Spatial correspondence between tactile projection patterns and the distribution of the antigenic Purkinje cell markers anti-zebrin I and anti-zebrin II in the cerebellar folium crus IIA of the rat.

We have compared the band-like distribution of the Purkinje cell-specific polypeptides zebrin I and zebrin II with the spatial organization of tactile projections to crus IIa in the cerebellar hemisphere of the rat. Maps of tactile responses in the granular layer of the cerebellar hemispheres are fractured into discontinuous regions, termed "patches". High-density micromapping was used to identify specific patches and their boundaries within this fractured somatotopic map. In one series of experiments, medial and lateral boundaries of the large central ipsilateral upper lip-related patch were identified and labeled with either Fast Blue or India Ink. Following immunocytochemical processing, the band-like distribution of immunostained Purkinje cells (zebrin-positive bands) and the identified patch boundaries were digitized and reconstructed in three dimensions. Comparisons between these two features demonstrate a spatial correspondence between zebrin transitions and the boundaries of the electrophysiologically defined upper lip-related patch. In another series of experiments, we outlined the boundaries or centers of several smaller patches consistently located in the medial portion of the folium. Again, we found a correspondence between the distribution of granule cell layer tactile patches and the zebrin staining pattern. The correspondence between tactile projection patterns and molecular features demonstrated in the present study implies that there is a distinct and largely fixed spatial pattern of organization in the cerebellar hemispheres. We discuss possible causal connections and developmental determinates, as well as the physiological significance of the correspondence between the two features.

Ascending granule cell axon: an important component of cerebellar cortical circuitry.

Physiologic evidence suggests that local activation of the cerebellar granule cell layer produces a much more restricted spatial activation of overlying Purkinje cells than would be expected from the parallel fiber system. These results have led to the suggestion that synapses associated with the ascending granule cell axon may provide a large, direct, excitatory input to Purkinje cells, whereas parallel fiber synapses may be more modulatory in nature. In the current experiments, serial electron microscopy was used to reconstruct synapses associated with these two segments of the granule cell axons in the cerebellar cortex of albino rats. The results indicate that there are significantly more presynaptic vesicles in ascending segment synapses than in parallel fiber synapses. Furthermore, a first-order linear regression analysis revealed positive correlations between all measures of pre- and postsynaptic morphology for parallel fibers, but not for ascending segment synapses. Perhaps most surprisingly, serial reconstructions of postsynaptic spines and their associated dendrites demonstrated that spines contacted by ascending segment synapses are located exclusively on the smallest diameter distal regions of the Purkinje cell dendrites, whereas parallel fiber synapses are found exclusively on intermediate- and large-diameter regions of the spiny branchlets. Based on two independent calculations, we estimate that 20% of the granule cell synapses onto a Purkinje cell are actually made by the ascending segment. By using computer simulations of a single Purkinje cell dendrite, we have also demonstrated that synchronous activation of these distal ascending segment inputs could produce a substantial somatic response. Taken together, these results suggest that the two different regions of granule cell axons may play very different physiologic roles in cerebellar cortex.