublamina - PMC

Some key Points:

  • In another barrel cortex study, Layer 6A contains a parvalbumin inhibitory interneuron (IL-PV IN) restricted to layer 6A upper.
  • This neuron receives strong thalamic input as well as local input and projects to all other cortical layers vertically within the column (layers 2/3, 4, 5, and layer 6A lower pyramidal neurons). Corticothalamic neurons (CThNs) and Corticocortical neurons (CCNs) in L6AUpper both strongly excite IL-PV INs.
  • "L6 Lower pyramidal neurons did not robustly excite IL-PV INs in L6U; only 2/21 tested connections were detected.”
  • VPM/POm L6Lower CThNs also project to L5a
  • The paper suggests that “the main interlaminar output of L6 U is columnar feedforward inhibition in response to the activity of local L6 U CThNs (corticothalamic neurons) and CCNs (cortico-cortico neurons) and long-range inputs to L6a. Our results indicate that an interlaminar inhibitory projection from a distinct sublamina in L6 U contributes to the canonical cortical circuit.”
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@EniAxon this is great and super relevant! Thank you for sharing those two papers and the key points :slight_smile:

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