Behavioural Brain Research, 28 (1988) 33-57 Elsevier BBR 00730 33 Anatomy Projections from the red nucleus and surrounding areas to brainstem and spinal cord in the cat. An HRP and autoradiographical tracing study the Gert Holstege and Joep Tan Department of Anatomy, Medical Faculty, Erasmus University of Rotterdam, Rotterdam (The Netherlands) (Received 1 February 1987) (Accepted 7 March 1987) Key words: Red nucleus; Motoneuron; Motor control; Facial nucleus; Inferior olive; Rostral interstitial nucleus of the MLF; Interstitial nucleus of Cajal; Cat HRP injections at the C2, T1 and S z spinal levels and in the medullary lateral tegmental field revealed that the contralaterally projecting rubro-bulbospinal neurons are located not only in the caudal but also to a certain extent in the rostral red nucleus (RN). These RN projections are somatotopically organized. Neurons projecting to the sacral cord are located in the ventrolateral RN, those projecting to the upper part of the spinal cord lie in the dorsomedial RN and those projecting to the medullary lateral tegmentum were found in the dorsal portions of the RN. These last neurons are smaller than many of the other RN neurons. The HRP results also revealed that the RN does not project to the caudal raphe nuclei. The autoradiographical results confirmed the HRP findings. They further indicated that the contralateral RN projections to the caudal brainstem precerebellar nuclei (nucleus corporis pontobulbaris, lateral reticular nucleus, lateral cuneate nucleus) and the dorsal column nuclei are also somatotopically organized. This was also true for the RN projections to the dorsomedial and intermediate facial subnuclei and the caudal pontine and medullary lateral tegmental field. These areas receive afferents from mainly the dorsal portions of the RN. Regarding the RN projections to the spinal cord, the autoradiographical tracing results revealed somatotopically organized contralateral RN projections to laminae V, VI and VII. Moreover, a small but distinct RN projection to a dorsolaterally located group ofmotoneurons at the Cs-T1 level was demonstrated. Ipsilaterally a minor projection to the cervical and upper thoracic lateral intermediate zone was observed. Finally, strong ipsilateral projections from the rostral mesencephalon to the inferior olive were seen. These projections were derived from various rostral mesencephalic areas, including the nucleus of Darkschewitsch, the nucleus accessorius medialis of Bechterew, the interstitial nucleus of Cajal and the area of the rostral interstitial nucleus of the medial longitudinal fasciculus. In the cat it was difficult to define which of the mesencephalic areas projecting to the inferior olive represented the parvocellular RN. A new subdivision of the RN is proposed based on its projections and not on the size of its cells. In this concept the first group is formed by the RN neurons projecting contralaterally to the caudal brainstem and spinal cord. The second group consists of RN neurons projecting to the inferior olive. However, it is difficult, especially in the cat, to determine the exact location of this second group of RN neurons, since they take part in a much wider mesencephalo-olivary projection system. Reprint requests: Dept. of Anatomy HB 13, Medical Faculty, Erasmus University, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. Correspondence: G. Holstege. Present address: NASA/Ames Research, Center Mail Stop 239-7, Moffett Field, CA 94035, U.S.A. 0166-4328/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)