The pontocerebellar system in the rat: an HRP study. I. Posterior vermis.

This study was undertaken to determine the origin of projections from the basilar pontine nuclei (BPN) and nucleus reticularis tegmentis pontis (NRTP) to the posterior vermal lobules VI-IX of the rat cerebellum. We describe the topographical organization of this component of the pontocerebellar projection, and the congruence of the cells of origin in the basilar pons with some of the major pontine afferent systems including the corticopontine and tectopontine projections. Horseradish peroxidase (HRP) was injected into the midline cerebellar vermal zones of Long-Evans hooded rats. The more sensitive chromogens, tetramethyl benzidine and benzidine dihydrochloride, were used to reveal the location of labeled neurons. With injections located near the midline, groups of labeled cells were observed bilaterally within the BPN. The basic trend of the projections noted was: lobule VIa receives a nonfocal projection from nearly all subdivisions of the BPN throughout its rostrocaudal extent, as well as a substantial input from NRTP. Lobules VIb-c receive input from NRTP, the rostral pons, and from the ventral, lateral, and medial groups of cells in the middle BPN project to lobule VII, in addition to projections from limited groups of cells in the rostral BPN. Lobule VIII receives afferents from the caudal aspect of the pontine gray. Lobules IXa-receive afferents from the medial and peduncular groups in the midline BPN, whereas lobule IXc receives inputs from a medial group and a small lateral cluster of cells in the caudal aspect of the BPN. Pontine neurons projecting to the posterior vermis originate from areas which appear to receive descending inputs from visual, auditory, and somatosensory regions of the cerebral cortex. However, a large number of pontine and NRTP neurons projecting to lobules VI and VII are located within the terminal fields of tectal neurons, perhaps indicating a stronger input from the tectum rather than visual and auditory cerebral cortical regions.