Clustered and laminar topographic patterns in rat cerebro-pontine pathways

Novel neuroanatomic approaches for investigating topographic maps at a systems level include combined use of sensitive neural tracing techniques and computerized methods for three-dimensional reconstruction from serial sections. Application of these methods have allowed discovery of new principles of topographic organization in the rat pontine nuclei. The pontine nuclei are intercalated in the large pathways connecting the cerebral cortex to the cerebellum. In rat, cerebropontine projections are characterized by multiple delineated terminal fields. It is generally accepted that these projections are topographically organized. The presence of widespread axonal clusters in the pontine nuclei is typically interpreted to represent a complex scheme of organization. In recent anatomic investigations of somatosensory corticopontine projections in young and adult rats, a somatotopic distribution of axonal clusters, concentrically organized in an inside-out fashion, has been reported. This review summarizes the topographic principles of organization proposed for somatosensory corticopontine projections, and discusses the possibility that widely segregated corticopontine terminal fields are located inside lamellar volumes. This organizational pattern may be explained by mechanisms operative during development, and resembles the patterns of organization previously described in cat and monkey. Possible implications of this architecture are discussed in relation to map transformations from the cerebral cortex to the cerebellum.