Tuesday, December 8, 2009

Cerebellum

The cerebellum lies in the posterior fossa. Its superior surface is covered by the tentorium cerebelli, a tentlike double fold of the dura mater that separates the cerebellum from the cerebrum. The surface of the cerebellum, unlike that of the cerebrum, display numerous small, horizontally running convolutions (folia), which are separated fromeach other by fissures. The narrow central portion of the cerebellum connecting the two hemispheres on either side is called the vermis because of its fancied resemblance to a worm.
A view of the cerebellum from below reveals the upper portion of the fourth ventricle lying between the cerebellar peduncles. The fourth ventricle communicates with the subarachnoid space through a single median aperture (foramen of Magendie) and two lateral apertures (foramina of Luschka). Caudal to the inferior andmiddle cerebral peduncles, there is a structure on either side called the flocculus; the two flocculi are connected across the midline through a portion of the vermis called the nodulus. Together, these structures constitute the flocculonodular lobe.
The subdivisions of the cerebellar vermis and hemispheres were given individual names by the old anatomists (culmen, declive, etc.), although they have little functional significance and are generally not clinically relevant. Today, it is more common to distinguish three major components of the cerebellum on phylogenetic and functionalgrounds:
The archicerebellum(phylogenetically oldest portion of the cerebellum) is intimately related to the vestibular apparatus. It receives most of its afferent input from the vestibular nuclei of the brainstem and is thus also called the vestibulocerebellum. Anatomically, it consists mainly of the flocculus and nodulus (flocculonodular lobe).
The paleocerebellum (next oldest portion of the cerebellum, after the archicerebellum) receives most of its afferent input from the spinal cord and is,therefore, also called the spinocerebellum (the term we will use in the following sections). It consists of the culmen and central lobule of the anterior lobe of the vermis, as well as the uvula and pyramid of its inferior lobe, and the paraflocculus. One can state, as a mild simplification, that the spinocerebellum is composed of most of the vermis and paravermian zone (pars intermedialis).
The neocerebellum (youngest portion of the cerebellum) is its largest part. Its phylogenetic development occurred together with the expansion of the cerebrum and the transition to an upright stance and gait. It is formed by the two cerebellar hemispheres and has an intimate functional connection to the cerebral cortex, which projects to it by way of the pontine nuclei. Thus, theneocerebellum is also termed the pontocerebellum or cerebrocerebellum,as we will call it in the following sections.

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