Interrelated modification of excitatory and inhibitory connections in the olivocerebellar neural network.

A model of plasticity is proposed for the olivocerebellar neural network in which the efficiency of the synaptic inputs to different neurons changes simultaneously and interdependently. This effect is based on the following functional characteristics of the network: simultaneous arrival of an afferent signal via mossy fibers to input granule cells and output neurons in the deep cerebellar nuclei; synchronous arrival of the signal from the inferior olive, via climbing fibers and their collaterals, at cells in the input and output layers, and to Purkinje cells, and the existence of local excitatory, inhibitory, and disinhibitory feedback circuits. Increases (decreases) in post-tetanic Ca2+ concentrations relative to the level evoked by the preceding stimulation in these cells are accompanied by decreases (increases) in the activity of cGMP-dependent protein kinase G, with increases (decreases) in the activity of protein phosphatase I. As a result, dephosphorylation (phosphorylation) of ionotropic receptors is accompanied by simultaneous depression (potentiation) of the excitatory input to a given neuron and potentiation (depression) of the inhibitory input to the same neuron. The depolarizing signal from the inferior olive affects synapse modification in different layers of the network in such a way that its presence (absence) depresses (potentiates) the signal sent from the output cells of the cerebellum to other structures.