TOPOGRAPHY OF FIBER ORGANIZATION IN THE CORTICOFUGAL PATHWAYS OF RATS

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.