Thalamic connections with limbic cortex. II. Corticothalamic projections.

The corticothalamic projections from the cat limbic cortex have been investigated with anterograde and retrograde axonal transport techniques. Five limbic cortical areas-the anterior limbic area, the cingular area, the granular and dysgranular retrosplenial areas, and the presubiculum-were identified on the basis of their cytoarchitecture. Emphasis was placed on determining the laminar distribution of the cells of origin of the efferent projections, the projection pathways, and the sites of termination within the thalamus. Projections to the thalamus originate in layers V and VI of limbic cortex. In the cingular region the cells of origin are predominantly in layer V and to a lesser extent in layer VI, while the majority of cells projecting from the more caudal retrosplenial areas and presubiculum are in layer VI. There are two fiber pathways from each cortical area to the thalamus. One system of fibers passes through the internal capsule and lateral thalamic peduncle, and a second system travels in the cingulate fasciculus before piercing the corpus callosum to join the postcommissural fornix. The lateral dorsal nucleus and the anterior nuclear group, including the anterior dorsal, anterior ventral, and anterior medial nuclei, are the major thalamic recipients of projections from limbic cortex. Corticothalamic projections also terminate sparsely in the midline and intralaminar nuclear complex, including the central lateral, central dorsal, paracentral, central medial, rhomboid, and reuniens nuclei. Projections from the anterior limbic area project predominantly to the anterior medial, central lateral, and paracentral nuclei. the anterior ventral nucleus, anterior medial nucleus, and lateral dorsal nucleus are the major thalamic recipients of projections from the cingular area, the granular and dysgranular retrosplenial areas, and the presubiculum. It appears that the anterior dorsal nucleus receives afferents only from the dysgranular retrosplenial area. Bilateral cortico-thalamic projections were found in the anterior medial, dorsal medial, central lateral, central medial, paracentral, and reuniens nuclei.