Da. Lozsadi et al., THE COURSE AND TERMINATION OF CORTICOTHALAMIC FIBERS ARISING IN THE VISUAL-CORTEX OF THE RAT, European journal of neuroscience, 8(11), 1996, pp. 2416-2427
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.