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Thalamocortical radiations
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Thalamocortical radiations
In neuroanatomy, thalamocortical radiations, also known as thalamocortical fibers, are the efferent fibers that project from the thalamus to distinct areas of the cerebral cortex. They form fiber bundles that emerge from the lateral surface of the thalamus.
Thalamocortical fibers (TC fibers) 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 thalamus supplies all parts of the neocortex with afferents. 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 fibers).
The thalamic reticular nucleus (TRN) receives incoming signals via corticothalamic pathways and regulates activity within the thalamus accordingly. Corticothalamic 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 nuclei as well as the intralaminar 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.
Thalamocortical signaling is primarily excitatory, causing the activation of corresponding areas of the cortex, but is mainly regulated by inhibitory mechanisms. The specific excitatory signaling is based upon glutamatergic signaling, and is dependent on the nature of the sensory information being processed. Recurrent oscillations in thalamocortical circuits also provide large-scale regulatory feedback inputs to the thalamus via GABAergic neurons that synapse in the TRN. 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.
Computational neuroscientists are particularly interested in thalamocortical circuits because they represent a structure that is disproportionally larger and more complex in humans than other mammals (when body size is taken into account), which may contribute to humans' special cognitive abilities. 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. It is proposed that core relay cells and matrix cells projecting from the dorsal thalamus allow for synchronization of cortical and thalamic cells during "high-frequency oscillations that underlie discrete conscious events", though this is a heavily debated area of research.
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Thalamocortical radiations
In neuroanatomy, thalamocortical radiations, also known as thalamocortical fibers, are the efferent fibers that project from the thalamus to distinct areas of the cerebral cortex. They form fiber bundles that emerge from the lateral surface of the thalamus.
Thalamocortical fibers (TC fibers) 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 thalamus supplies all parts of the neocortex with afferents. 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 fibers).
The thalamic reticular nucleus (TRN) receives incoming signals via corticothalamic pathways and regulates activity within the thalamus accordingly. Corticothalamic 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 nuclei as well as the intralaminar 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.
Thalamocortical signaling is primarily excitatory, causing the activation of corresponding areas of the cortex, but is mainly regulated by inhibitory mechanisms. The specific excitatory signaling is based upon glutamatergic signaling, and is dependent on the nature of the sensory information being processed. Recurrent oscillations in thalamocortical circuits also provide large-scale regulatory feedback inputs to the thalamus via GABAergic neurons that synapse in the TRN. 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.
Computational neuroscientists are particularly interested in thalamocortical circuits because they represent a structure that is disproportionally larger and more complex in humans than other mammals (when body size is taken into account), which may contribute to humans' special cognitive abilities. 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. It is proposed that core relay cells and matrix cells projecting from the dorsal thalamus allow for synchronization of cortical and thalamic cells during "high-frequency oscillations that underlie discrete conscious events", though this is a heavily debated area of research.
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