Organization in the somatosensory sector of the cat's thalamic reticular nucleus

This study describes the organization of cells in the thalamic reticular nucleus (TRN) that project to the somatosensory part of the dorsal thalamus in the cat. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and fluorescent dyes were made into the ventrobasal complex (VB) and the medial division of the posterior complex (POm) of the thalamus. The resultant retrograde labelling in TRN was analyzed. Large injections of a tracer in VB label many reticular cells that are restricted to a centroventral, or somatosensory, sector of TRN. Small injections of a tracer in VB produce narrow zones of labelled cells in this sector. In reconstructions these zones resemble thin "slabs," which lie parallel to the plane of TRN along its oblique rostrocaudal dimension and occupy only a fraction of its thickness. In comparisons of the zones of labelled cells in TRN resulting from tracer injections in different nuclei of VB, inner cells, intermediate cells, and outer cells across the thickness of TRN project to the ventral posteromedial, the medial division of the ventral posterolateral, and the lateral division of the ventral posterolateral nuclei, respectively. Furthermore, shifts in injected areas along the dorsoventral dimension of VB produce similar shifts in zones of labelled cells in TRN. Thus, reticular cells form an accurate map on the basis of their connections with VB. Large injections of a tracer in the ventral subdivision of POm label many reticular cells that are also restricted to the centroventral sector of TRN. Small injections of a tracer in ventral POm produce broad zones of labelled cells in this sector. In comparisons of the zones of labelled cells in TRN resulting from tracer injections in different regions of ventral POm, cells that project to these regions are scattered across the thickness of TRN and occupy overlapping territories. Large injections of a tracer in either VB or ventral POm also label cells in a restricted centroventral region of the perireticular nucleus. Double injections of different tracers in VB and ventral POm produce many cells in TRN that are labelled from both of these dorsal thalamic structures and fewer cells that are labelled from only one or the other of these structures. These results indicate that there is a dual organization in the projections of cells in the somatosensory sector of TRN to dorsal thalamus: Projections to VB are topographically organized whereas those to ventral POm lack a topographical organization. Furthermore, both of these mapped and nonmapped projections can arise from single reticular cells in the somatosensory sector.