The thalamus is a paired subcortical brain structure joined at the midline and sitting very near the center of the brain. In the human, each half is roughly the size and shape of a walnut. There are two major components. First is the dorsal thalamus, which is comprised of roughly 15 nuclei with relay cells that project to the cerebral cortex. (By "cortex" in this account, we mean "neocortex.") Second is the ventral thalamus, the major portion of which is the thalamic reticular nucleus, which sits like a shield flush against the lateral surface of the dorsal thalamus; reticular cells are GABAergic and project into the dorsal thalamus to inhibit relay cells. The other cellular component of thalamus, in addition to relay and reticular cells, is interneurons, which are also GABAergic, sit amongst the relay cells, and inhibit them. Generally, the relay cell to interneuron ratio is between 3 and 4 to one. An exception is found the mouse and rat, in which interneurons are essentially missing from all thalamic nuclei except the lateral geniculate nucleus (Arcelli et al, 1997).
Most of the relay nuclei topographically innervate the middle layers of cortex, but a few along the midline and extended between other nuclei project rather diffusely to upper cortical layers, including layer 1; rather little is known of these latter, diffusely-projecting nuclei, and they are not further considered in this account (for further details, see Sherman and Guillery, 2006; Jones, 2006). The remaining thalamic relay nuclei each innervates one or a small number of cortical areas. Indeed, all information reaching cortex passes through thalamus, and thus thalamus sits in a strategic position for brain processing.
The major role of thalamus is to gate and otherwise modulate the flow of information to cortex. For example, visual information from the retina is not sent directly to visual cortex but instead is relayed through the lateral geniculate nucleus of the thalamus. In the macaque monkey, there are roughly 1x10^6 geniculate relay cells (Williams and Rakic, 1988), but in primary visual cortex there are roughly 1.6x10^8 neurons (O'Kusky and Colonnier, 1982), which is typical of thalamocortical relationships. Thus thalamus represents the final bottleneck of information flow before it gets into cortex. In other words, to modify information flow for processes of attention and other behavioral requirements, it is more efficient to do this at the level of thalamus before it reaches cortex. While there is still much to learn about the cell and circuit properties of thalamus in this role, what we do know supports this general view of thalamic function. For further details of thalamus, see Jones (2006) and Sherman and Guillery (2006).
Adapted from S. Murray Sherman (2006), Scholarpedia, 1(9):1583.