Introduction

The cerebellum (literally, "little brain'') is located in the posterior cranial fossa. It represents 10% of the total brain volume and contains more than 50% of the total number of neurons of the central nervous system. Its general organization resembles that of the telence-phalon with an outer mantle of gray matter, the cerebellar cortex, that covers an internal white matter in which the deep nuclei (i.e., the three pairs of deep cerebellar nuclei) are embedded. The cellular organization of the cerebellum is quite simple; its basic structure has been well-known since the beginning of the 20th century due to the work of Ramon y Cajal (Fig. 1). Because of the simplicity of the cerebellar anatomical organization and its remarkable similarity in all mammals, it seemed to be an easy field for investigating the relationships between brain structure and function; thus, it has always been a field of extreme interest for neuroscien-tists. Since the pioneering work at the end of the 19th century, the number of studies devoted to the cerebellum and its function has increased enormously and many theories on the cerebellar function have been proposed. Despite all these efforts, we still do not know exactly what the cerebellum is. In recent years, particularly due to unpredicted clinical and functional neuroimaging findings, the classical theories focused on the role of the cerebellum in motor control and motor learning have been reconsidered and new hypotheses have been advanced. In this article, after a general update on the available data regarding the anatomical and functional cerebellar organization, recent theories on the functions of the cerebellum are reviewed.

Figure 1 Schematic reconstruction of a section cut perpendicular to the main axis of a cerebellar lobule. Reconstruction of the organization of the cerebellar cortex as drawn by Ramon y Cajal from observations of Golgi preparations. A, molecular layer; B, granular layer; C, white matter; a, Purkinje cells; b, basket cells; d, terminal arborization of the axons of the basket cells; e, stellate cells; f, Golgi cells; g, granule cells; h, mossy fibers; n, climbing fibers [reproduced with permission from Ramon y Cajal (1972). Histologie du Systeme Nerveux de L'Homme e des Vertebres, Consejo Superior de Investigagaciones Cientificas. Instituto Ramon y Cajal, Madrid].

Figure 1 Schematic reconstruction of a section cut perpendicular to the main axis of a cerebellar lobule. Reconstruction of the organization of the cerebellar cortex as drawn by Ramon y Cajal from observations of Golgi preparations. A, molecular layer; B, granular layer; C, white matter; a, Purkinje cells; b, basket cells; d, terminal arborization of the axons of the basket cells; e, stellate cells; f, Golgi cells; g, granule cells; h, mossy fibers; n, climbing fibers [reproduced with permission from Ramon y Cajal (1972). Histologie du Systeme Nerveux de L'Homme e des Vertebres, Consejo Superior de Investigagaciones Cientificas. Instituto Ramon y Cajal, Madrid].

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