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are found in the cortex. The main inputs into the cerebellar cortex are mossy fibers and
climbing fibers (Figure 1).
Figure 1. Main neural circuits of the cerebellar cortex. Sensory information is transmitted to the
cerebellar cortex through mossy fibers (MF). They are regarded as elements of the sensory layer in a
three-layered perceptron. Mossy fibers project to granule cells (GC), corresponding to elements of the
associative layer in the perceptron. Granule cells project to a Purkinje cell (PC), corresponding to
elements of the output layer. A climbing fiber (CF) relays the teaching signal to a Purkinje cell. LTD
occurs at the synapses between parallel fibers (PF) and a Purkinje cell, when the synaptic activity is
followed by the climbing fiber activity. CF: climbing fiber, DCN: deep cerebellar nuclei, GC: granule
cell, IO: inferior olivary nuclei, MF: mossy fiber, MO: medulla oblongata, PC: Purkinje cell, PF:
parallel fiber, PN: pontine nuclei.
On the other hand, the output from the cortex is confined to axons of Purkinje cells,
which form GABAergic inhibitory synapses on neurons in the deep cerebellar nuclei or in the
vestibular nuclei. The cerebellar cortex has three layers - the molecular, Purkinje and
granular layer. The outermost molecular layer contains two types of inhibitory interneurons,
basket and stellate cells, dendrites of Purkinje cells and axons of granule cells called parallel
fibers. Parallel fibers run laterally to the body axis, and perpendicularly to the flat dendrites of
Purkinje cells. Beneath the molecular layer is the Purkinje layer, where cell bodies of Purkinje
cells spread over in a single layer. The innermost granular layer contains cell bodies of
granule and Golgi cells. Climbing fibers originate from neurons in the inferior olivary nuclei
located in the ventral side of brainstem. A mature Purkinje cell receives synaptic inputs from
only one climbing fiber, which forms as many as 300 glutamatergic synapses and provides a
strong excitatory effect inducing a characteristic action potential called complex spike [3].
Mossy fibers arise from neurons in the pontine nuclei or in the medulla oblongata. They form
excitatory synapses on granule cells. An axon of a granule cell goes up to the molecular layer,
where it bifurcates into a parallel fiber. A parallel fiber forms excitatory glutamatergic
synapses on Purkinje cells. Each of parallel fiber - Purkinje cell synapse is weak, but a single
Purkinje cell has more than 100 thousand parallel fiber synapses. Parallel fibers also form
synapses on basket, stellate and Golgi cells. Both basket and stellate cells form inhibitory
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