The projection to the mesencephalon from the dorsal column nuclei. An anatomical study in the cat.

The terminal areas and cells of origin of the projection from the dorsal column nuclei to the mesencephalon were investigated by the intra-axonal transport method. Following injection of wheat germ agglutinin-horseradish peroxidase conjugate into the dorsal column nuclei, anterograde labeling was observed in several regions of the midbrain. The main terminal area was situated at the level of transition between the superior and inferior colliculus on the side contralateral to the injection site and comprised the intercollicular nucleus and part of the external and pericentral nuclei of the inferior colliculus and of the nucleus of the brachium of the inferior colliculus, but there were also projections to the caudal half of the deep and intermediate gray layers of the superior colliculus, the anterior and posterior pretectal nuclei, the nucleus of Darkschewitsch and nucleus ruber. Injections restricted to either the gracile nucleus or the cuneate nucleus revealed a somatotopic termination pattern in the intercollicular nucleus, superior colliculus and pretectal nuclei. The retrograde labeling seen after injection of tracer into the midbrain terminal areas showed that the cells of origin were located mainly in the rostral and caudal parts of the dorsal column nuclei, whereas the middle cell nest neurons were unlabeled, thus supporting previous observations that the neurons projecting to the midbrain constitute a population separate from that projecting to the thalamus. Cell counts revealed that the midbrain projection is of a considerable magnitude, involving between 10,000 and 15,000 neurons; its functional significance is, however, largely unknown.