Viewing the cerebellum through the eyes of Ramón Y Cajal.

The modern age in the study of the cerebellum started 120 years ago when Cajal published his first paper with Golgi-impregnated material. In this publication, he selected the cerebellum to initiate his gigantic work aimed at unraveling the complexity of the CNS organization. It was not by chance that he selected the cerebellum but because of the occurrence of specific types of fibers, particularly climbing and mossy afferents and basket fibers. The peculiarity of these fibers offered Cajal one of the clearest situations to envision his "neuron doctrine", which proposes that between the nerve cell processes there is no continuity, only contiguity. In 4 years of intense investigation, Cajal was able to untangle the whole cerebellar circuit, providing the roots of our present knowledge on cerebellar organization. This knowledge has greatly expanded in the last 40 years mainly because the application of new techniques, such as electron microscopy, axonal connection tracing techniques based upon axoplasmic transports, and especially modern immunohistochemical and in situ hybridization techniques allowing the correlation of the chemical constituents of the cells with their structural counterparts, as a valuable approach to better appraise function and organization of the cerebellum. These post-Cajal discoveries are briefly discussed to conclude that, even though we are still far from a complete understanding of its function, new important concepts have been developed, for instance that through its connections with the prefrontal cortex, the cerebellum does not only contribute to the planning and execution of the movement, but that has access also to higher cognitive functions.