Topographical organization of the afferent connections of the principal ventromedial thalamic nucleus in the cat.

The afferent connections to the principal division of the ventromedial thalamic nucleus (VMP) were studied in the cat by means of the HRP retrograde transport technique. The large (40 nl) and small (20 nl) injections of this enzyme were delivered into the VMP using different stereotaxic approaches. The main afferents to VMP emanated bilaterally from the prefrontal, premotor, and rostral agranular insular cortices. Another important group of afferents to the VMP were those originating in the rostral third of the reticular thalamic nucleus, the entopeduncular nucleus, the substantia nigra pars reticulata, and the deep cerebellar nuclei. From the cerebellar nuclei, the contralateral lateral nucleus and the caudal third of both (ipsi and contralateral) medial cerebellar nuclei were the origin of afferents to the VMP. Other cortical areas projecting (in a lower density) to the VMP were the motor cortex, the cortex along the anterior ectosylvian sulcus, the granular insular cortex, the posterior agranular insular area, the prelimbic area, and the cortex along the posterior rhinal sulcus (SRP). Among other subcortical prosencephalic structures projecting to the VMP are the dorsal claustrum, substantia innominata, hypothalamic formations, and the zona incerta. Projections originated from the brainstem in the lateral part of the intermediate and deep layers of the superior colliculus, the central gray matter, the locus coeruleus, and the reticular formation. The nucleus tegmenti pedunculopontinus pars compacta, parabrachial nuclei, the vestibular complex, and the spinal trigeminal nucleus were also origins of projections to the VMP. We conclude by emphasizing the important bilateral cortical modulation of the different functions attributed to the VMP: recruiting-response mediation, reticular-activating system participation, and extrapyramidal motor integration. In light of the connections just described, the VMP may be considered as a point for impulses coming from complex association cortical areas and limbic formations to converge with those emanating from cortical and subcortical motor structures.