Projections to layer VI of the posteromedial barrel field in the rat: a reappraisal of the role of corticothalamic pathways.

The present study bears 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 arborizations were revealed by targeting injections of biotinylated dextran amine in regions that contained retrogradely labeled neurons. Afferents to lamina VI arise from the thalamus (the ventral posteromedial, the posterior group and the intralaminar nuclei), the claustrum and the infragranular layers of other somatomotor regions of the neocortex (the motor, second somatosensory and perirhinal cortices). Among these afferent 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 approximately 50% of the neuronal population in lamina VI and 73% of their dendritic processes are deployed locally, it seems likely that afferents arising from the infragranular layers of the motor cortex may directly influence the firing of these neurons. These anatomical data suggest that the role of corticothalamic pathways should be studied from the viewpoint that sensory perception is an active process which operates under the guidance of motor activities.