The cerebellum, which stands for “little brain”, is a structure of the central nervous system. It has an important role in motor control. The cerebellum is the largest part of the hindbrain and second largest part of the brain as a whole. It weighs about 150 g. It is located in the posterior cranial fossa underneath the tentorium cerebelli and behind the pons and medulla oblongata. It is separated from the pons and medulla by a cavity of the fourth ventricle. Its surface bears numerous fissures separating narrow folia which are mostly transverse. The cerebellum is connected to the brainstem by three pairs of large fibre tracts called cerebellar peduncles.
The cerebellum arises from the rhombencephalon or hindbrain. More specifically, it arises from the rhombic lips or alar plates (dorsal thickening of the neural tube that forms the sensory areas of the nervous system) of the metencephalon (spans between the pontine flexure and the rhombencephalic isthmus).
The external features of the cerebellum comprise three parts, two surfaces, two notches, and three well-marked fissures. The cerebellum consists of two large lateral hemispherical lobes, the cerebellar hemispheres which are united to each other by a narrow median worm-like portion, called vermis.
The superior and inferior aspects of vermis are termed superior and inferior vermis respectively. The ridge-like superior vermis is continuous on either side with the superior surface of cerebellar hemisphere imperceptively. The inferior vermis is more clearly demarcated from the hemispheres in the floor of vallecula cerebelli. The superior surface of the cerebellum is convex. The twe cerebellar hemispheres are continuous with each other or this surface.
The inferior surface presents a deep median notch called vallecula which separates the two cerebella hemispheres. The floor of the vallecula is formed by inferio vermis and is limited on each side by sulcus valleculae.
The anterior aspect of cerebellum is marked by a wide shallow anterior cerebellar notch which accommodates pons and medulla. The posterior cerebellar notch is deep and narrow, and lodges the falx cerebelli.
The horizontal fissure is most conspicuous and runs along the lateral and posterior margins of the cerebellum. It marks the junction between the superior and inferior sufaces of the cerebellum.
The posterolateral fissure lies on the inferior surface the cerebellum and separates the flocculonodular lobe from the rest of the cerebellum (corpus cerebelli). The V-shaped fissura prima on the superior surface cuts the superior vermis at the junction of its anterior third and posterior one-third. It divides the corpus ebelli into anterior and posterior (middle) lobes.
Anatomically the cerebellum is divided into three lobes, viz. anterior, posterior and flocculonodular. The anterior lobe lies on the superior surface anterior to the fissura prima. The posterior/middle lobe lies between the fissura prima on the superior surface and posterolateral fissure on the inferior surface. The flocculonodular lobe is smallest of all and lies on the inferior surface in front of the posterolateral fissure.
Based on phylogenetic and functional criteria the cerebellum is divided into three parts, archicerebellum, paleocerebellum, and neocerebellum. Phylogenetically archicerebellumit is the oldest part of the cerebellum. The fishes and lower amphibians possess only this component of the cerebellum. It consists of flocculonodular lobe and lingula. The archicerebellum is chiefly vestibular in connections and concerned with the maintenance of equilibrium, tone and pasture of trunk muscles.
Phylogenetically paleocerebellumis next part of the cerebellum to appear in terrestrial vertebrates with the appearance of limbs. It consists of anterior lobe (except lingula) and pyramid and uvula of inferior vermis. The paleocerebellum is chiefly spinocerebellar in connections and concerned with the tone, posture and crude movements of the limbs.
Phylogenetically neocerebellum is the most recent part of the cerebellum to develop. It develops in primates in association with the enlargement of the telencephalon and the cerebral cortex. It is prominent in higher mammals. It is made up of middle lobe, the largest part of the cerebellum (except the pyramid and the uvula of inferior vermis). The neocerebellum is chiefly corticoponto-cerebellar in connections and is concerned with smooth performance of skilled voluntary movements.
The cerebellum is made up of a thin surface layer of grey matter, the cerebellar cortex and a central core of white matter. Embedded within the central core of white matter are masses of grey matter called intracerebellar nuclei. The cerebellar cortex is folded in such a way that the surface of cerebellum presents a series of parallel transverse fissures and intervening narrow leaf-like bands called folia. Each folium consists of a slender branched lamina of central core of white matter covered by a thin layer of grey matter.
The cerebellum plays a key role in accomplishing cut a smooth and coordinated movements by means of its comparator function. If the cerebral cortex decides on a movement to do, the impulses from motor cortex stimulate the anterior horn cells (LMNs) of the spinal cord to initiate voluntary movement, and at the same time, the motor cortex tell to the cerebellum about the intended movement, the cerebellum also receives proprioceptive information from muscles and joints, telling it what movement is actually occurring.
The cerebellum compares the intended movement with the actual movement occurring, and if a difference is detected, the cerebellum sends the information to the motor cortex and lower motor neurons to correct the discrepancy. The result is smooth and coordinated movements. With training, a person can develop highly skilled and rapid movements. The cerebellum thus plays an important role in learning of motor skills.
The vertebrobasilar system arises from the first part of the subclavian arteries ( and travels cranially through the transverse foramina of the upper six cervical vertebrae. The left and right vertebral arteries unite after they enter the cranial vault through the foramen magnum at the pontomedullary junction. Here it forms the basilar artery. Three main branches provide vascular supply to the cerebellum, two of which arise from the basilar component of the system and one arises from each vertebral branch of the system.
The first branch from the basilar artery is the superior cerebellar artery. It arises bilaterally and inferior to CN III. It provides perfusion for the superior cerebellar region along with the superior medullary velum, pineal gland and pons. The anterior inferior cerebellar artery (AICA) arises inferior to CN VI at the pontomedullary junction at the proximal part of the basilar artery. It then courses posterolateral to supply the inferior and anterior cerebellar regions. Thirdly, a posterior inferior cerebellar artery (PICA) arises from each vertebral artery, just superior to the rootlet of CN XI. It provides arterial blood to the cerebellar nuclei and its inferior surface (including the vermis).
