The isothalamus is a division used by some researchers in describing the thalamus.Percheron, G. (2003) "Thalamus". In Paxinos, G. and May, J. (eds). The human nervous system. 2d Ed. Elsevier. Amsterdam. pp.592-675
The isothalamus constitutes 90% or more of the thalamus, and despite the variety of functions it serves, follows a simple organizational scheme. The constituting neurons belong to two different neuronal genera. The first correspond to the thalamocortical neurons (or principal). They have a "tufted" (or radiate) morphology, as their dendritic arborisation is made up of straight dendritic distal branches starting from short and thick stems. The number of branches and the diameter of the arborisation are linked to the specific system of which they are a part, and to the animal species. They have the rather rare property of having no initial axonal collaterals, which implies that one emitting thalamocortical neuron does not send information to its neighbor. They send long-range glutamatergic projections to the cerebral cortex where they end electively at the layer IV (or around) level.
The other genus is made up of microneurons. These have short and thin dendrites and short axons and thus belong to local circuitry neurons. Their percentage in comparison to thalamocortical neurons varies across species, highly increasing with evolution. Their short axonal parts contact thalamocortical or other local circuitry neurons. Their neurotransmitter is GABA.
The dendrites of the two constituting genera receive synapses from a variety of afferent axons. The connections back to the thalamocortical neurons create "triads" modulating the thalamocortical output. One subcortical afference comes from the perithalamus (reticulate nucleus). This receives axonal branches from thalamocortical neurons. Its afferences are also GABAergic. The number of perithalamic neurons strongly decreases in evolution in opposition to the large increase in microneurons (Arcelli et al. 1997). To some extent the perithalamus plays a role in the local circuitry. The circuitous connection with corticothalamic neurons participates in the elaboration of thalamic rhythms.
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 form in 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),Burdach, K. F. (1822) Von Baue und Leben des Gehirns. Dyk, Leipzig constituted the classic thalamic nuclei. These have been later further subdivided. The Louvain symposium (in Dewulf, 1971)Attempt at standardization of nomenclature. In Dewulf, A. (1971) Anatomy of the normal human thalamus. Topometry and standardized nomenclature. Elsevier, Amsterdam pp.121-139 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 territories 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)Terminologia anatomica (1998) Thieme, Stuttgart. ISBN 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.
Superior region S (including the anterior A nucleus) The superior region comprises two elements that were linked during a long time and were later wrongly separated: the nucleus anterior and the nucleus superficialis, or superior (previous nucleus lateralis dorsalis). The nucleus anterior, divided into several entities in non-human species, is undivided in man. The two, anterior and superficial, nuclei are separated from the lateral and medial regions by the lamella superior and are everywhere surrounded by a capsule of white matter, including the lamina terminalis. The second nucleus (superficialis or superior) is posterior and in succession to the first. The two are constituted in the same manner. The main difference is their mode of afference. Both receives information from the subiculum of the hippocampus but in one case indirectly and in the other directly. The efferent axons of the subiculum follow the fornix. At the anterior part of the fornix, part of them go down to the mammillary body. The neurons of the mammillary bodies give axons forming the thick and dense mamillo-thalamic tract (of Vicq d'Azyr), which ends in the nucleus anterior. Another part of the subicular axons does not end in the mamillary body as, at the level of the foramen of Monro, they turn posteriorly. Some of them end into the anterior nucleus but a great quantity end in the nucleus superficialis. The selective target of the efferent axons from the anterior nucleus is the anterior cingulate cortex, that of the superfial nucleus is the posterior cingulate, with some overlap. The axons of these parts of the cingulate cortex, linked through the large cingulum (longitudinal bundle located at the base of the cingulate cortex), return to the parahippocampal gyrus. This circuit referred to as the Papez circuit (1937)Papez, J.W. (1937) A proposed mechanism of emotion. Arch. Neurol. Psychiat.38:725-743. was said by its author to be the substrate for emotion. There have been many further other elaborations (including the "limbic system"). Papez' circuit was in fact not close (at hippocampal level). In addition, the second nucleus, the superficial nucleus, not taken into consideration, has similar connections and participates in other close or linked circuits. The better known effect of the lesion of mamillary bodies, of the mamillothamic bundle and the fornix, if bilateral, is a particular (anterograde) amnesia (Korsakoff syndrome).
Medial region: Medial nucleus The medial dorsal nucleus corresponds to the part which is located medial to the lamina medialis. In the anterior part of the lamina, the oral intralaminar cellular part makes a clear border. This is no more true posteriorly with the pulvinar. Due to their constitution and connection, the two constitute a common set corresponding to the largest mass of the human thalamus. In non human primates, the medial nucleus (often named dorsomedian) is subdivided into several subnuclei. It is admitted that this is no longer the case in humans, which makes comparison even with old world monkeys difficult. Some subcortical afferences are documented in macaques (amygdalar, tectal). There are no arguments in favour of their existence in humans. The majority of the afferences comes from the cortex, reciprocated by corticothalamic efferences. In macaques, the spatial distribution of the connection was said to be "circunferential", and medial cortical areas being linked to medial parts of the nucleus and lateral dorsal to lateral dorsal. This is also true in humans. The strong interrelation between the medial nucleus and the frontal cortex is known for long. Lobotomies were intended to cut this connection. There are however other mediocortical connections; with the cingulate cortex, the insular cortex and also with the premotor cortex.
Posterior region. Pulvinar. Pu (with "LP" as a part) Pulvinar means pillow in Latin. It constitutes the posterior pole of the thalamus and its posterior border is indeed smooth. Anteriorly there is only an incomplete boundary with the medial nucleus. The two have in fact common connections both thalamocortical and corticothalamic. This is the case for instance for the frontal cortex. The usual subdivisions do not fit with the distribution of cortical afferent. It is common to find the description of a "nucleus lateralis posterior ( LP). This is simply a part of the pulvinar passing over the lateral region and giving in transverse sections the image of a ventral and a dorsal (or lateral) subdivision. Sagittal sections show that the pulvinar(LP) ensemble is a single curved entity. The whole receives in the same almost identical afferences. A main medial part receives flat islands of axonal terminations from the frontal, parietal, temporal and preoccipital cortex. Only one part of the pulvinar is particular, the intergeniculate or inferior pulvinar, which receives tectal afferents and which has a visuotopic map.
Basal region B