Organization of the thalamic projections of the striopallidum of the dog brain.

Experiments based on double luminescent labeling were performed to study the distribution of labeled neurons in the thalamic nuclei depending on the injection sites of luminescent markers into functionally similar or functionally different areas of the striopallidum of 16 dogs. The organizational characteristics of the thalamo-striopallidal projection system in dogs provide evidence for its high level of specificity, as not only the motor and limbic areas of the striopallidum, but also its functionally related areas, receive separate inputs mainly from diverse cellular groups. The centromedian nucleus contained groups of diffusely mixed cells, labeled with different markers and innervating functionally diverse segments of the caudate nucleus. In the centromedian, parafascicular, central medial, and medial dorsal nuclei of the thalamus, projection neurons form analogous cell populations innervating different segments of the striopallidal structures belonging to the same functional system. These striopallidal areas receive projections from small numbers of neurons via axon collaterals.