The Somatotopic Organization Within the Cat's Thalamic Reticular Nucleus

The organization of the somatosensory representation within the cat's thalamic reticular nucleus (TRN) was studied. Focal injections of horseradish peroxidase (HRP), wheatgerm agglutinin conjugated to HRP, and/or 3H]proline were made into somatosensory cortical areas 1 (S1) and 2 (S2). The resultant labelling in the thalamus was analysed. Single injections into S1 result in single zones of terminal labelling in TRN that are restricted to the centroventral part of the sheet-like nucleus. In reconstructions from horizontal sections these zones of labelling resemble thin 'slabs', which lie in the plane of the nucleus parallel to its borders, occupy only a fraction of the thickness of the reticular sheet, and are broadly elongated in the dorsoventral and oblique rostrocaudal dimensions. Thus, the slabs of S1 terminals, which represent large loci of the body surface, and the main distribution of the reticular dendrites have a similar orientation. In comparisons of the zones of labelling following single or double injections at different cortical sites in S1, an inner (medial) to outer (lateral) shift in labelling in the ventrobasal complex (VB) is accompanied by an inner (medial) to outer (lateral) shift in labelling along the thickness of the reticular sheet. Thus, like VB the reticular nucleus receives a topographically accurate projection from S1. Further, the somatotopic map conveyed from S1 to TRN is orientated perpendicular to the plane of the nucleus and repeats the spatial organization of the map in VB. S2 injections result in zones of terminal labelling in that part of TRN that receives S1 inputs. On the basis of these findings, together with those in other mammalian species, two conclusions can be reached about corticoreticular relations. First, although there can be continuity in individual maps of cortical inputs to TRN, there are discontinuities in cortical representations at the inner and outer borders of the reticular sheet. Second, TRN can receive a significant convergence of inputs from different cortical areas.