What Does the Cerebellum Do?
The cerebellum is a powerful processing device that we can use to improve the performance of our brains in a multitude of tasks. The cerebellum has broad connectivity and powerful intrinsic computational mechanisms that have the potential to help the forebrain with motor, sensory, cognitive, and social demands. The components of the cerebellum are arranged in a loop that is continuously active, ready to help out. Different cerebellar modules make recurrent connections with the brainstem and with the forebrain, including the parietal and frontal lobes. The cerebellum does computations on a scale that is found nowhere else in the brain. For this purpose, it has enormous numbers of neurons and synapses, as well as some of the largest and some of the smallest neurons in the brain. It has several specialized structures: layers of parallel axons; other axons that climb like ivy; dendritic arbors that resemble sea fans; glomeruli; and giant synapses. Even its ion channels are unusual. Theories of cerebellar function make use of the efficient, highly stereotyped, modular structure of the cerebellum for learning, for pattern recognition, and for timing. Modelers have tested several of these theories for plausibility, and some are also supported by experimental evidence. Key features of the successful models are synaptic plasticity; synchrony of firing; and filtering. Synaptic mechanisms for learning and memory in the cerebellum include cerebellar LTD and several other kinds of plasticity in the cerebellar cortex and in the deep cerebellar nuclei. Most, if not all, of the synapses in the cerebellum are plastic and, at least, some of the neurons can also modify their intrinsic excitability. Using such mechanisms, the cerebellum learns to carry out new motor tasks and to correct existing ones, making them quick and accurate. Inhibitory interneurons in the cerebellar cortex, including the unique basket cells, play a major role in timing cerebellar responses and in the learning process itself. Timing also depends on cells in the inferior olive that are interconnected in synchronous groups. In collaboration with other brain systems, the cerebellum uses sensory signals and internal models to control movement very precisely, time it accurately, and correct it immediately, contributing its computational power to make our movements efficient and accurate. Coordinated movement is a very complex endeavor, and is absolutely necessary for survival. The
The Cerebellum: Learning Movement, Language, and Social Skills, First Edition. Dianne M. Broussard. C 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
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What Does the Cerebellum Do?
cerebellum solves some of the more difficult aspects of movement, such as coordination across joints and directing movement at targets in the outside world. It balances us on our two (rather small) feet and allows us to leap, twirl, and run down stairs if we feel like it. Our manual dexterity may be largely due to the cerebellum and its collaboration with the cerebral cortex. Best of all, during routine tasks such as walking, it leaves our thoughts completely free to concentrate on other issues. But the cerebellum does not stay out of our thought processes. In fact, the increase in speed and accuracy that the cerebellum provides may extend across all realms of brain function. The posterior lobes and ventral dentate nucleus, which are hugely expanded in humans, seem to have taken on many non-motor tasks that were previously thought to be allocated exclusively to the cerebral cortex. This is a busy area of research, and in Section IV, I have outlined some of the exciting ideas that are beginning to garner support. The cerebellum may be involved in processes as diverse as emotion, language, working memory, and social behavior. I expect that in the next few years, considerably more will be learned about the functions of the human cerebellum. I was in graduate school in 1979, and learned about the cerebellum from Maurizio Mirolli, a talented and inspiring teacher. But I found it difficult to grasp just what it is that the cerebellum does. Before his next lecture, inundated by questions in the hallway (I was not the only one!), Dr. Mirolli emphatically reiterated his point: “The cerebellum does EVERYTHING!” This could not possibly be true, I thought. But in fact Mirolli, like Masao Ito and others, made an intuitive leap that has, largely, been supported. As I hope this book has convinced you, most brain functions, if defined broadly enough, make use of the cerebellum.
