The pontocerebellar projection onto the paramedian lobule in the cat: an experimental study with the use of horseradish peroxidase as a tracer.

Horseradish peroxidase (HRP) was injected into cerebellar cortex of the paramedian lobule in 12 cats, and the ensuing distribution of labeled cells in the pontine nuclei was mapped in some detail. The cells in the pontine gray which give origin to fibers to the paramedian lobule lie together, in part in groups, and in part in columns. The columns are situated both medial and ventrolateral to the peduncle, as well as in the dorsolateral pontine nucleus. The projection is bilateral with a clearcut contralateral preponderance, except in the lateralmost region in the dorsolateral nucleus, which projects mainly ipsilaterally. The column medial to the peduncle projects in a topographical pattern to the paramedian lobule. The dorsal part of this column projects to the rostral folia of the paramedian lobule, while successively more ventral parts in the column project to more caudal paramedian lobules. Within the other columns only a faint sign of a topographical organization is found. The location of the pontine columns projecting onto the paramedian lobule largely corresponds to the pontine terminal areas of fibers from the sensory cerebral cortex (SmI and SmII). The corresponding topography in these parts of the corticopontine and pontocerebellar pathways is suitable for a somatotopical impulse transmission from the sensory cortex to the paramedian lobule, in agreement with the results of physiological investigations. Furthermore, a correlation of the pontine areas projecting onto the paramedian lobule with the terminal areas of pontine afferents shows that the pons may be a relay station in mediating influences from other parts of the cortex (MsI, visual and acoustic), the cerebellar nuclei and the colliculi to the paramedian lobule.