Neural architecture of the rat medial geniculate body

The rat medial geniculate body was subdivided using Nissl preparations to e stablish nuclear boundaries, with Golgi-Cox impregnations to identify proje ction and local circuit neurons, and in fiber stained material to delineate the fiber tracts and their distribution. Three divisions were recognized ( ventral, dorsal and medial); the first two had subdivisions. The ventral di vision bad lateral and medial parts. The main cell type had bushy tufted de ndrites which, with the afferent axons, formed fibrodendritic laminae orien ted from dorso-lateral to ventro-medial; such laminae were not as regular m edially, in the ovoid nucleus. The dorsal division contained several nuclei (dorsal superficial, dorsal, deep dorsal, suprageniculate, and ventrolater al) and neurons with radiating or bushy dendrites; the nuclear subdivisions differed in the concentration of one cell type or another, and in packing density. A laminar organization was present only in the dorsal superficial nucleus. Medial division neurons were heterogeneous in size and shape, rang ing from tiny cells to magnocellular neurons; the various cell types interm ingled, so that no further subdivision could be made. This parcellation sch eme was consistent with, and supported by, the findings from plastic embedd ed or fiber stained material. There were very few small neurons with locall y ramifying axons and which could perform an intrinsic role like that of Go lgi type II cells. Their rarity was consistent with the small number of suc h profiles in plastic embedded or Nissl material and the few GABAergic medi al geniculate body neurons seen in prior immunocytochemical work. While sim ilar neuronal types and nuclear subdivisions are recognized in the rat and cat, there may be major interspecific differences with regard to interneuro nal organization in the auditory thalamus whose functional correlates are u nknown. (C) 1999 Elsevier Science B.V. All rights reserved.