Diencephalic projections of the subnucleus interpolaris of the brainstem trigeminal complex in the rat

In this study the projection fields of the subnucleus interpolaris were examined using anterograde degeneration techniques. The projection areas of the subnucleus interpolaris in the diencephalon include the arcuate nucleus of the ventrobasal complex, the zona incerta, parts of the posterior nuclear group and the medial portions of the medial geniculate nucleus. Within the arcuate nucleus of the ventrobasal complex, the subnucleus interpolaris afferents are not uniform but, rather, clumped into islets. In addition, these projections exhibit a distinct topography with respect to the face representation in both the subnucleus interpolaris and the arcuate nucleus of the ventrobasal complex. The subnucleus interpolaris afferents convey approximately a 225° rotated map of the subnucleus interpolaris to the arcuate nucleus.The projection of the subnucleus interpolaris to the diencephalon is very similar to that of the principal sensory nucleus of the brainstem trigeminal complex (Smith, 1973). Implications of two separate systems relaying information from the face to the same areas in the diencephalon are discussed.