The thalamus is more than just a relay

Abstract

The lateral geniculate nucleus and pulvinar are examples of two different types of relay: the former is a first order relay, transmitting information from a subcortical source (retina), while the latter is mostly a higher order relay, transmitting information from layer 5 of one cortical area to another cortical area. First and higher order thalamic relays can also be recognized for much of the rest of thalamus, and most of thalamus seems to be comprised of higher order relays. Higher order relays seem especially important to general corticocortical communication, and this challenges and extends the conventional view that such communication is based on direct corticocortical connections.

Figure 1

Schematic diagram of circuitry for the lateral geniculate nucleus. The inputs to relay cells are shown along with the relevant neurotransmitters and postsynaptic receptors (ionotropic and metabotropic) Abbreviations: ACh, acetylcholine; GABA, γ-aminobutyric acid; Glu, glutamate; LGN, lateral geniculate nucleus; PBR, parabrachial region; TRN, thalamic reticular nucleus.

Figure 2

Schematic diagrams showing organizational features of first and higher order thalamic nuclei. A first order nucleus (A) represents the first relay of a particular type of subcortical information to a first order or primary cortical area. A higher order nucleus (B, C) relays information from layer 5 of one cortical area to another cortical area. This relay can be from a primary area to a higher one (B) or between two higher order cortical areas (C). The important difference between them is the driver input, which is subcortical (A) for a first order thalamic nucleus and from layer 5 of cortex (B, C) for a higher order nucleus. Note that all thalamic nuclei receive an input from layer 6 of cortex, which is mostly feedback, but higher order nuclei in addition receive a layer 5 input from cortex, which is feedforward. Note in A–C that the driver inputs, both subcortical and from layer 5, are typically from branching axons, the significance of which is elaborated in the text. Abbreviations: FO, first order; HO, higher order; LGN, lateral geniculate nucleus; TRN, thalamic reticular nucleus. Redrawn from [23].

Figure 3

Comparison of conventional view (A) with the alternative view proposed here (B). The role of the direct corticocortical connections in B (dashed lines) is questioned (see text for details). Abbreviations: FO, first order; HO, higher order. Reproduced from [23].