Sources of descending afferents to the inferior olive from the upper brain stem in the cat as revealed by the retrograde transport of horseradish peroxidase.

The retrograde transport of horseradish peroxidase (HRP) was employed in 35 cats to trace the sources of descending afferents to the inferior olivary nucleus. The majority of HRP-labeled cells were located in the ipsilateral mesencephalon, from the rostral pole of the red nucleus to the caudomedial border of the thalamus. Among the heavily labeled nuclear groups were the the parvocellular red nucleus, the interstitial nucleus of Cajal, the nucleus of Darkschewitsch, and the caudomedial extremity of the subparafascicular nucleus. Occasional cells were also labeled in the reticular formation lateral to the interstitial nucleus of Cajal in the caudomedial parafascicular nucleus, in the nucleus of the fields of Forel, and in the central gray. It was not possible to recognize a topographic distribution of the labeled cells corresponding to the localization of the HRP deposits. Elsewhere in the midbrain, there were HRP-labeled cells in the deep layers of the superior colliculus (IV and VI), predominantly on the side contralateral to the injection and in the nucleus of the optic tract, and in the anterior and posterior pretectal nuclei, ipsilaterally. These groups were best labeled after caudally located injections in the olive. Certain areas previously reported to project to the inferior olive were devoid of labeled cells; these included the basal ganglia, the rostral raphe nuclei, and the nucleus of Edinger-Westphal. It appears that the mesencephalic nuclei bordering the central gray represent the major source of descending afferents to the olive.