Structure and connections of the thalamic reticular nucleus: Advancing views over half a century

Abstract

The advance of knowledge of the thalamic reticular nucleus and its connections has been reviewed and Max Cowan's contributions to this knowledge and to the methods used for studying the nucleus have been summarized. Whereas 50 years ago the nucleus was seen as a diffusely organized cell group closely related to the brain stem reticular formation, it can now be seen as a complex, tightly organized entity that has a significant inhibitory, modulatory action on the thalamic relay to cortex. The nucleus is under the control, on the one hand, of topographically organized afferents from the cerebral cortex and the thalamus, and on the other of more diffuse afferents from brain stem, basal forebrain, and other regions. Whereas the second group of afferents can be expected to have global actions on thalamocortical transmission, relevant for overall attentive state, the former group will have local actions, modulating transmission through the thalamus to cortex with highly specific local effects. Since it appears that all areas of cortex and all parts of the thalamus are linked directly to the reticular nucleus, it now becomes important to define how the several pathways that pass through the thalamus relate to each other in their reticular connections.

Spikes, synchrony, and attentive learning by laminar thalamocortical circuits

“…The SMART model proposes functional roles for both the specific and nonspecific projections of the TRN (Figures 2b and 2e). The specific inhibitory projections of the TRN to the LGN (Guillery et al 1998;Guillery and Harting, 2003) provide a detailed anatomical realization of the ART matching process that suppresses bottom-up inputs that mismatch cortical top-down excitatory expectations. In particular, the top-down excitatory on-center, adaptive pathway from layer 6 II to LGN core cells is supplemented by a TRN-mediated inhibitory offsurround (Figures 2b and 5).…”

“…Guillery and Harting (2003); Sherman and Guillery (2001). Table 1 Major simulated anatomical pathways.…”

“…The TRN forms a shell around the lateral and dorsal portions of the thalamus, lying in the axonal path connecting the specific and nonspecific thalamus and the cortex (Guillery and Harting, 2003). Afferents to the TRN are mainly branches of bottom-up axons from a specific thalamus to its target cortex, or branches of top-down axons from cortical layer 6 to its specific thalamic nucleus.…”

“…Top-down inhibitory feedback from the TRN to specific thalamic nuclei helps to balance top-down cortical layer 6 excitatory signals at their shared target cells (Figures 2b and 3a), and thereby enables the excitatory signals to have only a modulatory effect on these cells (Guillery and Harting, 2003) when these are the only active inputs. In addition to projecting to first-order and higher-order specific thalamic nuclei (Guillery and Harting, 2003), the TRN also projects to the nonspecific intralaminar and midline thalamic nuclei (Kolmac and Mitrofanis, 1997); see Figures 2e and 3c. TRN projections to the intralaminar and midline nuclei are more diffuse than the reticular projection to the specific dorsal thalamic nuclei.…”

Specific long-term memory traces in primary auditory cortex

“…(As noted above, the collicular shifts are reported to develop rapidly while the cortical shifts develop slowly 25 , which seems incompatible with the model's principle that cortical plasticity induces collicular shifts.) Termination of the positive feedback loop, which ends the short-lived collicular plasticity, is hypothesized to be caused by inhibition from the thalamic reticular nucleus (TRS) (presumably at the level of the medial geniculate nucleus 53,54 ), which receives cholinergic input from the NB (Fig. 5b).…”