Cerebellum Gross Anatomy

A wide hollow within the anterior cerebellum is occupied by the pons and upper medulla oblongata, which are separated from the cerebellum by the fourth ventricle. Posteriorly, there is a narrow median notch, lodging the falx cerebelli. The cerebellum is connected to the brainstem by three white matter tracts: the superior, middle, and inferior cerebellar peduncles (described more fully in Plate 8-3). The cerebellum’s superior and inferior surfaces meet within the caudal aspect of lobule crus I. The cerebellum forms a sphere, and therefore the vermal lobule I/II is separated anteriorly from lobule X by the fourth ventricle.

The cerebellum surfaces include numerous narrow folia separated by parallel, curved, deeply penetrating fissures. Each folium further consists of multiple, small subfolia. The folia are grouped into ten lobules divided by named fissures. These ten lobules form three lobes: the anterior, posterior, and flocculonodular lobes. Lobules I to V are the anterior lobe, lobules VI to IX are the posterior lobe, and lobule X is the flocculonodular lobe, including the flocculus, which is a small, semidetached portion lying close to the middle cerebellar peduncle. Earlier cerebellum nomenclatures were not uniform (one version is in the diagrams for comparison). These are replaced by a simplified, coherent numeric system existing across different species’ brains. All lobules are identifiable at the vermis; lobules III to X are continuous across the hemispheres.

The primary fissure separating the anterior from the posterior lobe is deepest and most evident in the midsagittal plane but not as readily identifiable externally. The superior posterior fissure separating lobule VI from lobule VII is well seen on the posterior superior surface. The horizontal fissure, prominent on the posterior, inferior, and lateral hemisphere aspects divides lobule VIIA into two major components: lobule VIIAf at the vermis/crus I in the hemisphere, and lobule VIIAt at the vermis/crus II in the hemisphere. The paravermian sulcus on each side of the superior cerebellum surface is an indentation formed by the superior cerebellar artery medial branch. The retrotonsillar groove at the inferior and medial aspect of the cerebellum is caused by the rim of the foramen magnum and delineates the tonsil, a gross morphologic feature comprising lobule IX and part of lobule VIIIB that becomes clinically relevant with herniation syndromes.

The interior of the cerebellum contains a central mass of white matter, the medullary core, surrounded by the deeply folded cerebellar folia. The relationship of the folia to the white matter has a tree branch appearance, hence arbor vitae. The white matter core extends into the folia as narrow laminae, surrounded by the three-layered cerebellar cortex. The white matter consists largely of mossy and climbing fibers entering the cerebellum, and axons of Purkinje cells leaving the cerebellar cortex to the nuclei. There are no association fibers in the cerebellum linking cerebellar cortical areas with each other. The cerebellar nuclei within the medullary core include, medial to lateral, the fastigial, globose, emboliform, and dentate. These nuclei, together with other minor nuclei in the medullary core and vestibular nuclei in the posterior pons and medulla, are linked with the cerebellar cortex serving as the cerebellum’s functional unit, namely, the corticonuclear microcomplex. Except for the vestibulocerebellum, these nuclei are the primary source of cerebellar efferents. These have highly organized connections with extracerebellar structures. The large, folded dentate nucleus is U-shaped. Its open end, or hilus, points medially, conveying fibers that, together with those from the fastigial, globose, and emboliform nuclei, form the superior cerebellar peduncle.