Abstract
The activity of the GABAergic neurons of the thalamic reticular nucleus (TRN) has long been known to play important roles in modulating the flow of information through the thalamus and in generating changes in thalamic activity during transitions from wakefulness to sleep. Recently, technological advances have considerably expanded our understanding of the functional organization of TRN. These have identified an impressive array of functionally distinct subnetworks in TRN that participate in sensory, motor, and/or cognitive processes through their different functional connections with thalamic projection neurons. Accordingly, “first order” projection neurons receive “driver” inputs from subcortical sources and are usually connected to a densely distributed TRN subnetwork composed of multiple elongated neural clusters that are topographically organized and incorporate spatially corresponding electrically connected neurons—first order projection neurons are also connected to TRN subnetworks exhibiting different state-dependent activity profiles. “Higher order” projection neurons receive driver inputs from cortical layer 5 and are mainly connected to a densely distributed TRN subnetwork composed of multiple broad neural clusters that are non-topographically organized and incorporate spatially corresponding electrically connected neurons. And projection neurons receiving “driver-like” inputs from the superior colliculus or basal ganglia are connected to TRN subnetworks composed of either elongated or broad neural clusters. Furthermore, TRN subnetworks that mediate interactions among neurons within groups of thalamic nuclei are connected to all three types of thalamic projection neurons. In addition, several TRN subnetworks mediate various bottom-up, top-down, and internuclear attentional processes: some bottom-up and top-down attentional mechanisms are specifically related to first order projection neurons whereas internuclear attentional mechanisms engage all three types of projection neurons. The TRN subnetworks formed by elongated and broad neural clusters may act as templates to guide the operations of the TRN subnetworks related to attentional processes. In this review article, the evidence revealing the functional TRN subnetworks will be evaluated and will be discussed in relation to the functions of the various sensory and motor thalamic nuclei with which these subnetworks are connected.
Highlights
The thalamus is a prominent diencephalic structure that contains a large number of nuclei (Figure 1), each of which engages in its own specialized functions (Jones, 2007)
Thalamoreticular terminals and/or thalamic reticular nucleus (TRN) neurons related to two or more thalamic nuclei are often situated in close proximity within overlapping territories (Cicirata et al, 1990; Pinault et al, 1995; Crabtree, 1996, 1998; Kolmac and Mitrofanis, 1997; Crabtree et al, 1998; Pinault and Deschênes, 1998a), TRN neurons can project to more than one thalamic nucleus (Pinault et al, 1995; Crabtree, 1996, 1998; Kolmac and Mitrofanis, 1997), and axonal fasciculi that stream across the thalamus have parallel trajectories that relate to particular subsets of thalamic nuclei and TRN regions (Crabtree and Isaac, 2002)
ventroposterior medial (VPM), posterior medial (POm) and ILc make up a caudal group of thalamic nuclei (Figure 1) whose topographically organized reciprocal interactions are most likely mediated by neurons in a broad central region of TRN (Kolmac and Mitrofanis, 1997; Crabtree et al, 1998; Crabtree and Isaac, 2002)
Summary
Technological advances have considerably expanded our understanding of the functional organization of TRN These have identified an impressive array of functionally distinct subnetworks in TRN that participate in sensory, motor, and/or cognitive processes through their different functional connections with thalamic projection neurons. “first order” projection neurons receive “driver” inputs from subcortical sources and are usually connected to a densely distributed TRN subnetwork composed of multiple elongated neural clusters that are topographically organized and incorporate spatially corresponding electrically connected neurons—first order projection neurons are connected to TRN subnetworks exhibiting different state-dependent activity profiles. “Higher order” projection neurons receive driver inputs from cortical layer 5 and are mainly connected to a densely distributed TRN subnetwork composed of multiple broad neural clusters that are non-topographically organized and incorporate spatially corresponding electrically connected neurons.
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