Isothalamus

The isothalamus is a division used by some researchers in describing the thalamus.

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 of, 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 axon(s) 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 connection 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.

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.

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