Distinguishing rat brainstem reticulospinal nuclei by their neuronal morphology. I. Medullary nuclei.

While cytoarchitectonic and hodological investigations suggest that the brainstem reticulospinal nuclei (BRN) are complexly organized, previous Golgi studies claimed that BRN comprise a homogeneous population with respect to neuronal morphology. To determine whether this is indeed the case, neurons of the various BRN of adult albino or hooded rats were either backfilled with horseradish peroxidase (HRP) from spinal injections, stained with a Nissl method or impregnated with a Golgi-Kopsch variant. The results suggest that at least thirteen BRN can be distinguished in the medulla. Some medullary BRN contain neurons whose dendritic arborizations (DA) are radially symmetric (e.g., nucleus reticularis (NR) ventralis pars beta (RVb), NR gigantocellularis (RGc) and nucleus raphe magnus (RaM]. Some BRN contain neurons whose DA exhibit a pronounced dorsomedial to ventrolateral slant (e.g., NR dorsalis (RD) and NR parvocellularis (RPc). The DA of NR paragigantocellularis dorsalis (RPgcd) neurons tend to course dorsally. The DA of nucleus raphe obscurus (RaO) neurons course vertically, while those of NR magnocellularis pars alpha (RMca) and NR magnocellularis pars beta (RMcb) course horizontally. The DA of NR ventralis pars alpha (RVa) may be oriented horizontally also, but sometimes slant from dorsolateral to ventromedial. The DA of NR paramedianus neurons (RPm) are cruciform. The neurons of NR paragigantocellularis lateralis (RPgcl) and of the nucleus raphe pallidus (RaP) exhibit a variety of DA patterns. The neurons of RD, RVa, RMcb and RMca project to the spinal cord with a strong ipsilateral predominance, while those of RVb, RPgcl and RGc project to the spinal cord with a weak ipsilateral predominance. The axons of RPc, RaO, RaP, and RaM neurons exhibit no lateral predominance. RPm neurons project to the cord with a weak contralateral predominance, and RPgcd neurons project to the cord with a strong contralateral predominance. Most medullary BRN project to the spinal cord via the medial longitudinal fasciculus (mlf) and sulcomarginal fasciculus. However, RPgcl, RMca and RMcb also project to the spinal cord via the lateral funiculus. The neurons of RD, RPm and RaM project to the spinal cord exclusively via the lateral or dorsolateral funiculus. Since the various medullary BRN are distinguishable on the basis of neuronal morphology, they may play distinct roles in brainstem modulation of spinal motor, sensory or autonomic activity.