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

Abstract

This study describes the organization of cells in the thalamic reticular nucleus (TRN) that project to the auditory part of the cat's dorsal thalamus. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and fluorescent dyes were made into the medial geniculate complex (MG). The resultant retrograde labelling in the TRN was analyzed. Injections of WGA-HRP into the ventral (MGv), dorsal (MGd), or medial (MGm) nuclei of the MG label zones of cells that are restricted to a caudoventral sector of the TRN. In reconstructions, these zones resemble "slabs" that are elongated in the dorsoventral and oblique rostrocaudal dimensions of the nucleus. In comparisons of the zones of labelling in the TRN following tracer injections into different nuclei of the MG, inner and caudal cells project to the pars lateralis of the MGv (MGvl) or to the MGd, and outer and rostral cells project to the pars ovoidea of the MGv (MGvo) or to the MGm. Thus, cells projecting to the MGvl or MGd or to the MGvo or MGm occupy overlapping territories. Double injections of different fluorescent dyes into selected pairs of MG nuclei result in reticular cells that are labelled from either both nuclei or only one or the other nucleus in each pair. These results indicate that the projections of cells in the auditory sector of the TRN to the MGvl or MGvo or to the MGd or MGm are topographically organized. Furthermore, projections to more than one MG nucleus can arise from single reticular cells.

Auditory System

“…The auditory sector of Rt is located in the caudoventral region of the nucleus (Jones, 1983;Shosaku and Sumitomo, 1983). Cells in the auditory sector are topographically segregated according to the region/ nuclear subdivision of the MG they innervate (Crabtree, 1998). The auditory sector of the Rt projects to the MG and acts via GABA A and GABA B receptor mechanisms on MGV, MGD and MGM cells (Bartlett and Smith, 1999).…”

Intrathalamic sensory connections mediated by the thalamic reticular nucleus

“…Once again, the organizational components in this sector are 'slabs,' formed by reticular cells that project to the dorsal thalamus, and these slabs are also oriented parallel to the plane of TRN [151; see also 152]. As demonstrated in the cat's auditory sector [151], figure 9 shows horizontal sections through slabs of labelled TRN cells following an injection of WGA-HRP in the middle-to high-frequency representation in MGv (fig. 9A) and in the middle-to low-frequency representation in MGv (fig.…”

“…Thus, outputs to MGvm and MGm arise from overlapping zones, as do outputs to MGvl and MGd. In addition, many cells in the cat's auditory sector project either to both MGvm and MGm or to both MGvl and MGd [151]. MGm and MGd are presumably higher-order nuclei.…”

“…MGm and MGd are presumably higher-order nuclei. The auditory sector also receives afferents from first-order and higher-order nuclei of the dorsal thalamus [24,151]. There appears to be a well-organized map of ventral medial geniculate inputs in the auditory sector [151; see also 152], which is in register with the map formed by reticular cells that project to MGv.…”

“…However, in the cat's somatosensory [120] and auditory [151] sectors of TRN, cells projecting to different modality-related nuclei of the dorsal thalamus occupy overlapping territories. Furthermore, many of these cells project to at least two dorsal thalamic nuclei.…”