PROJECTIONS TO LAYER-VI OF THE POSTEROMEDIAL BARREL FIELD IN THE RAT - A REAPPRAISAL OF THE ROLE OF CORTICOTHALAMIC PATHWAYS

The present study hears on afferents that terminate in layer VI of the posteromedial barrel field in the rat. Their origin was determined by the retrograde transport of cholera toxin, and their axonal arborizat ions were revealed by targeting injections of biotinylated dextran ami ne in regions that contained retrogradely labeled neurons. Afferents t o lamina VI arise from the thalamus (the ventral posteromedial, the po sterior group and the intralaminar nuclei), the claustrum and the infr agranular layers of other somatomotor regions of the neocortex (the mo tor, second somatosensory and perirhinal cortices). Among these affere nt systems, corticocortical axons, particularly those issuing from the motor cortex, give rise to the most profuse projections in layer VI, whereas thalamic and claustral afferents form sparse terminal fields. Because corticothalamic cells represent similar to 50% of the neuronal population in lamina VI and 73% of their dendritic processes are depl oyed locally, it seems likely that afferents arising from the infragra nular layers of the motor cortex may directly influence the firing of these neurons. These anatomical data suggest that the role of corticot halamic pathways should he studied from the viewpoint that sensory per ception is an active process which operates under the guidance of moto r activities.