Topographical organization of the thalamic afferent connections to the motor cortex in the cat.

The topographical distribution of the cortical afferent connections to the different subdivisions of the motor cortex (MC) was studied in adult cats. The retrograde axonal transport of horseradish peroxidase technique was used. Small single injections of the enzyme were made in the entire MC, including the hidden regions in the depth of the sulcus cruciatus. The areal location and density of the subsequent thalamic neuronal labeling were evaluated in each case. Comparison of the results obtained in the various cases shows that the following: (1) The ventral anterior-ventral lateral complex is the principal thalamic source of afferents to the MC. (2) The ventral medial, dorsal medial, the different components of the posterior thalamic group (lateral, medial, and ventral posteroinferior and suprageniculate nuclei), and the intralaminar, lateral anterior, lateral intermediate, lateral medial, and anteromedial thalamic nuclei are also thalamic sites in which neural projections to the MC arise. (3) The thalamocortical projections to the MC are sequentially organized. The connections arising from the lateral part of the thalamus end in the region of area 4 that is situated medially in the superior lip of the sulcus cruciatus and in the posterior sigmoid gyrus. The projections originating in the most medial thalamic regions terminate in that region of area 6a beta which is located in the medial part of the inferior lip of the cruciate sulcus, and in the anterior sigmoid gyrus. Moreover, the ventral thalamic areas send connections to the most anteriorly located zones of the MC, while the most dorsal thalamic ones project to the most posteriorly located parts of the MC. (4) This shift in the thalamocortical connections is not restrained by cytoarchitectonic boundaries, either in the thalamus or in the cortex. (5) The populations of thalamocortical cells which project to neighboring MC subdivisions exhibit consistent overlapping among themselves. (6) These findings suggest, moreover, that the basal ganglia and the cerebellar projections to the MC through the thalamus are arranged in a number of parallel pathways, which may occasionally overlap.