Principles of organization of a cerebro-cerebellar circuit. Micromapping the projections from cerebral (SI) to cerebellar (granule cell layer) tactile areas of rats.

We defined spatial patterns of organization of projections from somatosensory cerebral cortex (SI) to the somatosensory cerebellar cortex of anesthetized albino rats using microelectrode (stimulation and recording) micromapping methods and low-threshold cutaneous (tactile) stimulation. Two sampling strategies were used: (1) a single cerebral SI locus in layers V-VI was stimulated electrically, while a responding region of the cerebellar granule cell (GC) layer was systematically mapped with a recording electrode; (2) the SI stimulating electrode was used as the mapping electrode while the cerebellar GC electrode remain fixed. We found highly specific patterns of connections between somatotopically organized SI cortex and the somatotopically fractured tactile cerebellar cortex. Using threshold stimulating currents in SI, the projections from small populations of neural elements were found to be highly restricted, terminating within the confines of only those tactile cerebellar hemispheric locations having the same receptive fields (RFs). These SI-GC projections conform to the patchy mosaic pattern of organization previously shown for peripheral tactile projections. SI projections to GC patches were either contralateral or ipsilateral, depending on the laterality of the peripheral projections to that patch. Each SI focus projected to only a portion of a patch; projections from several adjacent SI loci overlapped serially within a patch. As with the peripherally evoked GC layer responses, SI-evoked GC responses were organized in a columnar fashion and were maximal at middle levels of the GC layer; SI-GC latencies were 5-8 ms. These data reveal that this tactile-related cerebro-cerebellar circuit exhibits precisely organized patterns of projection.