The Cerebellum

The brain structure most frequently associated with motor learning is the cerebellum. Tucked beneath the visual cortex in the posterior cranial fossa, the cerebellum is the largest subcortical structure, constituting approximately 15% of the brain's mass and containing more than half of the entire neurons in the brain. The structure is divided into three lobes, which are composed of 10 lobules. Given its tremendous size, it is likely that the cerebellum contributes to many behaviors, and much recent speculation has focused on nonmotor functions of the cerebellum. Nonetheless, as emphasized in the classic observations of the British neurologist Gordon Holmes, the hallmark of cerebel-lar dysfunction in humans is the breakdown of skilled movement.

Some remarkable features of the cerebellum serve as a useful background for considering how this structure might perform essential computations for motor learning. Perhaps the most striking feature of the cerebellum is its uniformity. Inputs to the cerebellar cortex come from two distinct pathways, the mossy fibers and the climbing fibers. Mossy fibers originate in various brain stem nuclei and synapse onto the cerebellar granule cells. The axons of the granule cells, or parallel fibers, extend for as long as a few centimeters, synapsing with various cell types within the cerebellar cortex. Principal among these are the Purkinje cells, the sole output of the cerebellar cortex, providing an inhibitory signal on the deep cerebellar and vestibular nuclei. The parallel fibers can also influence the Purkinje cells indirectly via inhibitory interneurons called basket cells and Golgi cells. A given Purkinje cell may receive input from 200,000 parallel fibers, although each parallel fiber will only synapse once on a particular Purkinje cell. Climbing fibers originate in the inferior olive. In stark contrast to the extreme divergence of information deriving from the mossy fibers, each Purkinje cell receives input from a single climbing fiber.

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