THE PROJECTIONS OF THE LATERAL RETICULAR NUCLEUS TO THE DEEP CEREBELLAR NUCLEI - AN EXPERIMENTAL-ANALYSIS IN THE RAT

The projections of the lateral reticular nucleus (LRN) to the cerebell ar nuclei were studied using the retrograde axonal transport of tetram ethyl rhodamine dextran amine (10% solution in 0.01 M neutral phosphat e buffer) in 19 adult Wistar strain rats. The cerebellar nuclei receiv e topographically organized projections from the LRN. The projections are bilateral with an ipsilateral predominance and they are symmetrica l. The contralateral component is progressively larger for projections to the nuclei interpositalis, to the nucleus lateralis and to the nuc leus medialis. The projections to the various cerebellar nuclei arise from rostrocaudally oriented columns of neurons located in different ( partly overlapping) areas of the magnocellular division of the LRN. Th e nucleus lateralis receives terminals from the dorsomedial area (main ly from the rostral level of the LRN), the nuclei interpositalis from the dorsolateral area (mainly from the central level) and the nucleus medialis from the intermedioventral area (mainly from the caudal level ). Afferent fibres from the small subtrigeminal division were traced t o the three cerebellar nuclei and from the parvocellular division to t he nuclei interpositalis and medialis. The density of the projections from the LRN to the nuclei interpositalis increases progressively with the shift of the terminal field from the rostrolateral to the caudome dial part of the nucleus. The projections to the nucleus lateralis rea ch principally the dorsolateral hump, whereas only a few neurons proje ct to the other divisions (parvo- and magnocellular). The projections to the various regions of the nucleus medialis show different densitie s. The highest density was found for projections to the caudal part, i n particular to the dorsolateral protuberance and to the ventrolateral area of the middle division. Conversely, a low density of projections was found for the other areas of the middle division. The regions of the magnocellular division of the LRN which project to the nuclei late ralis (and are thus related to the cerebral cortex), interpositalis (r elated to the red nucleus) and medialis (related to the spinal cord) a lso receive afferent terminals from the cerebral cortex, the red nucle us and the spinal cord respectively, in addition to various afferent i nputs. Thus, each of these areas is apparently concerned with integrat ing some spinal and supraspinal information in reverberating circuits.