We defined spatial patterns of tactile projections to cerebellar cortex of anesthetized albino rats using microelectrode micromapping methods. Low threshold natural stimulation of cutaneous mechanoreceptors of specific body structures evokes brisk, short latency, localized responses within the granule cell layer. These projections terminate within columnar assemblies of granule cells within the folia of crus I, crus II and paramedian lobule of the cerebellar hemispheres. These columnar inputs appear as individuated patches and the overall array of patches make up mosaics. The somatotopic organization of single patches is precise but that among the patches within the mosaic is fractured. Electrode puncture densities of as high as 65/mm were required to delineate the miniature projectional features within the patches and mosaics. The cutaneous sources contributing to the bulk of the granule cell inputs were smallest from perioral structures which are those prominently utilized in discriminative action sequences of rats. This suggests an important perceptual function of these circuits. Ipsilateral projections were predominant but bilateral and contralateral projections were also found. These data reinforce Snider'searly contention that tactile afferents (rather than just proprioceptive inputs) play an important role in cerebellar functions. All these findings necessitate reexamination of several current views of cerebellar functions.

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