Cerebellar Thalamic and Thalamocortical Projections
Although it is well known that the major output of the cerebellum is directed to the thalamus and ultimately to the cerebral cortex, the anatomical details and functional organization of this system remains unclear. Here, the current status of the cytoarc
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Sharleen T. Sakai
Abstract
Although it is well known that the major output of the cerebellum is directed to the thalamus and ultimately to the cerebral cortex, the anatomical details and functional organization of this system remains unclear. Here, the current status of the cytoarchitecture of the motor thalamus, its afferents and efferent cortical projections are reviewed. The distribution of the cerebellothalamic and pallidothalamic projections to motor cortical areas is also discussed and the functional importance of these motor systems is highlighted.
Introduction Thalamus as the gateway to the cerebral cortex occupies a pivotal place in the processing of incoming and outgoing signals. Over the past 20 years, interest in the organization of the motor thalamus increased due to its role in the amelioration of tremor and rigidity following either thalamotomy (Ohye and Narabayashi 1979; Tasker et al. 1982; Ohye 1997) or deep brain stimulation (Benabid et al. 1996; Hubble et al. 1997; Starr et al. 1998). Despite the importance of the motor thalamus in motor control functions, details of its anatomical organization including its afferent and efferent connections still remain to be addressed. Thalamic studies are often stymied by the difficulty in defining and clearly delineating its constituent nuclei and their borders. The lack of agreement on thalamic terminology and nomenclature has also contributed to the confusion (for review, see Percheron et al. 1996, Ilinsky and Kultas-Ilinsky 2002; Jones 2007). Thalamic nuclei can be defined based on cytoarchitecture and chemoarchitecture but the term motor
S.T. Sakai Department of Psychology and Neuroscience Program, Michigan State University, 108 Giltner Hall, East Lansing, MI, 48824, USA e-mail: [email protected] M. Manto, D.L. Gruol, J.D. Schmahmann, N. Koibuchi, F. Rossi (eds.), 529 Handbook of the Cerebellum and Cerebellar Disorders, DOI 10.1007/978-94-007-1333-8_24, # Springer Science+Business Media Dordrecht 2013
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S.T. Sakai
thalamus refers to the projection territory of the basal ganglia (efferent projections of the substantia nigra and globus pallidus) and the deep cerebellar nuclei. Since these projections may not strictly adhere to nuclear boundaries, comparisons made from different experiments and based on different species are problematic. This is due, in part, to the difficulty in reliably delineating the boundaries of the motor thalamus across mammalian species since both the number and cytoarchitectural details of the constituent subnuclei vary across species (Jones 1985, 2007). Many studies compared results across different experiments and animals in concluding that the cerebellum and basal ganglia projections distributed to separate thalamic nuclei and that the thalamocortical projections to different motor cortical areas arose from separate thalamic nuclei (Schell and Strick 1984; Alexander et al. 1986; Jones 1985, 2007; Percheron et al. 1996). The primary problem with such comparisons is the uncertainty of applying the same thalamic nuclear bo
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