A subpopulation of L6b neurons provides driver‐like input to the posteromedial thalamus

Corticothalamic Pathways in Auditory Processing: Recent Advances and Insights From Other Sensory Systems

“…Recent studies that selectively targeted and manipulated distinct neuronal subtypes in L6 suggest that the previous classification might be incomplete, because not all L6 CT neurons send a reciprocal feedback to thalamus or provide modulator-like input. A subpopulation of neurons in L6b seems to be involved in a CT circuit that differs anatomically and physiologically from both L5 and L6a, the canonical L6 feedback circuit, and likely represent a third type of CT circuit ( Figure 1 ; Hoerder-Suabedissen et al, 2018 ; Ansorge et al, 2020 ; Buchan, 2020 ; Zolnik et al, 2020 ), as we will explain in section L6b non-reciprocal Corticothalamic Projections.…”

“…Perhaps the most surprising findings were that L6b Drd1a neurons receive their main input from long range intracortical neurons (e.g., from motor, auditory, and visual regions) with little or no contribution from thalamic input (Zolnik et al, 2020 ), and do not have side branches or synapses in TRN (Hoerder-Suabedissen et al, 2018 ), as opposed to L6a neurons ( Figure 1 ). Functionally, Drd1 neurons seem to carry a driver-like signature, like L5 CT neurons, because their synapses are depressing (somatosensory system; Ansorge et al, 2020 ; Buchan, 2020 ), although L6b axon terminals are significantly smaller and simpler than the majority from L5 axons [somatosensory system, posterior thalamic nucleus; Figure 1 , compare photomicrographs in plots D and F; Hoerder-Suabedissen et al ( 2018 )].…”

“…In contrast, the giant terminals carried by L5 axons (Bajo et al, 1995 ; Bartlett et al, 2000 ) can drive their own messages to thalamic neurons via non-reciprocal projections, and then feedforward these messages to a different, hierarchically higher cortical area, forming a transthalamic corticocortical circuit (Llano and Sherman, 2008 ; Theyel et al, 2010 ; Mo and Sherman, 2019 ). Very recent studies suggest that some CT neurons emanating from deep layer 6 (L6b) have distinct anatomical and physiological properties from neurons emanating from both L6a and L5 and could represent a third CT circuit (Hoerder-Suabedissen et al, 2018 ; Ansorge et al, 2020 ; Buchan, 2020 ; Zolnik et al, 2020 ).…”