Isothalamus - Isothalamic Parts or Regions

Isothalamic Parts or Regions

The different functional modalities represented in the thalamus are segregated in specific anatomical regions, differentiated by the cerebral systems from where they receive their afferent projections. There are more corticothalamic than thalamocortical axons. Corticothalamic endings are of two kinds. The "classical" projection emanates from layer VI of the cortex,the axons are thin and have a long, almost straight, trajectory through the thalamus, not respecting intrathalamic borders. They emit only short perpendicular collaterals (the arborization formin a thin cylinder (Globus and Scheibel). Their terminal synapses are glutamatergic. The second kind of corticothamic axons is the Rockland type II (1994). This emanates from larger pyramidal cells and is much thicker. Its ending is small, dense and globular. Its synapses are located close to the soma of the thalamic neuron, often forming the center of glomerular complexes. The isothalamus serves the function of transforming and distributing "prethalamic" information to the cortex.

The thalamic parts delineated by the lamellar and cellular "limiting" elements, according to the founding system of Burdach (1822), constituted the classic thalamic nuclei. These have been later further subdivided. The Louvain symposium (in Dewulf, 1971) made the recommendation to call the classical subdivisions "region". One region may be made up of one or several nuclei. These may have one (or several) partes, if there is a particular coafference for instance.

The region separated by the superior lamella is the Anterior region (A). The region separated medially by the medial lamina is the Lateral region (L). Almost separated from the thalamic mass are the Geniculate bodies (G). The remaining isothalamus is made up of the medial region (M, medial to the medial lamina) and posteriorly, with no complete separation in man, of the posterior regio or pulvinar (Pu). The last two represent a huge medioposterior ensemble. The classical separation into relay nuclei, receiving "specific" subcortical afferences or association nuclei, which would not, cannot be retained as absolute. The lateral region and the geniculate bodies indeed receive strong lower "specific" afferences and can be seen as the "sensorimotor" part of the thalamus. The medioposterior ensemble, in most of its volume does not receive subcortical afferents and abundant afferences from the "associative" cortex but in some, essentially ventral parts, in fact receives subcortical afferences, such as tectal, spinothalamic or amygdalar. The anterior region receives a particular afference that is not entirely subcortical (directly or indirectly from the subiculum).

Thalamic regions may be functionally inhomogeneous. The elements of the lateral region have been frequently separated into ventral and dorsal (in fact named lateral) nuclei. This subdivision no more hold true. Cytoarchitectonics have partly failed. What differentiates anatomofunctional parts are the major afferent systems present in the thalamus as terminal parts of axons and axonal arborisations. Three-dimensional analyses of the distribution of all the axonal ending coming from the same source show that they occupy together an own space in the thalamus, which is called a territory. Such a main territory do no mix or overlap in primates with neighbouring territories (Percheron et al. 1998). This is what made possible a solid partition of the thalamus. These territories may cover one or several nuclei. The analyses of the three-dimensional geometry of the main afferent terrirories in macaques have shown that a dorsal element on transverse sections is simply the posterior part of the preceding territory. There are thus no "dorsal nuclei". This is one reason why the nomenclature selected by the Nomina anatomica and the Terminologia anatomica (1998) is hardly applicable. The evolution of the thalamus follows that of the cortex and there are differences including between primates (new world monkeys and old world; old world and humans), which means that a universal nomenclature valid in all species is not simply reachable.