Extensive Divergence and Convergence in the Thalamocortical Projection to Monkey Somatosensory Cortex

Abstract

This study examined the extent of thalamocortical divergence as a potential determinant of activity-dependent representational plasticity in area 3b of adult monkey somatosensory cortex. Single or paired injections of anterogradely transported tracers, of varying anteroposterior extent, were made horizontally from behind in defined parts of the body representation in the ventral posterior lateral (VPL) and/or ventral posterior medial (VPM) thalamic nuclei, and the distribution and density of labeled thalamocortical terminations were mapped in cortex. Injections of increasing size in any dimension of VPL or VPM resulted in increasing accumulation of labeled terminals within the region of projection, implying extensive convergence of individual axons. Anteroposteriorly elongated injections labeled mediolaterally extended but anteroposteriorly restricted zones in cortex. Dorsoventral placement of an injection in VPL or VPM determined anteroposterior location of labeling in cortex.Dual injections separated mediolaterally or dorsoventrally by ϳ1 mm, and in different parts of the thalamic body or head-face representation gave rise to labeled thalamocortical terminations that overlapped extensively. For injection sites at different anteroposterior levels in VPL or VPM, the area of cortical convergence was related to their extent of anteroposterior coincidence. Labeled terminations arising from injections in immediately adjacent parts of VPL and VPM did not overlap in cortex.The extent of thalamocortical divergence and convergence revealed by these experiments is greater than that predictable from labeling of single axons and is sufficiently great to account for representational plasticity that exceeds the 1.5 mm cortical "distance limit."

Thalamocortical and the dual pattern of corticothalamic projections of the posterior parietal cortex in macaque monkeys

“…The ventroposterior nucleus has been shown to have a somatotopic organization (Welker, 1974;Kaas et al, 1984;Kaas and Pons, 1988;Rausell et al, 1998). In the present study in particular, retrograde labeling was found in VPL after injections in areas PE and PEa dorsally.…”

Referred phantom sensations and cortical reorganization after spinal cord injury in humans

“…These findings suggest that adult sprouting of corticocortical projections could contribute to the segregated pattern of activation reported here, although the most dramatic example of intracortical growth recorded in these studies projected only Ϸ50% of the more extensive nonactivated cortical distance observed (2 cm). The divergence of thalamocortical projections from a given representation within the ventral posterior nucleus has recently been demonstrated to extend further than previously appreciated, and these terminals typically coalesce into periodic densities (44). Whether this divergence can account for the cortical reorganization observed in the present study is an open question.…”

Studies of μ-, κ-, and δ-Opioid Receptor Density and G Protein Activation in the Cortex and Thalamus of Monkeys

“…It is worth noting that in the spinal cord of monkeys, MOR, KOR, and DOR are approximately 68, 25, and 7% of the total binding sites, respectively, and that SNC80 is not able to stimulate [ 35 S]GTP␥S binding in this region (Lee et al, 2002). The thalamus is the major neural substrate modulating nociceptive signals from both spinothalamic and thalamocortical pathways (Bushnell et al, 1993;Craig et al, 1994;Shi and Apkarian, 1995;Rausell et al, 1998). Thus, it is highly possible that DOR agonists act as weak antinociceptive agents in monkeys due to the lower DOR density and consequent limited G protein activation in the spinothalamic pathway.…”

“…In particular, the thalamus is the main neural substrate in nociceptive pathways and plays an integral role for both spinothalamic and thalamocortical pathways (Bushnell et al, 1993;Craig et al, 1994;Shi and Apkarian, 1995;Rausell et al, 1998). The aim of this present study was to measure the density of MOR, KOR, and DOR in the monkey cortex and thalamus.…”