Tactile cutaneous representation in cerebellar granule cell layer of the opossum, Didelphis virginiana

Recent studies of the albino rat revealed extensive cutaneous somatosensory projections to the granule cell (GC) layer of the cerebellar hemispheres and the caudal vermis. These projections are organized asomatotopically in patchy mosaics. To determine whether similar projections exist in a marsupial, we explored the GC layer of the cerebellar cortex of anesthetized Virginia opossums using in-depth microelectrode micromapping and juxtathreshold cutaneous natural stimulation techniques. We found: Somatosensory projections to the GC layer exist throughout the mediolateral extent of the folia of the posterior lobe. The anterior lobe was not explored. The submodality of most receptive fields was 'gentle-touch' cutaneous, but some were located in muscle, joint, or other deep-lying structures. Peripheral projections to the GC layer are organized asomatotopically. Adjacent body parts project disjunctively to nonadjacent GC regions, and the overall pattern of peripheral projections forms a patchy columnar mosaic. Many body parts send projections to multiple loci. Ipsilateral projections predominate. Mechanoreceptors from face, snout, mouth and teeth activate the bulk of GC loci on crus I and crus II. The paramedian lobule receives projections from the entire ipsilateral body; the pyramis is activated from hindlimb and forelimb; the uvula from the upper arm and vibrissae. Different folia have different combinations and arrangements of disjunctive patchy peripheral projections. Individual differences in pattern of foliation and body representation occur. Except for differences in mosaic pattern and relative size of different projections, these findings are similar to those in rats and cats. These data suggest that somatosensory (especially cutaneous) inputs to the cerebellum are not only functionally significant, but that they exist widely among mammals.