Studies on the projections of the superior cerebellar peduncle and the rubrothalamic and nigrothalamic projections in the rat, and their relevance to the homologies of the anterior parts of the ventral nucleus of the thalamus in the mammalian brain

Abstract

Since there has been no detailed study of the projections of the superior cerebellar peduncle (S.C.P.) with the most recently developed silver impregnation technique - the Fink-Heimer method - such a study has been carried out in the albino rat following complete unilateral destruction of the S.C.P. In order to assist with the interpretation of the thalamic terminations of the S.C.P., studies have also been made of the thalamic projections of the red nucleus and substantia nigra. The results are based on experiments in 56 animals: 22 for the investigation of the Projections of the S.C.P., 12 for the investigation of the rubrothalamic projections, and 22 for the investigation of the nigrothalamic projections. Oblique electrode approaches were devised which enabled the placement of discrete peduncular, rubral or nigral lesions with only the minimum of incidental damage to the surrounding Structures. The rubrothalamic projections were studied by the method of successive degeneration. The animals were generally killed after a survival period of 5 to 7 days and the axon degeneration was demonstrated with the Fink-Heimer silver impregnation technique. To analyse and record the experimental results, the patterns of degeneration were identified with the light microscope and meticulously plotted on diagrams of the brain using a projection system. To gain the fullest possible picture of the fibre projections, experimental material was analysed in coronal, sagittal and horizontal planes. The principal results of this investigation in the rat are as follows: (i) The confirmation of Ramon y Cajal’s (1903) original description that the fibres in the S.C.P. project caudally via ipsilateral and contralateral descending pathways to the pons and medulla and rostrally via a contralateral ascending pathway to the midbrain and diencephalon. (ii) The demonstration: (a) that the ipsilateral descending pathway terminates in the parvocellular reticular formation of the pons and medulla; (b) that the contralateral descending pathway terminates topically in the nucleus reticularis tegmenti pontis, the pontine nuclei and the inferior olive, and also in the magnocellular nuclei of the reticular formation and the nucleus reticularis paramedianus;(c) that the contralateral ascending pathway terminates mainly in the red nucleus in the midbrain and throughout the Vm-V1 complex of the thalamus, and also in various other midbrain, subthalamic and thalamic nuclei. (iii) The demonstration: (a) that the cells of the red nucleus project to the Vm-Vl complex; (b) that the substantia nigra projects by a dorsal tegmental route to Vm and to adjacent parts of the V1 complex in the thalamus, and that the pars compacta of the substantia nigra projects to the striatum. (iv) The identification of the Vm-V1 complex as the homologue in the rat thalamus of the nuclei ventralis lateralis et anterior thalami of the higher primates on the basis of the distribution of cerebellar, rubral and nigral afferents to these nuclei.