Thalamocortical organization in the cerebello-thalamo-cortical system.

In the cat, the cerebellum projects via the ventroanterior-ventrolateral (VA-VL) complex of the thalamus to the motor and premotor cortices and also to the parietal association cortex. Cerebellar inputs to each of these regions have been characterized electrophysiologically by depth profiles of cortical potentials following stimulation of the brachium conjunctivum and of the VA-VL complex, and morphologically by the laminar distribution of thalamocortical (TC) terminations, in aggregate and at the single-axon level. One population of TC neurons terminated mainly in layer I and was associated with late surface negative potentials. A second population, with terminations in layers III and IV, was associated with early deep negative potentials. Terminations in layer III of the motor cortex formed multiple patches about 1-1.5 mm wide (mediolateral), which aligned to form 2-5 mm stripes extended rostrocaudally. This pattern correlates with the configuration of individual TC axons, which have two to six separate terminal patches distributed over 6 mm (rostrocaudal). The wide morphological divergence of single TC axons in the cortex may imply functionally multiple innervation of different efferent columns. Alternately, along with other inputs, it may permit a highly dynamic output selection from multiple representations, for example, of a variety of muscle groups in different combinations.