Do you have references on this particular point? I am looking for materials on connections between L6 CT axon terminals and the dendrites of thalamic relay cells (TRC).
Nearly every paper I read take for granted that L6 CT cells have a modulatory role, but I’m not sure they have fully investigated this point.
This paper from Sherman and Guillery (1998) seems to be the most cited paper when referring to the modulatory vs driver role of L5 PT cells and L6 CT cells.
On the actions that one nerve cell can have on another: Distinguishing ‘‘drivers’’ from ‘‘modulators’’:
https://www.pnas.org/content/pnas/95/12/7121.full.pdf
Extract about the axonal termination on proximal (from L5 PT cells) or distal dendrites (from L6 CT cells):
A first order relay receives its
driver inputs on proximal dendrites from subcortical sources via
ascending pathways whereas a higher order relay receives its driver
inputs from cells in cortical layer 5 (see ref. 40). The first order relay
sends a driver input to layer 4 of cortical area A (thick line), and that
same cortical area sends a modulator input (thin line with small
terminals onto distal dendrites of the thalamic relay cell) from layer 6
back to the same first order thalamic nucleus.
Next question: if we admit this anatomical observation, could many L6 CT cells give an above-threshold input driving the thalamic relay cells? The quantity may compensate for the individual strength.
Randall O’Reilly (the author of the 3VS paper) would answer “yes”!
I haven’t heard of any studies activating many L6 CT cells and causing thalamocortical cells to fire. It’s hard to tell how many L6 CT cells they activate though. L6 CT cells also activate the thalamic reticular nucleus a lot, which can cause overall inhibition, so it’s not like just activating all the L6 CT cells which target one particular thalamocortical cell. Just activating those CT cells might not cause much disynaptic inhibition via the thalamic reticular nucleus.
A Corticothalamic Circuit for Dynamic Switching between Feature Detection and Discrimination
A corticothalamic switch: controlling the thalamus with dynamic synapses
If you’re looking for search terms, there are loads of studies which use NTSR1-cre mice to optogenetically activate L6 CT cells.
One issue is that L6 CT cells might fire too sparsely to cause thalamocortical cells to fire. L6 CT cells have fairly broad axons in the thalamus, so thalamocortical cells might receive input from a good chunk of L6 CT cells if they form a lot of synapses. For example, in one case their axon arbors cover the whole part of the thalamus corresponding to the CT cell’s cortical column.
Activating a lot of the L6 CT cells which are presynaptic to a given thalamocortical cell might not happen outside experiments, since that could be like 1/2 of all L6 CT cells. I have no idea what the numbers are like though.
@Casey: Thanks for your answer, it helps!
The most enlightening info is in this series of videos.
In any case, the most interesting paper is:
[1] S. M. Sherman, “Functioning of circuits connecting thalamus and cortex,” Compr. Physiol. , vol. 7, no. 2, pp. 713–739, 2017. SCI-hub