Thalamocortical radiations

Thalamocortical radiations
Deep dissection of brain-stem. Lateral view. (Thalamocortical fibers labeled at center top.)
Dissection of brain-stem. Dorsal view. (Thalamocortical fibers labeled at upper left.)
Details
Identifiers
Latin	Radiatio thalamocorticalis, tractus thalamocorticalis
NeuroNames	ancil-399
Dorlands /Elsevier	f_05/12361630
Anatomical terms of neuroanatomy []

Thalamocortical radiations are the fibers between the thalamus and the cerebral cortex.

Thalamocortical (TC) fibers have been referred to as one of the two constituents of the isothalamus, the other being micro neurons. Thalamocortical fibers have a bush or tree-like appearance as they extend into the internal capsule and project to the layers of the cortex. The main thalamocortical fibers extend from different nuclei of the thalamus and project to the visual cortex, somatosensory (and associated sensori-motor) cortex, and the auditory cortex in the brain. Thalamocortical radiations also innervate the gustatory pathways, as well as pre-frontal motor areas. Visual input from the optic tract is processed by the lateral geniculate nucleus of the thalamus, auditory input in the medial geniculate nucleus, and somatosensory input in the ventral posterior nucleus of the thalamus. Thalamic nuclei project to cortical areas of distinct architectural organization and relay the processed information back to the area of original activity in the thalamus via corticothalamic (CT) fibers . The thalamic reticular nucleus (TRN) receives incoming signals via corticothalamic pathways and regulates activity within the thalamus accordingly. Cortico-thalamic feedback neurons are mostly found in layer VI of the cortex . Reciprocal CT projections to the thalamus are of a higher order than, and synapse with, the TRN in much greater number than do thalamocortical projections to cortex. This suggests that the cortex has a much bigger role in top down processing and regulation of thalamic activity than do the processes originating in thalamic interneurons. Large-scale frequency oscillations and electrical rhythms have also been shown to regulate TC activity for long periods of time, as is evident during the sleep cycle. Other evidence suggests CT modulation of TC rhythms can occur over different time scales, adding even more complexity to their function.

Thalamic interneurons process sensory information and signal different regions of the thalamic nuclei. These nuclei extend to relay cells, which in turn innervate distinct areas of the cortex via thalamocortical fibers. Either specifically or nonspecifically, TC relay cells project specifically to organized areas of the cortex directly and nonspecifically project to large areas of cortex through the innervation of many interconnected collateral axons. According to Jones (2001) there are two primary types of relay neurons in the thalamus of primates–core cells and matrix cells–each creating distinct pathways to various parts and layers throughout the cerebral cortex. Matrix cells of the thalamus, or calbindin-immuno-reactive neurons (CIR neurons), are widely distributed and diffusely dispersed in each of the nuclei of the dorsal thalamus. In comparison, parvalbumin immuno-reactive neurons (PIR neurons) can be found only in principal sensory and motor relay nuclei, and in the pulvinar as well the asintralaminar nuclei. The PIR neurons cluster together creating "densely terminating afferent fibers…forming a core imposed on a diffuse background matrix of PIR cells" (Jones 2001). PIR cells tend to project upon the cerebral cortex and terminate in an organized topographic manner in specifically localized zones (in deep layer III and in the middle layer IV). In contrast, CIR cells have dispersed projections wherein various adjacent cells connect to non-specific different cortical areas. CIR axons seem to terminate primarily in the superficial layers of the cortex: layers I, II, and upper III.