Thalamocortical radiations

Neural pathways between the thalamus and cerebral cortex

In neuroanatomy, thalamocortical radiations, also known as thalamocortical fibres, are the efferent fibres that project from the thalamus to distinct areas of the cerebral cortex. They form fibre bundles that emerge from the lateral surface of the thalamus.

Thalamocortical fibers (TC fibres) have been referred to as one of the two constituents of the isothalamus, the other being microneurons. 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 gustatory and olfactory 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 fibers (CT fibres). 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. 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. In a study done by Gibbs, Zhang, Shumate, and Coulter (1998) it was found that endogenously released zinc blocked GABA responses within the TC system specifically by interrupting communication between the thalamus and the connected TRN. Evidence from one study (Arcelli et al. 1996) offers partial support to this claim by suggesting that thalamic GABAergic local circuit neurons in mammalian brains relate more to processing ability compared to sensorimotor ability, as they reflect an increasing complexity of local information processing in the thalamus. Activation of the thalamocortical neurons relies heavily on the direct and indirect effects of glutamate, which causes excitatory postsynaptic potentials (EPSPs) at terminal branches in the primary sensory cortices.

Primarily, thalamocortical somatosensory radiation from the VPL, VPM and LP nuclei extends to the primary and secondary somatosensory areas, terminating in cortical layers of the lateral postcentral gyrus. S1 receives parallel thalamocortical radiations from the posterior medial nucleus and the VPN. Projections from the VPN to the postcentral gyrus account for the transfer of sensory information concerning touch and pain. Several studies indicate that parallel innervations to S1 and also S2 via thalamocortical pathways result in the processing of nociceptive and non nociceptive information. Non specific projections to sensori motor areas of the cortex may in part have to do with the relationship between non noci receptive processing and motor functions. Past research shows a link between S1 and M1, creating a thalamocortical sensori motor circuit. When this circuit becomes disrupted symptoms are produced similar to those that accompany Multiple sclerosis, suggesting thalamocortical rhythms are involved in regulating sensori motor pathways in a highly specialized manner. TC CT rhythms evident during sleep act to inhibit these thalamocortical fibers so as to maintain the tonic cycling of low frequency waves and the subsequent suppression of motor activity. Models of the pulvinar projections to the visual cortex have been proposed by several imaging studies, though their mapping has been difficult due to the fact that pulvinar subdivisions are not conventionally organized and have been difficult to visualize using structural MRI. MMGN radiations terminate in specific locations while thalamocortical fibers from the VMGN terminate in nonspecific clusters of cells and form collateral connections to neighboring cells. Research done by staining the brains of macaque monkeys reveals projections from the ventral nucleus mainly terminating in layers IV and IIIB, with some nonspecific clusters of PIR cells terminating in layers I, II, IIIA, and VI. Fibers from the dorsal nuclei were found to project more directly to the primary auditory area, with most axons terminating in layer IIIB. The magnocellular nucleus projected a small amount of PIR cells with axons mainly terminating in layer 1, though large regions of the middle cortical layers were innervated through collaterally connected CIR neurons. Past research suggests that the thalamocortical auditory pathway may be the only neural correlate that can explain a direct translation of frequency information to the cortex via specific pathways.

The primary motor cortex receives terminating thalamocortical fibers from the VL nucleus of the thalamus. This is the primary pathway involved in the transference of cerebellar input to the primary motor cortex. The VA projects widely across the inferior parietal and premotor cortex. Other Non specific thalamocortical projections, those that originate in the dorsal medial nuclei of the thalamus, terminate in the prefrontal cortex and have subsequent projections to associative premotor areas via collateral connections. The cortico basal ganglia thalamo cortical loop has been traditionally associated with reward learning and though has also been noted by some researchers to have a modulatory effect on thalamocortical network functioning–this is due to inherent activation of the premotor areas connecting the VA nucleus with the cortex. Another study done on rats suggests during spike and wave seizures, thalamic rhythms are mediated by local thalamic connections, while the cortex controls the synchronization of these rhythms over extended periods of time. Thalamocortical dysrhythmia is a term associated with spontaneously reoccurring low frequency spike and wave activity in the thalamus, which causes symptoms normally associated with impulse control disorders such as obsessive compulsive disorder, Parkinson's disease, attention deficit hyperactivity disorder, and other forms of chronic psychosis. Other evidence has shown how reductions in the distribution of connections of nonspecific thalamocortical systems is heavily associated with loss of consciousness, as can be seen with individuals in a vegetative state, or coma.

The bilateral interruption or severing of the connection between thalamocortical radiations the medial and anterior thalamic nuclei results in a prefrontal lobotomy, which causes a drastic personality change and a subdued behavioral disposition without cortical injury.