Anatomy of frontal and sensory cortico-thalamic pathyways
While, traditionally, the major role assigned to the thalamus is to relay subcortical sensory information to the cortex, large part of the thalamus receives driver inputs from the cortex and projects back there. In this study we compared the anatomical properties of cortico-thalamic driver inputs, originating from the 5th layer (L5) in the sensory and frontal cortices. The (L5) cortical inputs to the thalamus were studied in the RBP4-Cre mice with conditional viral tracing. The terminals of L5 pyramidal cells in the primer sensory (S1) cortex formed large terminals in the Posterior nucleus of the thalamus, as described before. In addition, however, we observed large number of small S1 terminals as well. In contrast, L5 pyramidal cells of the secondary motor (M2) cortex formed exclusively small boutons in the ventro-medial nucleus of the thalamus (VM). All L5 fibers passed through the reticular nucleus (TRN) to reach the thalamus, but only the M2 axons emitted collaterals there. The later pathway, newly described by our group, is restricted to the anterior sector of the TRN. Retrograde tracing from the VM in wild type animals, back-labeled TRN cells only within this TRN sector. Our data indicate that the logic of cortico-thalamic communication is fundamentally different in frontal and sensory cortico-thalamic circuits. Since the size of the terminals and the presence or absence of feed forward inhibition via TRN critically affect the impact of cortical influence on thalamus S1 and M2 cortical activity will recruit thalamic cell populations in a distinctively different manner.