Thalamic afferents from the brain stem. An experimental study using retrograde single and double labelling with HRP and iron-dextran in the rat. I. Medial and lateral reticular formation.

The reticulo-thalamic projection of the rat was studied in single and double experiments using 51 injections of HRP and/or iron-dextran. The medial reticular formation (excluding the ncl. pedunculo-pontinus, subncl. compactus, see Part II) sends out projections to the ncll. intralaminares (IL), ncl. mediodorsalis (MD), ncl. ventralis medialis (VM), ncl. ventralis lateralis (VL), ncl. ventralis basalis (VB) and ncl. posterior (PO) of the thalamus. The mesencephalic RF (ncl. cuneiformis and ncl. subcuneiformis) projects to all the above mentioned nuclei. The rhombencephalic RF (ncl. pontis oralis, ncl. pontis caudalis, ncl. gigantocellularis et paragigantocellularis dorsalis et lateralis) projects to the posterior IL (ncl. parafascicularis and ncl. centromedianus) and to the MD, pars paralaminaris. The pontine RF (ncl. pontis oralis et caudalis) projects to the posterior IL and to the medial part of the ncl. parafascicularis. The RF of the pons and the upper oblongata projects to the VL, VB and PO. The lateral RF (ncl. parvocellularis) projects solely from the tiers around the ncl. n. VII. to the posterior IL in what is a scanty projection. We found no connections projecting from the medial or lateral RF to the midline nuclei, to the lateral nuclei or to the ncl. commissurae posterioris; the projection to the ncl. reticularis thalami was left unstudied. The reticulo-thalamic pathways exhibit marked laterality. The mesencephalic RF projects to the thalamus in what is pronounced ipsilateral predominance, the pontine RF in ipsilateral predominance, the upper oblongata RF equally ipsi- and contralaterally, and the lower oblongata RF with a predominance of the contralateral component. Collateralization (double labelled cells) was found projecting solely to the IL and MD regions. It was both ipsilateral and contralateral. The discoveries included the topographical organization of the reticulo-thalamic projection: a major antero-posterior in the rhombencephalon and a mediolateral in the projections from the mesencephalon and from the pons. The most substantial projection was found aiming at the posterior IL nuclei (ncl. parafascicularis-centrum medianum), a less substantial one to the ventral and posterior nuclei, and the scantiest to the anterior IL nuclei (ncl. paracentralis, ncl. centralis lateralis).