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CHAPTER32CerebellumThe cerebellum is a softball-sized structure located at the base of the skull. Grab the back of your head just above where it meets your neck: your hand is now cupped around your cerebellum. As with most brain systems, much of what we know about the cerebellum stems from symptoms of damage or pathology and from its connectivity with the rest of the brain. From such evidence it has long been clear that the cerebel-lum is an important component of the motor system. Severe abnormalities of movement are produced by pathologies of the cerebellum. Cerebellar outputs infl uence systems that are unambiguously motor, such as the rubro-spinal tract, and inputs to the cerebellum convey information known to be essential for move-ment such as joint angles and loads on muscles. More recently it has become equally clear that the role of the cerebellum is not limited to movement. This is indi-cated by its interconnections with nonmotor structures, by the more subtle, nonmotor defi cits seen with cere-bellar lesions, and by functional imaging studies where regions of the cerebellum show activation during nonmotor tasks. In moving toward a stronger under-standing of the cerebellum one obvious task will be to identify the common aspects or computational demands that these motor and nonmotor functions share.Research on the anatomy, physiology, and function of the cerebellum has been complemented and enhanced by computational approaches that empha-size the rules for input/output transformation and how the cerebellar neurons and synapses implement this transformation. This interdisciplinary approach was made possible by the seminal work of Eccles, Ito, and Szentagothai, who in 1967 published “The cerebel-lum as a neuronal machine,” which described most of the essential aspects of the cellular and synaptic orga-nization of the cerebellum. Soon after, a remarkable paper was published in 1969 by David Marr who inferred some basic computational properties of the cerebellum solely on the basis of the wiring diagram that Eccles and associates had worked out. Since then, the numerous conceptual and practical advantages of working on the cerebellum have enabled a more detailed understanding of what the cerebellum com-putes and how its neurons and synapses produce this computation.ANATOMY AND PHYLOGENETIC DEVELOPMENT OF THE CEREBELLUMThe cerebellum is present in all vertebrates and in the most primitive prevertebrates (myxinoids) up through primates (Jansen and Brodal, 1954; Larsell, 1967, 1970, 1972). In agnathans (lampreys and hagfi sh), it is a rudimentary structure that assists the functions of the well-developed vestibulo-ocular, vestibulospi-nal, and reticulospinal systems. The cerebellum is somewhat larger in fi shes where, on the input side, it processes sensory information from the vestibular, lateral line, and to a lesser extent proprioceptive and somatosensory systems. On the output side it is con-nected to the vestibular and reticular nuclei (Box 32.1). In amphibians the region of the cerebellum that receives proprioceptive and other sensory information is expanded. This region, called the corpus cerebella, increases further in reptiles, birds, and mammals. In these vertebrate classes, it constitutes the largest portion of the cerebellum, receiving proprioceptive, somatosensory, visual, and auditory information and projecting to the tectum, the red nucleus, and the Fundamental Neuroscience, Third Edition 751 © 2008, 2003, 1999 Elsevier Inc.752 32. CEREBELLUM V. MOTOR SYSTEMScerebral cortex via the thalamus. In primates, the hugely expanded lateral hemispheres of the corpus cerebella are connected with the enlarged cerebral cortex (Fig. 32.1). The hemispheres receive information from the frontal parietal, and visual cortices by way of pontine nuclei and project to the motor and premotor cortices as well as to more anterior portions of the frontal lobe (Fig. 32.2). (Asanuma et al., 1983a, 1983b, 1983c, 1983d; Middleton and Strick, 1994; Orioli and Strick, 1989; Sasaki et al., 1976; Schell and Strick, 1984)The Cerebellum Can Be Subdivided on the Basis of Phylogeny, Anatomy, and the Effect of LesionsSuperfi cially, the cerebellum consists of a three-layered cortex folded in thin, parallel strips called folia (leaves), which in most species run roughly transverse to the long axis of the body (Fig. 32.2). The cerebellar cortex surrounds three pairs of deep cerebellar nuclei. From medial to lateral, the deep cerebellar nuclei are the fastigius, the interpositus (which is further divided into the globose and emboliform nuclei in humans), and the dentate. The deep cerebellar nuclei and the vestibular nuclei constitute the output structures of the cerebellum. An inner mass of white matter contains axons that run between the cerebellar cortex and the deep cerebellar nuclei.The cerebellum is divided into three lobes (Jansen and Brodal, 1954). The fl occulonodular lobe (vestibu-locerebellum) is located on the inferior surface and is separated from the posterior lobe via the posterolateral fi ssure. Superior to the posterior lobe is the anterior lobe: these two lobes are separated by the primary fi ssure. The lobes are divided further into lobules (Larsell, 1967, 1970, 1972) (Fig. 32.2), which are num-bered I–X beginning at the dorsal anterior vermis and ending at the inferior posterior vermis. Each lobule contains a number of folia. Lobulation is fairly consis-The weakly electric fi sh (family Mormyridae) have an enormous cerebellar structure called the gigantocerebel-lum. On a per body-weight basis, the gigantocerebellum is comparable in weight to the human cerebellum. The largest portion of the gigantocerebellum, called the valvula, has a Purkinje cell layer that would stretch to approximately 1 m if unfolded. That length is similar to the length of the Purkinje cell layer in the entire human cerebellum. The valvula receives inputs from the electro-sensory organs, the lateral line, and possibly the visual system. The functional signifi cance of this massive expan-sion of the cerebellum is a mystery.BOX 32.1THE GIGANTOCEREBELLUM7. quadrangular lobule8. primary fissure9. simplex lobule10. superior posterior fissure11. superior semilunar lobule12. horizontal fissure13. inferior semilunar lobule14. ansoparamedian fissure15. gracile lobule16. prebiventral fissure17. biventral lobule18. secondary fissure19. cerebellar tonsil1. culmen2.declive3.vermal folium4. vermal tuber5. vermal


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UT PSY 394U - Cerebellum

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