The organisation of the long descending corticofugal pathways is poorl
y understood. We have examined these pathways to determine the fibre r
elationships along the extent of their course through the internal cap
sule, cerebral peduncle, longitudinal pontine fasciculus, pyramid, pyr
amidal decussation, and dorsal column of the spinal cord. Different cy
toarchitectonic regions (e.g., lateral agranular and granular) of the
rat's neocortex were injected with the axonal tracer biotinylated dext
ran. In other experiments, each animal had different coloured fluoresc
ent tracers (Fluoro Ruby and dextran-fluorescein) injected into separa
te cortical areas. Our results show that in the anterior and posterior
limbs of the internal capsule, axons arising from spatially separate
sites in rat neocortex occupy distinct regions of the cross-sectional
area of the pathway. More caudally, within the cerebral peduncle and t
he longitudinal pontine fasciculus, axons from more distant cortical a
reas remain largely separate, but those from adjacent cortical areas b
egin to overlap. By the medullary pyramid, the pyramidal decussation,
and the dorsal column of the spinal cord, the representations of all t
he cortical regions injected overlap completely; in these structures,
the axons arising from each cortical area are widely intermingled. Thu
s, along the rostral-to-caudal course of the corticofugal pathways, th
ere is a change in the organisation of axons. At rostral levels, the o
rder corresponds roughly to the spatial distribution of the cells of o
rigin, hut more caudally, this changes to an arrangement of axons that
has no readily apparent order. A similar change has been observed alo
ng the course of the retinofugal pathway, where a decrease of spatial
order in the fibre distribution has been associated with a reordering
of axons according to their temporal sequence of outgrowth. (C) 1997 W
iley-Liss, Inc.