Axonal branching in the projections from the paramedian reticular nucleus to the cerebellar cortex.

Injections of fluorescent tracers into cat cerebellar cortex gave evidence of collateral axonal branching of neurons situated in the paramedian reticular nucleus. These branched reticulocerebellar projections were distributed to opposing sides of the cerebellum, in particular the anterior lobe and the ansiform lobule. No topographical organization was observed in the PRN. Less than 30% of ipsilaterally projecting reticulocerebellar fibers had contralaterally directed collateral branches. These results are in keeping with a bilateral fastigial projection to the PRN forming a feedback loop circuit through which orthostatic reflexes may be mediated.