Thalamocortical Radiations - Relay Cells

Relay Cells

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.