[1] The isothalamus constitutes 90% or more of the thalamus, and despite the variety of functions it serves, follows a simple organizational scheme.
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 number of perithalamic neurons strongly decreases in evolution in opposition to the large increase in microneurons (Arcelli et al.
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.
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).
Its synapses are located close to the soma of the thalamic neuron, often forming the center of glomerular complexes.
The lateral region and the geniculate bodies indeed receive strong lower "specific" afferences and can be seen as the "sensorimotor" part of the thalamus.
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.
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 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.
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).
Sagittal sections show that the pulvinar(LP) ensemble is a single curved entity.
A main medial part receives flat islands of axonal terminations from the frontal, parietal, temporal and preoccipital cortex.
They differentiate early in ontogenesis and totally, for the lateral or partially for the medial separate from the thalamic mass.
The thalamocortical axons from the medial geniculate nucleus end in the primary auditory cortex located in the center of the superior temporal plane.
The lateral geniculate nucleus is made up of different cellular strata separated by lamellae, parallel to the surface.
After the chiasma, axons form the visual tracts turn around the peduncles and arrive into the polar anterior part of the lateral geniculate nucleus.
The axons from the lateral geniculate nucleus, through the optic radiation, end in the primary visual cortex around the calcarine fissure.
This corresponds to the part of isothalamus located laterally to the medial lamina and in front of the pulvinar (the noyau externe of Dejerine after Burdach).
Its thalamocortical neurons send axons to the primary gustatory area located in the opercule of the insula.
Within the somesthetic nucleus, physiological maps, including in humans, have found a spatial separation between the representation of the tactile and the deep stimuli.
Friedman and Jones (1986)[10] designated the deep region the "shell" as opposed to the tactile "core".
In primates, the dentate nucleus is subdivided into two nuclei: one anterior and the other posteroventral, the first "motor" and the other not (Dum and Strick, 2002).
Starting from cercopithecidae, the two sources from the basal ganglia system medial pallidum and nigra have distinct, spatially separate, thalamic territories.
In the whole territory axons expand widely (François et al., 2002)[18] allowing no precise map, which is confirmed by physiology (Wichemann and Kliem, 2004).
It is important to stress the necessity from now to clearly distinguish the pallidal VO and the nigral VA territories.