Distinguishing rat brainstem reticulospinal nuclei by their neuronal morphology. II. Pontine and mesencephalic nuclei.

The purpose of this study was to determine whether pontine and mesencephalic reticulospinal nuclei like those of the medulla, can be differentiated on the basis of neuronal morphology. Accordingly, neurons of the various pontine and mesencephalic brainstem reticulospinal nuclei (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 13 BRN can be distinguished in the pons and mesencephalon of the rat on the basis of neuronal morphology. The dendritic arborizations (DA) of neurons in nucleus reticularis (NR) pedunculopontinus pars compacta (RPpc), NR pedunculopontinus pars dissipatus (RPpd), NR cuneiformis (RCf) and NR subcuneiformis (RScf) are radially symmetrical. The DA of neurons in NR pontis caudalis pars beta (RPoCb) and alpha (RPoCa), as well as those of NR subcoeruleus (RSc) and the A5 cell group, exhibit a pronounced dorsomedial to ventrolateral slant. The dendrites of locus coeruleus (LC) neurons slant from dorsolateral to ventromedial. The DA of NR pontis oralis pars medialis (RPoOm) neurons course medially, while those of NR pontis oralis pars lateralis (RPoOl) course laterally. The DA of Kolliker-Fuse neurons (KF) course horizontally. Finally, the DA of the nucleus raphe dorsalis (RaD) are either symmetrically multipolar or fusiform. The neurons of RPoCb, RPpc, KF and RCf project to the spinal cord with a strong ipsilateral predominance, while those of LC, RSc, RPoOm and RPpd project to the spinal cord with a weak ipsilateral predominance. The axons of A5, RPoOl and RaD neurons exhibit no lateral predominance in their spinal projections. Finally, RPoCa neurons, as well as neurons in RScf, project to the spinal cord with a strong contralateral predominance. The neurons of RPoCb, RPoCa, RPoOl, RPpd and RScf project to the spinal cord via the mlf and sulcomarginal fasciculus. The neurons of RSc, KF, RPoOm, RPpc and RCf project to the spinal cord via the lateral funiculus.