The course and termination of corticothalamic fibres arising in the visual cortex of the rat

Corticothalamic axons have been studied in adult Lister hooded rats with single or dual injections of tracers into the visual cortex. Labelled axons leave medial and lateral injection sites in separate or partially overlapping bundles along parallel trajectories in the subcortical 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 medial injection site innervate more lateral parts of the TRN than do the axons from lateral injection sites. The most medial third of the TRN is not innervated from area 17 but receives a topographically arranged input from peristriate cortex (Crabtree and Killackey, 1989, Eur. J. Neurosci., 1, 94-109; Coleman and Mitrofanis, 1996, Eur. J. Neurosci., 8, 388-404). The two groups of axons then separate in the dorsal thalamus, axons from medial parts of visual cortex turning caudally into lateral regions of the lateral geniculate nucleus, whereas fibres from more lateral cortex continue into medial parts of the nucleus. Connolly and van Essen (1984, J. Comp. Neurol., 226, 544-564) and Nelson and LeVay (1985, J. Comp. Neurol., 240, 322-330) have shown that in the geniculocortical pathway the two groups of fibres cross over in the subcortical white matter, probably in the region of the subplate. We show that the corticothalamic pathway also has a crossing, but it occurs in, or close to, the diencephalon itself, in the region of the perireticular nucleus. This result suggests that each of these pathways, the geniculocortical and the corticogeniculate, may undergo reorganization within distinct cerebral zones, one diencephalic for the corticothalamic axons and the other telencephalic for the thalamocortical axons.