Thalamic afferents of area 4 and 6 in the dog: a multiple retrograde fluorescent dye study

In the present study, we compared the distribution of thalamocortical afferents of cortical area 4 to that of cortical area 6 in the dog, using fluorescent tracers. Multiple injections of combinations of two dyes (diamidino yellow dihydrochloride, Evans blue, fast blue, granular blue) were made into either the anterior and posterior sigmoid gyri or into the medial and lateral regions of the anterior sigmoid gyrus in the anesthetized dog. We found that the thalamic afferents of areas 4 and 6 arise from topographically organized bands of cells that traverse several thalamic nuclei and extend throughout the rostrocaudal extent of the thalamus. The most medial band included area 6-projecting neurons in the anterior nuclei, the rhomboid nucleus, the ventral anterior nucleus (VA), ventromedial nucleus (VM) and mediodorsal nucleus (MD). Within this band, cells projecting to medial area 6a tended to be more numerous in the anterior nuclei, anterior parts of VA and VM and anterior and caudal parts of MD. Fewer cells in MD but more cells in caudal parts of VA and VM projected to lateral area 6 a. Lateral bands of cells in central through lateral parts of VA and VL projected topographically to lateral area 4 on the anterior sigmoid gyrus and lateral through medial parts of postcruciate area 4. The most lateral band of cells in VL continued ventrally into the zona incerta. Area 4 also received input from VM and the central lateral (CL) and centrum medianum (CM) nuclei. Within regions of VA, VL and VM, cells from one band interspersed with cells from another, but there were very few double-labeled cells projecting to two cortical sites. When the present results are compared with our previous findings on the distribution of subcortical afferents to the motor thalamus, it appears that separate motor cortical areas may receive predominantly separate but also partially over-lapping pathways in the dog.Abbreviations AV Anterior ventral nucleus - AM anterior medial nucleus - Cb cerebellar nuclei - CeM central medial nucleus - CL central lateral nucleus - CM centrum medianum nucleus - EN entopeduncular nucleus - Hb habenula - LD lateral dorsal nucleus - MD mediodorsal nucleus - mt mammillothalamic tract - MV medioventral nucleus - Pf parafascicular nucleus - R reticular thalamic nucleus - rf retroflex fasciculus - Rh rhomboid nucleus - SN substantia nigra - VA ventral anterior nucleus - VL ventral lateral nucleus, principal division - VLd ventral lateral nucleus, dorsal division - VM ventral medial nucleus - VPL ventral posterior lateral nucleus - ZI zona incerta