Thalamocortical Radiations - Overview

Overview

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.