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COMPARISON OF POSTERIOR CINGULATE CORTEX PROJECTIONS TO
THE PULVINAR AND LATERODORSAL NUCLEI. |
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The posterior cingulate cortex (pCG) is thought to be
involved in orientation within and interpretation of the environment. The pCG
projects to the pulvinar and laterodorsal (LD) nuclei, which play important
roles in visual attention and spatial memory. To better understand the
thalamic connections contributing to these related functions, we injected
anterograde tracers into the pCG of cats and monkeys (Macaca mulatta) and
examined the distribution, morphology and ultrastructure of corticothalamic
terminals. In the cat, where the pulvinar and LD nuclei are of similar size,
the pCG projections were dense in the LD but relatively sparse in the
pulvinar. In the monkey, the pCG contributed dense projections to dorsomedial
portions of the vastly enlarged pulvinar nucleus, and relatively sparse
projections to the LD nucleus. In both species, the majority of pCG terminals
were small terminals, but larger terminals were also observed. Electron
microscopic examination of pCG terminals revealed similar ultrastructural
features in both nuclei. As described for other thalamic nuclei, small pCG
terminals contained round vesicles (RS profiles) and primarily contacted
small caliber dendrites of projection cells. Larger terminals also contained
round vesicles (RL profiles) and innervated approximately equal numbers of
interneurons and projection neurons in more complex synaptic arrangements.
Given the proposed functions of RS and RL terminals as "modulatory"
and "driver" inputs respectively, these results suggest the pCG is
capable of modulating and driving pulvinar and LD activity. Furthermore,
these results suggest that in comparison to the cat, the pCG may play a
relatively more important role in the functions of the primate pulvinar
nucleus. |