The pontocerebellar efferents from the pontine reticular tegmental nucleus in the rabbit: differential projections to the paramedian sublobules as revealed with tracing of retrograde axonal transport of horseradish peroxidase.
Projections from the nucleus reticularis tegmenti (NRTP) pontis to the cerebellar paramedian lobule were studied using a horseradish peroxidase retrograde transport technique. The rabbit's NRTP consists of a medial principal part (the nucleus papillioformis: PLF) and smaller lateral part (the processus tegmentosus lateralis: PTL). The PLF can be subdivided into a dorsomedial part--zone A, a ventrolateral part--zone C, and a main part--zone B. Zone B is furthermore divisible into the subzones: B1, B2, and B3. In addition, some cells of zone B3 invade the fiber bundles of the medial lemniscus. Experimental data have indicated a topographically organized bilateral projection from the NRTP to the anterior, the middle and the posterior folia of the paramedian lobule with some degree of contralateral dominance. In contrast to an earlier report in cats we find that not only the ventral, but also the dorsal region of the NRTP send projections onto the paramedian lobule. The neurons sending axons to the paramedian lobule are localized mainly in the middle and to a some degree in the posterior thirds of the NRTP. Zones A, B1 and B3 project onto all of sublobules of the paramedian lobule. While zone A projects primarily onto the anterior sublobules (e, f), zone B1 emanates to the middle paramedian sublobules (c, d), Zone B3 sends fibers mainly into the posterior sublobules (a, b) and to a some degree to the middle paramedian sublobules. Zone B2 distributes a few axons exclusively to the middle folia of the paramedian lobule, while the middle part of the PTL gives rise to small bilateral projection to sublobules c and d of the paramedian lobule. Neurons of zone C send projections into the middle folia of the paramedian lobule. These findings are discussed in relationship to other morphological and physiological data on neurons of the pontine reticular tegmental nucleus.