Parallel fibers arise from granule cells in the cerebellar cortex. They form excitatory synapses onto the dendrites of Purkinje cells (the output neurons of the cerebellum).
Granule cells are very small and very numerous. They are thought to make up as many as half of the neurons in the brain. Granule cells have axons which rise up and then branch out into parallel fibers. These fibers intersect the Purkinje cell dendrites.
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