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Gross Anatomy of the Cerebellum By Prof. Dr. Muhammad Imran Qureshi Normal Functions Coordination Balance Posture Eye movements Motor learning Introduction It is known as the Little Brain and constitutes 11% of the total brain mass. In the posterior cranial fossa, it lies dorsal to pons and medulla oblongata, from which it is separated by the cavity of the fourth ventricle. The occipital lobe of the cerebral hemisphere lies above it. The two are separated by a fold of dura mater called the tentorium cerebelli. Part of the cavity of the fourth ventricle extends into it as a transverse cleft, which is bounded cranially by a lamina of white mater called the Superior Medullary Velum. Architecture It has a superficial layer of gray mater, while the white mater lies deeply. Discrete masses of Gray mater are present in the central region of the cerebellum in the form of FOUR nuclei. The cerebellum consists of TWO lateral hemispheres, separated by a central zone called the Vermis. It has Two surfaces, the Superior, and the Inferior. On the superior surface, there is no line of demarcation between the vermis and the hemispheres. However, on the inferior surface, the two hemispheres are well demarcated from one another by a deep depression, called the Vallecula Cerebelli. The Vermis lies in the depth of the Vallecula. Here, the vermis is separated from each cerebellar hemisphere by a Paramedian sulcus. Anteriorly and posteriorly, the hemispheres extend beyond the vermis and are separated by anterior and posterior cerebellar notches. The falx cerebelli lies in the posterior cerebellar notch. The surface of cerebellum is marked by a series of fissures that run almost parallel to one another. These fissures subdivide the cerebellum into narrow leaf like bands, called the Folia. The long axis of the majority of folia is transverse. The outer rind of each folium is made up of Grey mater, while within it runs a very thin core of white mater.A section cut at right angle to the folium gives it a characteristic Tree like appearance and is called Arbor Vitae (Tree of Life) Fissures and Lobes Some of the fissures on the surface of the cerebellum are deeper than the others and are named. These fissures divide the cerebellum into lobes. Thus the Primary fissure (Fissura Prima), which separates it into a smaller anterior and a larger posterior lobes, runs transversely on the superior surface. Another fissure, called the Posterolateral fissure is present on the inferior aspect and separates the Posterior lobe from the Flocculonodular lobe. The anterior and posterior lobes together form the Corpus Cerebelli. Another fissure, called the Horizontal fissure divides the cerebellum into upper and lower halves. Thus the part above this fissure is the superior surface and the part below it is the inferior surface. Within the corpus cerebelli, other less deep (un named) fissures divide each lobe into lobules. To see various divisions of the cerebellum in a single view, the cerebellum is represented as if it has been opened out, so that the superior and inferior surfaces are visible together. From above downwards, different parts of the vermis are: Lingula Central Lobule Culmen Declive Folium Tuber Pyramis Uvula Nodule Parts of Hemispheres With the exception of lingula, each subdivision of the vermis is related laterally to a part of the hemisphere. Thus Central lobule is related to the Ala of central lobule. Culmen to Anterior quadrangular lobule Declive to Posterior quadrangular lobule Folium to Superior semilunar lobule Tuber to Inferior semilunar lobule Pyramis to Biventral lobule Uvula to tonsil, and Nodule to Flocculus Phylogeny On the basis of Phylogeny, THREE divisions of the cerebellum are recognized: The Archycerebellum, which is the only component of cerebellum in the fishes and lower amphibians. It consists of the flocculonodular lobe and the lingula. The Paleocerebellum makes its first appearance in higher amphibians and is larger in reptiles and birds. In the human, it is represented by the superior vermis in the anterior lobe and part of the inferior vermis in the posterior lobe The cerebellar hemispheres together with the superior vermis in the posterior lobe constitute the Neocerebellum, which is found in mammals and is largest in the humans Functional Divisions These phylogenetic divisions correspond largely with the divisions based on major sources of afferent fibers. Thus: The archycerebellum is identical to the Vestibulocerebellum, which receives input from the vestibular nerve and nuclei Those parts of the vermis that constitute the paleocerebellum, together with adjacent parts of the hemispheres (that belong to neocerebellum) make up the Spinocerebellum. This is the site of termination of the spinocerebellar tracts and cuneocerebellar fibers, which convey proprioceptive and other sensory information. The remainder of the neocerebellum (i.e. the large lateral parts of the hemispheres, and the superior vermis in the posterior lobe) constitute the pontocerebellum. The contralateral pontine nuclei send afferent fibers to this area. Some overlapping occurs in these divisions Grey matter of the Cerebellum Most of the grey matter of the cerebellum is arranged as a thin layer covering the central core of the white mater---- Cerebellar cortex. Unlike the Cerebral Grey matter, it is uniform throughout. Embedded within the central core of white mater are masses of grey matter called the cerebellar nuclei. These are, from medial to lateral side: Fastigeal Nucleus Globose Nucleus Emboliform Nucleus, Dentate Nucleus Mnemonic: Fat Goat Eats Dandelion Cerebellar Peduncles Superior Cerebellar Peduncle: Connects the cerebellum to Mid brain Middle Cerebellar Peduncle: Connects the cerebellum to the Pons Inferior Cerebellar Peduncle: Connects the cerebellum to The Medulla and Spinal cord Regulation of muscle tone, coordination of skilled voluntary movement Planning and initiation of voluntary activity Maintenance of balance, control of eye movements Vestibulocerebellum Spinocerebellum Cerebrocerebelum