New insights on vertebrate olivo-cerebellar climbing fibers from computerized morphological reconstructions

Characterization of neuronal connectivity is essential to understanding the architecture of the animal nervous system. Specific labeling and imaging techniques can visualize axons and dendrites of single nerve cells. Two-dimensional manual drawing has long been used to describe the morphology of labeled neuronal elements. However, quantitative morphometry, which is essential to understanding functional significance, cannot be readily extracted unless the detailed neuronal geometry is comprehensively reconstructed in three-dimensional space. We have recently applied an accurate and robust digital reconstruction system to cerebellar climbing fibers, which form highly dense and complex terminal arbors as one of the strongest presynaptic endings in the vertebrate nervous system. Resulting statistical analysis has shown how climbing fibers morphology is special in comparison to other axonal terminals. While thick primary branches may convey excitation quickly and faithfully to the far ends, thin tendril branches, which have a larger bouton density, form the majority of presynaptic outputs. This data set, now publicly available from NeuroMorpho.Org for further modeling and analysis, may constitute the first detailed and comprehensive digital reconstruction of the complete axonal terminal field with identified branch types and full accounting of boutons for any neuronal class in the vertebrate brain.