Elsevier

Brain Research

Volume 20, Issue 1, 20 May 1970, Pages 19-32
Brain Research

The fasciculus cinguli in the rat

https://doi.org/10.1016/0006-8993(70)90150-2Get rights and content

Abstract

The white matter of the medial cortex in the rat was examined along its longitudinal extent from the genu to the splenium of the corpus callosum. The problem of distinguishing thalamic from cortical contributions to this fiber complex was approached experimentally by tracing axon degeneration elicited by lesions in the anterior thalamus and the cingulate cortex6, respectively. The results indicate that the fasciculus cinguli is primarily a thalamocortical fiber stratum, contiguous with that of the remaining cortex, and forming with the latter the external medullary stratum of the cortical white matter. In their course to the medial cortex, thalamocingulate fibers pass rostralward from the thalamus. At levels extending from the anterior commissure forward beyond the genu of the corpus callosum, the fibers first turn dorsalward, penetrating the callosal fiber sheet, and then medialward to become enclosed in the fasciculus cinguli. The appearance of these fibers is most striking at levels anterior to the genu where they take an almost vertical sweep dorsalward into the fasciculus cinguli. The thalamocingulate projection system remains enclosed in the fasciculus cinguli throughout the latter's rostrocaudal extent, continually issuing fibers to the medial cortex, including the presubicular cortex. In comparison to its thalamocortical components the fasciculus cinguli of the rat appears only sparsely populated by corticocortical fibers. Moreover, efferents from the cingulate cortex projecting to subcortical structures do not become enclosed within the fasciculus cinguli, but traverse the bundle and accumulate in a deeper fiber stratum (the internal sagittal stratum of the white matter) from which they penetrate the layer of callosal fibers to join the internal capsule. Of this category of cortical efferents, the corticothalamic fibers originating from the caudal half of the cingulate cortex turn medialward around the stria terminalis and enter the thalamus as components of the lateral thalamic peduncle. The present finding that the thalamocingulate fibers, including the projection to the caudal cingulate region, do not travel by way of the lateral thalamic peduncle reveals a marked contrast in the respective trajectories of the thalamocingulate and cingulothalamic pathways.

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*

Present address: Neurological Unit, Boston City Hospital, Boston, Mass. 02118, U.S.A.

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