Sizes, laminar and topographic origins of cortical projections to the major divisions of the red nucleus in the monkey

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Abstract
The retrograde transport of horseradish peroxidase was used to study the topographic and laminar origins of the cortical projections to the parvocellular and the magnocellular divisions of the red nucleus in Macaca mulatto and Macaca fascicularis. Approximately 90% of the corticorubral projection is directed to the parvocellular division of the nucleus. Corticoparvocellular division of the nucleus. Corticoparvocellular (CRp) neurons are pyramidally shaped, are smaller in size than corticospinal neurons, and are more numerous. They are found principally in sublamina Va of cytoarchitectonic areas 4 and 6, and in moderate quantities in sublamina Vb of posterior area 8 and area 5. In areas 4 and 6, the cells are grouped in clusters of three to 15 neurons each and are arranged in cellular bands of varying rostrocaudal thickness which course mediolaterally. With respect functionally define zones, CRp neurons are found through the supplementary motor area and the precentral motor cortex. In addition, they are found in parts of area 5, 6, and 24 that project to these cortical motor areas, and that are thought to have “premotor” or movement programmming functions.The corticomagnocellular (CRm) projection arises principally from cells in sublamina Vb of the precentral arm and leg areas (area 4), and from adjacent parts of posterior area 6. CRm cells are pyramidally shaped and their size distribution is bimodal, with peaks that correspond, respectively, to the modal diameters of CRp and of corticospinal neurons.These result and those of previous studies that CRm neurons are involved principally in the control of hand and foot movements, with little effect on proximal musculature. The massive CRp projection, however, is clearly part of a large cerebrocerebellar communications system, with motor and/or movement programming functions that have yet to be clearly defined.