Reciprocal connections are formed between the dorsal lateral geniculate nucleus and the striate cortex in the mammalian visual system. The question of whether fibers of these corticopetal and corticofugal pathways are segregated or intermingled within the white matter is still open. In order to examine the organization of these fiber tracts within the white matter, we have used orthograde axonal transport of radiolabelled proteins and neuronal degeneration following kainic acid lesions in the geniculocortical and corticofugal pathways of the rat. Within the white matter the two pathways reside in different layers and are segregated from one another over a significant portion of their course, geniculocortical fibers lying in the external sagittal stratum and corticofugal fibers lying in the internal sagittal stratum of the white matter. In addition, the corticofugal pathways projecting to subcortical structures appear fasciculated in both the transport and the degeneration studies suggesting that axons of cortical output neurons are organized into fiber bundles. The separation of fibers within the white matter may be of potential use for selectively stimulating afferent and efferent pathways in electrophysiological studies in situ and in cortical slice preparations. In addition, the corticofugal fiber bundles may play an important role in guiding axons therein to appropriate targets during axonogeneis and may carry the output of columnar units within visual cortex.
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