Like the basal ganglia, the cerebellum provides a motor loop pathway for fine-tuning motor commands generated in the cortex, particularly those involving multijoint movement and learning of new motor skills.
Purkinje cells in the spinocerebellar division of the cerebellum synapse within deep cerebellar nuclei and send information to the spinal cord to control limb movement. Purkinje cells in the vestibulocerebellum project directly to vestibular nuclei in the brain stem, coordinating their control of postural movements in response to gravity. Purkinje cells in the cerebrocerebellum provide smooth coordination of complex motor programs via synapses in the dentate nucleus that project to the spinal cord via the red nucleus and rubro-spinal tract, and to the cortex via the thalamus.
Projections, called climbing fibers, originating from the inferior olivary complex in the medulla, relay sensory information from the spinal cord to Purkinje cells.
Information from the cortex is sent to the cerebellum through synaptic relays in the pontine nuclei, which project as mossy fibers, then to granule cells of the cerebellum, and finally through connections called parallel fibers between granule cell axons and Purkinje cells.
Mossy fiber activity is modulated by climbing fiber input; local inhibitory neurons provide additional control over both granule cells and Purkinje cells.
Lesions of the cerebellum lead to specific motor deficits associated with loss of coordination, or ataxia.
The cerebellum, or "little brain,'' appears as a relatively separate appendage to the cerebrum (telen-cephalon/diencephalon) and brain stem. There is no anatomic linkage between the cerebellum and cerebrum; the two are separated by a dense sheet of connective tissue, the tentorium. Connections are made from the cerebellum to the brain stem through three bridges, or peduncles (superior, middle, and inferior). Through these connections, the cerebellum receives both motor and sensory information from all levels of the neural axis and, in turn, sends motor commands back to the spinal cord, brain stem, diencephalon, and motor cortex. Like the basal ganglia, the cerebellum functions as a major relay within motor loop pathways (Fig. 1). Its function is to allow individual components of a given motor plan to be carried out in a smooth fashion, in particular those requiring synergistic, multijoint movement. A general appreciation of cerebellar function can be gained by observing the uncoordinated movements of someone who is intoxicated. The clumsiness, inability to accomplish targeted movement, and loss of equilibrium are largely a direct consequence of depression of cerebellar circuits. The cerebellum also plays a critical role in learning new motor skills (e.g., learning to play a specific piece of music on the piano). With repeated practice of a motor sequence, the cerebellum helps formulate the required programs and then generates appropriate movement sequences on demand without further need for conscious control.
Superficially, the cerebellum looks like a wedge-shaped diminutive cerebrum (Figs. 2 and 3). Gray matter forms the superficial cortical layer, which sends processes to nuclei located within the inner core of the structure. The cerebellar cortex is highly convoluted, more so than the cerebral cortex. The slender compacted folds, called folia, are necessary to accommodate the disproportionately large number of cells present. While accounting for only 10% of the volume of the brain, the cerebellum contains 50% of the total number of neurons.
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