THE COURSE AND TERMINATION OF CORTICOTHALAMIC FIBERS ARISING IN THE VISUAL-CORTEX OF THE RAT

Corticothalamic axons have been studied in adult Lister hooded rats wi th single or dual injections of tracers into the visual cortex. Labell ed axons leave medial and lateral injection sites in separate or parti ally overlapping bundles along parallel trajectories in the subcortica l white matter, In the internal capsule they converge and both bundles enter roughly the same sector of the thalamic reticular nucleus (TRN) . Their reticular terminal fields, however, differ. Axons from a media l injection site innervate more lateral parts of the TRN than do the a xons from lateral injection sites. The most medial third of the TRN is not innervated from area 17 but receives a topographically arranged i nput from peristriate cortex (Crabtree and Killackey, 1989, Eur: J. Ne urosci., 1, 94-109; Coleman and Mitrofanis, 1996, Eur. J. Neurosci., 8 , 388-404). The two groups of axons then separate in the dorsal thalam us, axons from medial parts of visual cortex turning caudally into lat eral regions of the lateral geniculate nucleus, whereas fibres from mo re lateral cortex continue into medial parts of the nucleus, Connolly and van Essen (1984, J. Comp. Neurol., 226, 544-564) and Nelson and Le Vay (1985, J. Comp. Neurol., 240, 322-330) have shown that in the geni culocortical pathway the two groups of fibres cross over in the subcor tical white matter, probably in the region of the subplate. We show th at the corticothalamic pathway also has a crossing, but it occurs in, or close to, the diencephalon itself, in the region of the perireticul ar nucleus. This result suggests that each of these pathways, the geni culocortical and the corticogeniculate, may undergo reorganization wit hin distinct cerebral zones, one diencephalic for the corticothalamic axons and the other telencephalic for the thalamocortical axons.