Quantitative analysis of converging spinal and cuneate mossy fibre afferent projections to the rat cerebellar anterior lobe

Abstract

The convergence/divergence of mossy fibre afferent projections to the cerebellar anterior lobe from a single lumbar segment, from adjacent or widely separated lower thoracic and lumbar segments, and finally from the lower thoracic-upper lumbar spinal cord and the brainstem cuneate nuclei was quantitatively analysed in adult rats. Spinal and cuneate mossy fibre terminals were differentially labelled with biotinylated dextran amine and cholera toxin subunit B, immunohistochemically identified in the same histological sections, and their spatial distributions quantitatively plotted in computer reconstructions of the unfolded anterior lobe cortex. Afferent convergence was quantified by calculating the number of biotinylated dextran amine-labelled terminals that radially overlapped with cholera toxin-labelled terminals at points on the unfolded cortical map that represented theoretical Purkinje cells. Spino- and cuneocerebellar mossy fibre terminals are organized in patches that are oriented in parasagittally-oriented stripes or transversely oriented bands. Afferent convergence was greatest following biotinylated dextran amine and cholera toxin injections in the same or adjacent spinal lumbar segments (60 and 52%, respectively). When biotinylated dextran amine and cholera toxin were injected in a single segment differentially labelled terminals appeared randomly intermingled in common patches. There was a trend for terminals labelled from adjacent lumbar segments to be more segregated in the patches. Segmentally separated biotinylated dextran amine and cholera toxin spinal cord injections (four lumbar segments) resulted in clearly segregated (80%) biotinylated dextran amine from cholera toxin-labelled terminal patches or patches with distinct divergence of the differentially labelled terminals in the patch. Cuneocerebellar terminals labelled with biotinylated dextran amine were located in patches, stripes, and bands spatially segregated from terminal patches, stripes, and bands of cholera toxin-labelled spinal afferents except at their immediate borders where some radial overlap occurred (9-22%). These anatomical findings for a fractured somatotopy of spinal and cuneate inputs to the cerebellar anterior lobe complement neurophysiological findings for a very similar pattern of organization of cutaneous inputs to the posterior lobe, and are discussed in light of potential mechanisms for anterior lobe processing of somatosensory information.