The organization of neocortical projections from the ventroposterior thalamic complex in the marsupial brush-tailed possum, Trichosurus vulpecula: a horseradish peroxidase study.

Retrograde transport of horseradish peroxidase (HRP) was used to determine the extent and some of the organizational details of the cortical projection of the ventroposterior thalamic complex (VP) in the marsupial brush-tailed possum, Trichosurus vulpecula. The cortical projection field of VP is coincident with SI as determined by electrophysiological methods, and would appear not to overlap fully the primary motor cortex. Thus, in Trichosurus it appears that the motor and somatic sensory cortical regions are not fully congruent, unlike those of the American opossum, Didelphis, which are. Each division of VP projects discretely, in a non-overlapping manner, to various regions within SI. The ventrolateral subdivision or VPL projects medially and in a strict homotypic manner, though the proportion of VPL cells projecting to cortex is subject to a large amount of variation. The dorsomedial division of VP or VPM projects uniformly to cortex from all areas of that subnucleus, but the strict homotypy characteristic of VPL's projection was not as apparent. VPM also projects to two distinct regions within its cortical field. The posteromedial division of VP or VPP projects to an area of cortex that receives no other VP input but, on the basis of cortical mapping studies, appears to belong to SI. Projections from VPL (and presumably from VPM) to a small area of cortex in the extreme posterolateral part of the VP field correspond to the position expected for, and electrophysiologically confirmed to be, SII.