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Corticobulbar projections from distinct motor cortical areas to the reticular formation in macaque monkeys

Fregosi, M;Contestabile, A;Hamadjida, A;Rouiller, EM;

Corticospinal and corticobulbar descending pathways act in parallel with brainstem systems, such as the reticulospinal tact, to ensure the control of voluntary movements via direct or indirect influences onto spinal motoneurons. The aim of the present study was to investigate the corticobulbar projections from distinct motor cortical areas onto different nuclei of the reticular formation. Seven adult macaque monkeys were analyzed for the location of corticobulbar axonal boutons, and one monkey for reticulospinal neurons’ location. The anterograde tracer BDA was injected in the premotor cortex (PM), in the primary motor cortex (M1) or in the supplementary motor area (SMA), in 3, 3 and 1 monkeys, respectively. BDA anterograde labelling of corticobulbar axons were analyzed on brainstem histological sections and overlapped with adjacent Nissl-stained sections for cytoarchitecture. One adult monkey was analyzed for retrograde CB tracer injected in C5-C8 hemispinal cord to visualise reticulospinal neurons. The corticobulbar axons formed bilateral terminal fields with boutons terminaux and en passant, which were quantified in various nuclei belonging to the Ponto-Medullary Reticular Formation (PMRF). The corticobulbar projections from both PM and SMA tended to end mainly ipsilaterally in PMRF, but contralaterally when originating from M1. Furthermore the corticobulbar projection was less dense when originating from M1 than from non-primary motor areas (PM, SMA). The main nuclei of bouton terminals corresponded to the regions where reticulospinal neurons were located with CB retrograde tracing. In conclusion, the corticobulbar projection differs according to the motor cortical area of origin in density and laterality. This article is protected by copyright. All rights reserved.