THALAMOCORTICAL PROJECTION FROM THE PARAFASCICULAR NUCLEUS TO LAYER-VPYRAMIDAL CELLS IN FRONTAL AND CINGULATE AREAS OF THE RAT

Thalamocortical projections originating from the parafascicular nucleu s were reinvestigated using biocytin or biotylinated dextran amine as anterograde tracers in the rat. After stereotaxic injection of the mar ker in the lateral part of the parafascicular nucleus, labelled ascend ing fibres were observed running ipsilaterally to the frontal motor an d anterior cingulate areas. Labelled fibres gave rise in layer VI to a plexus of thin ramifications ending in layer V, where sparse boutons en passant and terminaux were seen in close apposition to pyramidal ce lls. Few retrogradely labelled pyramidal somata, contacted by labelled varicosities, were also observed. Electron microscopy demonstrated th e synaptic nature of the labelled contacts, displaying asymmetrical ju nctions and a round vesicular content. The direct loop parafascicular- motor cortex-parafascicular may be of great functional significance in motor control.