Feedback Control in the Olivo-Cerebellar Loop
About 40 years ago, a subpopulation of small neurons in the deep cerebellar nuclei were shown to project to the inferior olive, the source of the climbing fiber input to the cerebellum. This nucleo-olivary projection follows the zonal and probably also mi
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Fredrik Bengtsson and Germund Hesslow
Abstract
About 40 years ago, a subpopulation of small neurons in the deep cerebellar nuclei were shown to project to the inferior olive, the source of the climbing fiber input to the cerebellum. This nucleo-olivary projection follows the zonal and probably also microzonal arrangement of the cerebellum so that closed loops are formed between the neurons in the olive, the cerebellar cortex, and the deep nuclei. Although it was first thought to be excitatory, it was independently shown that the cells were GABAergic and that activity in the nucleo-olivary pathway inhibits olivary activity. A number of functions have been suggested for this inhibition: (a) feedback control of learning, (b) gating of olivary input in general, and (c) feedback control of background activity in Purkinje cells. Evidence is consistent with (a) and (c). Activity in the nucleo-olivary pathway suppresses both synaptic transmission and background activity in the olive. When conditioned blink responses develop, the blink-related part of the olive is inhibited. When the nucleo-olivary pathway is interrupted, there is a corresponding increase in complex spike discharge in Purkinje cells followed by a strong suppression of simple spike firing. Stimulation of the pathway has the opposite results. It is concluded that the nucleo-olivary fibers are inhibitory and that they form a number of independent feedback loops, each one specific for a microcomplex, in a system that regulates cerebellar learning as well as
F. Bengtsson (*) Department of Experimental Medical Science, Division for Neuroscience, University of Lund, BMC F10, Tornav€agen 10, Lund, SE-22184, Sweden e-mail: [email protected] G. Hesslow Department of Experimental Medical Science, Section for Neuroscience, University of Lund, Lund, 22100, Sweden e-mail: [email protected] M. Manto, D.L. Gruol, J.D. Schmahmann, N. Koibuchi, F. Rossi (eds.), 1079 Handbook of the Cerebellum and Cerebellar Disorders, DOI 10.1007/978-94-007-1333-8_45, # Springer Science+Business Media Dordrecht 2013
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F. Bengtsson and G. Hesslow
spontaneous activity in the cerebello-olivocerebellar circuit. Besides these inhibitory effects, it has been argued that the nucleo-olivary pathway regulates electrotonic coupling between olivary cells.
Introduction The inferior olive receives inhibitory input from several different sources. The focus here will be on the inhibitory fibers from the deep cerebellar nuclei to the inferior olive, the nucleo-olivary (NO) pathway. The motive for emphasizing this pathway is that it appears to form a feedback loop for controlling background firing of Purkinje cells as well as learning in the cerebellar cortex and that it is therefore of particular importance for understanding the operation of the cerebellum in general. Climbing fibers, originating in the inferior olive, contact Purkinje cells and interneurons in the cerebellar cortex as well as cells in the deep cerebellar nuclei. The deep cerebellar nuclei project to the cer
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