Lower thoracic upper lumbar spinocerebellar projections in rats: a complex topography revealed in computer reconstructions of the unfolded anterior lobe.

The topography of wheatgerm agglutinin-horseradish peroxidase/horseradish peroxidase-labeled mossy fiber terminals of lower thoracic-upper lumbar (T12-L3) spinal projections to the cerebellar anterior lobe was quantitatively analysed in adult rats. Computer-based image analysis mapped the orthogonal (parallel to the surface) distribution of labeled terminals in two-dimensional reconstructions of the unfoled anterior lobe cortex. The radial (perpendicular to the surface) distribution of terminals within the granule cell layer was mapped by computing whether the terminals were in either the outer- or inner-halves of this layer. The number of labeled terminals in each lobule was calculated. In the anterior lobe, lower thoracic-upper lumbar spinocerebellar projections terminate primarily in lobules II (mean 27.14%), III (mean 38.68%), and IV (mean 19.31%). Different-sized bilateral injections restricted to L1 were used to study the organization of intrasegmental spinocerebellar projections. Small injections into L1 labeled a limited number of terminals which were located either in clusters or were spatially isolated. Intermediate-sized intrasegmental injections resulted in additional clusters of labeled terminals. Many of the terminal clusters were spatially related and formed larger irregularly shaped patches. Large intrasegmental injections labeled terminal clusters and patches that were discontinuous but aligned parallel to the longitudinal (transverse) axis of lobules II-IV. Injections including segments rostral and caudal to L1 were used to study the topography of intersegmental lower thoracic-upper lumbar spinocerebellar projections. Multisegmental injections increased the number of labeled terminal clusters and patches which obscured the pattern of segmental input, but there was still a transversely oriented pattern of termination. Distinct transversely aligned terminal free areas remained apparent. Lower thoracic-upper lumbar spinocerebellar projections terminated in both the outer- and inner-halves of the granule cell layer, but overall were more numerous in the outer-half of this layer. In serially spaced sagittal sections, however, the majority of terminals alternated between the outer- and inner-halves of the granule cell layer. Outer- and inner-terminals were not spatially segregated in their orthogonal distribution. These results indicate lower thoracic-upper lumbar spinocerebellar projections have a complex three-dimensional topography in the anterior lobe. These findings are discussed in relation to previous findings for a sagittally oriented topography for lower thoracic-upper lumbar spinocerebellar projections and in the context of how cerebellar somatosensory afferent input may be organized.