Spatial arrangement of cerebro-pontine terminals

Understanding the interaction of the cerebral cortex and cerebellum require s knowledge of the highly complex spatial characteristics of cerebro-cerebe llar signal transfer. Cerebro pontine fibers from one neocortical site term inate in several sharply demarcated patches across large parts of the ponti ne nuclei (PN), and fibers from different neocortical areas terminate in th e same pontine region. To determine whether projections from segregated neo cortical sites overlap in the PN, we studied double anterograde tracing of cerebro-pontine terminals from large parts of rat neocortex. In none of the se experiments, including double injection into two functionally related ar eas, were we able to demonstrate overlapping patches, although close spatia l relationships were always detected. This non-overlapping distribution is consistent with a compartmentalized organization of the cerebro-pontine pro jection and may be the basis of the fractured type of maps found in the cer ebellar granular layer. The critical distance between two sites on the neoc ortical surface that project to non-overlapping patches in the PN was found to be 600 mum, by using double injection within the whisker representation of the primary somatosensory area. This matches the diameter of dendritic trees of layer 5 projection neurons, indicating that non-overlapping popula tions of neocortical projection neurons possess non-overlapping patches of pontine terminals. Estimations based on this critical distance and the pont ine volume anterogradely labeled by one injection site indicate that the si ze of the PN may be well suited to accommodate a complete set of non-overla pping pontine patches from all possible neocortical sites. (C) 2001 Wiley-L iss, Inc.