Rat Somatosensory Cerebropontocerebellar Pathways: Spatial Relationships of the Somatotopic Map of the Primary Somatosensory Cortex Are Preserved in a Three- Dimensional Clustered Pontine Map TRYGVE B. LEERGAARD, 1 KJERSTI A. LYNGSTAD, 1 JOHN H. THOMPSON, 2 SOFIE TAEYMANS, 3 BART P. VOS, 3 ERIK DE SCHUTTER, 3 JAMES M. BOWER, 2 AND JAN G. BJAALIE 1 * 1 Department of Anatomy, Institute of Basic Medical Sciences, University of Oslo, N-0317 Oslo, Norway 2 Division of Biology, California Institute of Technology, Pasadena, California 91125 3 Born-Bunge Foundation, University of Antwerp, 2610 Antwerp, Belgium ABSTRACT In the primary somatosensory cortex (SI), the body surface is mapped in a relatively continuous fashion, with adjacent body regions represented in adjacent cortical domains. In contrast, somatosensory maps found in regions of the cerebellar hemispheres, which are influ- enced by the SI through a monosynaptic link in the pontine nuclei, are discontinuous (“fractured”) in organization. To elucidate this map transformation, the authors studied the organization of the first link in the SI-cerebellar pathway, the SI-pontine projection. After injecting anterograde axonal tracers into electrophysiologically defined parts of the SI, three-dimensional reconstruc- tion and computer-graphic visualization techniques were used to analyze the spatial distribution of labeled fibers. Several target regions in the pontine nuclei were identified for each major body representation. The labeled axons formed sharply delineated clusters that were distributed in an inside-out, shell-like fashion. Upper lip and other perioral representations were located in a central core, whereas extremity and trunk representations were found more externally. The multiple clusters suggest that the pontine nuclei contain several representations of the SI map. Within each representation, the spatial relationships of the SI map are largely preserved. This corticopontine projection pattern is compatible with recently proposed principles for the estab- lishment of subcortical topographic patterns during development. The largely preserved spatial relationships in the pontine somatotopic map also suggest that the transformation from an organized topography in SI to a fractured map in the cerebellum takes place primarily in the mossy fiber pontocerebellar projection. J. Comp. Neurol. 422:246 –266, 2000. © 2000 Wiley-Liss, Inc. Indexing terms: three-dimensional reconstruction; anterograde tracing; cerebellum; corticopontine; electrophysiology; topography Judging by the number of axons alone, the cerebropon- tocerebellar pathway is one of the largest pathways in the mammalian brain (Tomasch, 1968, 1969). This pathway, which originates from large parts of the cerebral cortex, reaches almost all regions of the cerebellum (for reviews, see rodal and Bjaalie, 1997; Schmahmann and Pandya, 1997). With this report, we initiate a detailed study in the rat of the projection topography of the pathway that orig- inates in the primary somatosensory cortex (SI) and ter- minates in the granule cell layer of the cerebellum. The Grant sponsor: European Community; Grant number: Bio4 CT98-0182; Grant sponsor: Research Council of Norway; Grant sponsor: National Institutes of Health; Grant number: NS37109. *Correspondence to: Dr. Jan G. Bjaalie, Department of Anatomy, Insti- tute of Basic Medical Sciences, University of Oslo, P.O. Box 1105 Blindern, N-0317 Oslo, Norway. E-mail: j.g.bjaalie@basalmed.uio.no Received 24 September 1999; Revised 6 January 2000; Accepted 21 February 2000 THE JOURNAL OF COMPARATIVE NEUROLOGY 422:246 –266 (2000) © 2000 WILEY-LISS, INC.