Anatomie fonctionnelle des voies corticothalamiques du chat

Introduction. Cortico-cortical information can travel via both a direct route or via a high order thalamic relay (HO) to make a cortico-thalamo-cortical pathway. HO convey information that already been processed by the cortex by opposition to first order thalamic relay (FO) which convey information for a first time to the cortex. In the visual system, the FO is the lateral geniculate nuclei and the HO is the LP-pulvinar. Functionally, corticothalamic pathways is known to play two major roles (1) modify the gating functions of relay cells (modulatory role), or (2) transfer the information (driving role) to thalamic relay cells. These two roles rely on two distinct morphologies (Rockland 1996), type I axon terminal permit a modulatory influence on post-synaptic neurons and type II permit the transfer of information. Type I is observed in the feedback way has the area 17 to lateral geniculate nuclei and type II is observed in the feedforward way has the area 17 to the LP-pulvinar. Type I and type II originate from cortical cells in layer 6 and 5 respectively (Deschenes et al., 1994; Bourassa et Deschenes, 1995). All extrastriates visual areas send projection from layer 5 and 6 to the LP-pulvinar (Abramson et Chalupa, 1985). Together, these two studies allow the hypothesis those two modes of communication (modulation and drive) demonstrated by Sherman and Guillery (2001) can be applied to the entire cortical visual areas of the cat brains.; Methods. In order to test this hypothesis three different levels of the hierarchical organisation of visual areas (Scannell et al. 1999, Cereb Cortex 9(3):277) were injected with tracer (BDA 3000MW by pressure or iontophoretic injection). These areas were (1) the lowest visual area of the hierarchy, the primary visual cortex (area 17), (2) the midpoint of the visual hierarchical organisation, the posteromedian part of the lateral suprasylvian sulcus (PMLS), and (3) the highest cortical visual area, that is the ectosylvian sulcus (AES). Seven to ten days were allowed for the transport of the dye, animals were then perfused and the brains were cryoprotected and sliced on cryostat.; Results. Histological analysis allowed the quantification of corticothalamic (type I or II) terminals from the different cortical visual areas. Our results indicate that the thalamus receives projections from these areas: (1) Area VI (n = 2) sends a small number (9%) of type I axon terminals in the LPl region. (2) Area PMLS (n = 7) projects a majority of type I (57%) and a large proportion of singletons (30%) in LPl (LPl1 and 2) and LPm. (3) Area AES (n = 6) sends almost exclusively type I (84%) axon terminals to LPm, MG and LM-Sg. Retrograde labelled cells were also observed following every area injected. These cells were located in the same location as axon terminals.; Discussion. The presence of these retrograde labelled cells in the terminal fields suggests cortico-thalamo-cortical loops from a single area. The present results confirm that two modes of communication state by Sherman and Guillery's theory may be applied to the entire cortical visual system of the cats. This organisation of the corticothalamic pathways suggests a general scheme of organisation in which the higher hierarchical levels of the visual cortical area tend to modulate the activity of the thalamus. This may represent an example of top-down regulation.; Keywords. Cat, Corticothalamic, Terminal, Thalamus, Cortex, Photonic microscopy, Electronic microscopy, Single axon reconstruction.