A Continuous Time Model of Synaptic Plasticity in the Cerebellar Cortex
Patients with cerebellar damage exhibit a variety of motor deficits1, and both human and animal studies indicate that cerebellar lesions disrupt several forms of motor learning2. A number of competing hypotheses regarding the nature of cerebellar involvement in motor function have been proposed3–8. One particularly influential class of models, based on the original proposals of Marr9, and shortly thereafter amended by Albus10, asserts that motor memories are stored in the cerebellar cortex at synapses from parallel fibers onto Purkinje cells (pf*Pkj). This class of models can be characterized by the following four hypotheses. I) Climbing fiber inputs to the cerebellum, which originate from the inferior olive, are topographically organized by motor function. Each climbing fiber conveys a precise motor instruction to a group of target Purkinje cells, which in turn project to cells in the cerebellar nuclei capable of generating the instructed movement via projections to brain stem motor nuclei. 2) Mossy fiber inputs to the cerebellum, which originate from the pontine and other brain stem nuclei, are activated by virtually every sensory modality, as well as by direct inputs from the cerebral cortex. Granule cells, whose axons give rise to parallel fibers, receive a highly divergent input from mossy fiber afferents. The precise pattern of parallel fibers active at any given moment provides a sparse distributed representation of the mossy fiber input. 3) For any given Purkinje cell, each climbing fiber input induces a cell wide signal that causes the weights of any coactive pf*Pkj synapses to be depressed. 4) By repeatedly pairing a particular movement context with the appropriate pattern of climbing fiber activity, the occurrence of that context alone becomes sufficient to initiate the instructed movement. In particular, the occurrence of a previously paired movement context leads to a reduction of Purkinje cell firing, thereby initiating the instructed movement by releasing the appropriate target neurons in the cerebellar nuclei from tonic Purkinje cell inhibition. In order to account for the reversibility of learned motor behavior, the above model is typically augmented by an additional hypothesis: 5) When pf*Pkj synapses are active in the absence of a climbing fiber input, their synaptic weights are potentiated11,12. Thus, repeated presentations of a previously established movement context in the absence of activity in those climbing fibers which instruct for that movement will eventually eliminate the motor response associated with that context.
KeywordsPurkinje Cell Cerebellar Cortex Synaptic Weight Cerebellar Nucleus Parallel Fiber
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