Anterograde projections of the cortical tongue area of the tree shrew (Tupaia belangeri).

In altogether seven tree shrews, Tupaia belangeri, the anterograde projections of the motorcortical tongue area were investigated as part of a larger comparative study. Identification of the tongue area was carried out by electrical brain stimulation. Three different tracers were used: biotin-conjugated dextranamine (BDA), Phaseolus vulgaris leucoagglutinin (PHA-L) and 3H-leucine. Intracortical projections were found to the motor cortex around the injection site, the premotor cortex, supplementary motor area, the homologues of the primate frontoparietal operculum and insula, the anterior cingulate and agrannular retrosplenial cortex, the somatosensory and bordering inferior parietal cortex as well as to the perirhinal cortex. Except the very weak projections into the retrosplenial, posterior parietal and perirhinal region which were ipsilateral, all other projections were bilateral. Subcortically, there was a projection into the ventral putamen, rostrodorsal claustrum and, very sparsely, into the caudate nucleus. In the thalamus, terminal labeling was found in the nuclei reticularis, anteroventralis, anteromedialis, ventralis lateralis, ventralis posterior medialis, ventralis posterior inferior, medialis dorsalis, in the intralaminar nuclei paracentralis, centralis lateralis, centrum medianum and parafascicularis, in the midline thalamus and in the nuclei posterior and pulvinaris. Further diencephalic projections, however all of them wak, could be traced into the zona incerta, dorsolateral subthalamus, dorsomedial, lateral and supraoptic hypothalamus. In the midbrain, labeling was found in the deep layers of the lateral superior colliculus, in the bordering reticular formation and, very sparsely, in the periaqueductal grey. In the lower brain-stem, fibres ended in the griseum pontis, dorsolateral reticular formation, principal and spinal trigeminal nucleus and, sparsely, in the lateral parabrachial region, solitary tract nucleus, inferior olive and magnocellular reticular formation. No terminals were found in the hypoglossal nucleus. The projection system revealed with PHA-L was less extensive than that demonstrated with BDA and 3H-leucine, both of which were similar.