Congruence of mossy fiber and climbing fiber tactile projections in the lateral hemispheres of the rat cerebellum

We have examined the spatial relationship between the mossy fiber and climb ing fiber projections to crus IIa in the lateral hemispheres of the rat cer ebellum. Experiments were performed in ketamine/xylazine anesthetized rats using extracellular recordings and high-density micromapping techniques. Re sponses were elicited using small, tactile stimuli applied to the perioral and forelimb regions at a rate of 0.5 Hz. In our first series of experiment s we demonstrate that the primary (i.e., strongest) receptive field for a s ingle Purkinje cell's complex spike is similar to the primary receptive fie ld of the granule cells immediately subjacent to that Purkinje cell. In our second series of experiments we demonstrate that the granule cell region m ost strongly activated by a particular peripheral stimulus is immediately s ubjacent to the Purkinje cells whose complex spikes are also activated most strongly by the same stimulus. The region of climbing fibers activated by a localized peripheral stimulus is "patchy"; it clearly does not conform to the notion of a continuous microzone. These results support original obser vations first reported in the 1960s using evoked potential recording techni ques that the messy fiber and climbing fiber pathways converge in cerebella r cortex. However, we extend this earlier work to show that the two pathway s converge at the level of single Purkinje cells. Many cerebellar theories assume that messy fiber and climbing fiber pathways carry information hem d ifferent peripheral locations or different modalities to cerebellar Purkinj e cells. Our results appear to contradict this basic assumption for at leas t the tactile regions of the lateral hemispheres. (C) 2001 Wiley-Liss, Inc.