The Somatotopic Organization Within the Cat's Thalamic Reticular Nucleus

Abstract

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.

Subcellular distribution of AMPA and NMDA receptor subunit immunoreactivity in ventral posterior and reticular nuclei of rat and cat thalamus

“…No immunoreactivity was detected in the postsynaptic densities of symmetrical synapses made by GABAergic F type terminals mainly from the RTN in both species (Ohara et al, 1980;Montero and Scott, 1981; see reviews by Jones, 1985;Liu et al, 1995b) or by GABAergic PSD on other neuronal profiles. The presence of the receptor proteins at postsynaptic densities of glutamate containing asymmetrical synapses on RTN cells is also indicative of the presence of collateral thalamocortical and corticothalamic excitatory synapses on these cells (Jones, 1975;Yen et al, 1985;Crabtree, 1992). Ionotropic effects have been demonstrated physiologically on these cells (de Curtis et al, 1989), but to date NMDA receptor effects have not been shown.…”

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

“…Thus, the visual (Montero et al, 1977;Crabtree and Killackey, 1989;Conley and Diamond, 1990;Lozsá di et al, 1996) and somatosensory (Hoogland et al, 1987;Crabtree, 1992a,b) sectors receive mapped inputs from modality-related primary cortical areas. Furthermore, inputs from two or more modality-related cortical areas are largely segregated in the visual sector (Crabtree and Killackey, 1989;Conley and Diamond, 1990;Coleman and Mitrofanis, 1996;Lozsádi et al, 1996), whereas such inputs overlap in the somatosensory sector (Crabtree, 1992b).…”

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

“…However, the projections of cells in this sector to the ventral subdivision of POm are not topographically organized even though most of these projections arise from the same cells that project to VB. A preliminary description of the data has been reported (Crabtree, 1994).…”