Thalamocortical Relations

The thalamus is a paired structure joined at the midline and located at the center of the brain (Figure 10.1 ). Each half is roughly the size of a walnut. The main part of the thalamus is divided into a number of discrete regions, known as relay nuclei. These contain the relay cells that project to the cerebral cortex. (In this chapter, cortex refers to neocortex, which does not include the hippocampal for mation or olfactory cortex.) Lateral to this main body of the thalamus is the thalamic reticular nucleus ( TRN in Figure 10.1 ), which fits like a shield alongside the body of the main relay nuclei of the thalamus. The thalamic reticular nucleus is comprised entirely of GABAergic neurons that do not innervate cortex but instead innervate thalamic relay cells. In Figure 10.1 , all but the front part of the thalamic reticular nucleus has been cut away to reveal the relay nuclei. Strictly speaking, the relay nuclei are the dorsal thalamus, while the thalamic reticular nucleus is part of the ventral thalamus; here, dorsal and ventral reflect embryonic origin rather than relative location in the adult, meaning that the relay nuclei and thalamic reticular nucleus have different developmental origins. Unless otherwise specified, thalamus refers to the relay nuclei of the dorsal thalamus. Virtually all information reaching the cortex must pass through and be relayed by the thalamus. Thus anything we are consciously aware of and all of our perceptions of the outside world depend on thalamic relays. This relay is dynamically controlled by behavioral states and processes, including attentional demands. Each of the main relay nuclei shown in Figure 10.1 innervate one or a small number of cortical areas and, as far as we know, every area of cortex receives a thalamic input. The thalamus is there not just to get peripheral information to the cortex, but it continues to play a vital role in the further processing of this information by the cortex. Where thalamocortical relationships are understood (e.g., the projection of the lateral geniculate nucleus to the primary visual cortex), the thalamic input plays a major role in determining the functional properties of the cortical target area. It has been shown that if retinal inputs in the ferret are diverted into the medial geniculate nucleus instead of the normal auditory inputs, the auditory cortex acquires visual responsiveness and organizes orientation specific domains normally seen only in the visual cortex (Sharma, Angelucci, & Sur, 2000 ). Since all thalamic nuclei innervate cortex and all cortical areas are thus innervated, this might suggest that the functional properties of any cortical area follow its thalamic input rather than inputs from other cortical areas. This is a rather subversive idea that runs counter to the traditional dogma that cortical processing depends solely on direct cortico cortical pathways. Cortico thalamo cortical pathways play a heretofore neglected and perhaps dominant role in cortical functioning. Figure 10.1 Schematic view of the right thalamus of the human. Shown are the main relay nuclei plus the thalamic reticular nucleus (TRN), of which only the anterior portion is visible; the remainder has been removed to reveal the thalamic relay nuclei. Normally, the thalamic reticular nucleus extends the length of thalamus as a thin shield closely apposed to the lateral surface of the relay nuclei. Abbreviations: A, Anterior Nuclei; CM, Central Medial Nucleus; IL, Intralaminar Nuclei; LD, Lateral Dorsal Nucleus; LGN, Lateral geniculate Nucleus; LP or PO, Lateral Posterior or Posterior Nucleus; MD, Medial Dorsal Nucleus; MGN, Medial Geniculate Nucleus; MI, Midline Nuclei; P, pulvinar; TRN, thalamic reticular nucleus; VA, Ventral Anterior Nucleus; VL, Ventral Lateral Nucleus; VPL, Ventral Posterolateral Nucleus; VPM, Ventral Posteromedial Nucleus.